Routing Protocols- Dynamic Routing

Packet routing in the Internet is divided into two general groups

Interior and Exterior Routing.

Interior routing with (IGP’s) Interior Gateway protocols happens inside independent network systems also known as autonomous systems. Within an autonomous system (AS), routing information is exchanged using an interior routing protocols RIP, RIPv2, IGRP, EIGRP, OSPF, IS-IS.

The Exterior routing protocols (EGP’s) External Gateway Protocols are used between the autonomous systems such as Boarder Gateway protocol, BGP.

There are two main types of algorithms for IP routing: Distance Vector and Link State Routing

Within each autonomous system routing is done separately from other autonomous systems, Routing between autonomous systems is performed by running BGP (Border Gateway Protocol).

IGP (Interior Gateway Protocol) is a protocol for routing within a single autonomous system These protocols define how to route to networks within the AS, and can also distribute routing information for networks outside the AS (that information will come, naturally, from the routers at the edges of the AS, which also run the BGP protocol).

Distance Vector Routing and Link State Routing.

Distance Vector protocols determine best path on how far the destination is.Link State protocols are more sophisticated taking into consideration link variables, such as bandwidth, delay, reliability and load.  Distance Vector protocols judge best path on how far it is. Distance can be hops or a combination of metrics calculated to represent a distance value. The Distance Vector routing protocols are: Routing Information Protocol (RIP v1 and v2) and (IGRP) Interior Gateway Routing Protocol.

Distance-vector routing protocols are simple and efficient in small networks, and require little, if any management. However, they do not scale well, and have poor convergence properties, which has led to the development of more complex but more scalable link-state routing protocols for use in large networks.

A Link-state routing is a concept used in routing of packet-switched networks in computer communications. Link-state routing works by having the routers tell every router on the

network about its closest neighbors. The entire routing table is not distributed from any router, only the part of the table containing its neighbors. 

Link-state routing protocols are the OSPF, IS-IS, EIGRP and Novell's NLSP (NetWare Link State Protocol) which only supports IPX. This type of routing protocol requires each router to maintain at least a partial map of the network. When a network link changes state (up to down, or vice versa), a notification, called a link state advertisement (LSA) is flooded throughout the network. All the routers note the change, and re-compute their routes accordingly.

Link State Routing protocols provide greater flexibility and sophistication than the Distance Vector routing protocols. They reduce overall broadcast traffic and make better decisions about routing by taking characteristics such as bandwidth, delay, reliability, and load into consideration, instead of just distance or hop count.

Exploring the update behavior of distance-vector and link-state protocols from the perspective of a single router R.

For the following scenarios, consider whether R will "always", "never", or "sometimes" transmit routing data as a result of a certain event in the network

 

Interior Gateway Routing Protocol (IGRP)

IGRP is a Cisco proprietary distance vector protocol. This means to use IGRP all routers must be Cisco. IGRP has a max hop count of 255 with the default being 100. IGRP uses bandwith and delay as the default metric this is called a composite metric. Reliability, load and MTU maximum transmission unit can also be used.

QuestionYou are configuring a router. In particular, a routing table that contains static, RIP, and IGRP routers for the same destination network with each set to its default administrative distance. He asks you which route will be used to forward data?

A. The IGRP routeB. The static routeC. The RIP routeD. All three will load balance.

Answer B

ExplanationTo decide which route to use, IOS uses a concept called Administrative Distance. Administrative distance is a number that denotes how believable an entire routing protocol is on a single router. The lower the number, the better, or more believable the routing protocol.By default, static routes have an administrative distance of 1.

QuestionYou want to configure a router for load balancing across 4 unequal cost paths on your network. Which of the following routing protocols can you use? (Choose two)

A. RIP v1 B. RIP v2 C. OSPF D. IGRP E. EIGRP F. VLSM Answer D & E

Routing Basics

Routing is the process of directing packets from a source node to a destination node on a different network. Getting packets to their next hop requires a router to perform two basic activities: path determination and packet switching.

Path determination Involves reviewing all paths to a destination network and choosing the optimal route.  To determine the optimal route, information is put in a route table, which includes information such as destination network, the next hop, and an associated metric.

Packet switching Involves changing a packet's physical destination address to that of the next hop (the packet's logical destination and source addresses will stay the same).

Information a Router needs to know to route a packet Destination address. Neighbor routers. Possible routes to all remote networks. The best route to each network. How to maintain and verify routing information.

There are two main types of routing, static and dynamic, the third type of routing is called Hybrid. Static routing involves the cumbersome process of manually configuring and maintaining route tables by an administrator. Dynamic routing enables routers to "talk" to each other and automatically update their routing tables. This process occurs through the use of broadcasts.

Static Routing

Advantages No over head on CPU No Bandwidth between routers

Increased security as the administrator can choose which networks to allow to route.

Disadvantages Administrative overhead - adding a router will mean adding its route to all routers

manually.

Dynamic Routing

Routing protocols are protocols that implement routing algorithms, to build tables used in determining path selection for routed protocols. Dynamic routing protocols assist in the automatic creation of routing tables. Network topologies are subject to change at any time. A link may fail unexpectedly, or a new link may be added. A dynamic routing protocol must discover these changes, automatically adjust its routing tables, and inform other routers of the changes. The process of rebuilding the routing tables based on new information is called convergence.

Interior gateway protocols (IGPs) are used to exchange routing information with routers in the same autonomous system AS.

Autonomous System (AS) is a collection of networks under a common administrative domain this means all routers sharing the same routing table information are in the same AS.

Exterior gateway protocols (EGPs) are used to communicate between ASes.An example of an EGP is the Border Gateway Protocol (BGP)

Administrative Distances

The administrative distance (AD) is used to rate the trustworthiness of routing information received by a router from a neighboring router. 0 is most trusted and 255 is not trusted and will not be used.

The lower the number, the more trustworthy the type of route isStatic Route to a connected interface = 0Static Route to a IP address = 1Internal EIGRP = 90OSPF = 110RIP = 120

If a router receives two updates listing the same remote network the router will check the AD’s and accept the route with lowest AD to be included in the routing table. If both advertised routes have the same AD then routing metrics such as hop count or bandwidth are taken into account and the lowest metric is used for the best path. If the advertised route has the same AD and metrics then the router will load-balance to the remote network.

If a network is directly connected the router always uses the interface connected to the network if an administrator configures a static route that is given an AD of 1 so if you have a static route a RIP advertised route and a IGRP advertised route listing the same network the router always uses the static route unless you change the AD.

Routing Protocols

Most routing algorithms can be classified into one of two categories distance vector link-state

The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork. The link-state approach, also called shortest path first, recreates the exact topology of the entire internetwork

Distance-Vector Routing Protocols

Distance-vector routing refers to a method for exchanging route information. A router will advertise a route as a vector of direction and distance. Direction refers to a port that leads to the next router along the path to the destination, distance is a metric that indicates the number of hops to the destination. Each router learns routes from its neighbouring routers' perspectives and then advertises the routes from its own perspective. Because each router depends on its neighbours for information, which they in turn may have learned from their neighbours, and so on, distance vector routing is also referred to as "routing by rumour."

distance vector routing protocols all send out complete routing tables periodically.

(RIP) Routing Information Protocol uses hop count its only metric

Cisco's propriety (IGRP) Internet Gateway Routing Protocol includes bandwith and delay when calculating metrics.

Problems with Distance -Vector  The main problem with distance -vector is that it updates the network step by step so it requires more bandwidth to process the information.More problems include: Routing loops and Counting to infinity.Distance vector algorithms also do not allow a router to know the exact topology of an internetwork as each router only sees its neighbouring routers.

Each router that uses distance vector routing begins by identifying its own neighbors. The interface that leads to each directly connected network is shown as having a distance of 0. As the distance vector network discovery process proceeds, routers discover the best path to destination networks based on the information they receive from each neighbor. Router A learns about other networks based on the information that it receives from Router B. Each of the other network entries in the routing table has an accumulated distance vector to show how far away that network is in a given direction.

Route Invalidation Timers

When an internetwork is fully converged, how will it handle reconvergence when some part of the topology changes? If network 10.1.5.0 goes down, the answer is simple enough—router A, in its next scheduled update, flags the network as unreachable and passes the information along.

10.1.1.0 10.1.2.0 10.1.3.0 10.1.4.0 10.1.5.0

But what if, instead of 10.1.5.0 going down, router A fails? Routers B, C, and D still have entries in their route tables about 10.1.5.0; the information is no longer valid, but there's no router to inform them of this fact. They will unknowingly forward packets to an unreachable destination—a black hole has opened in the internetwork.

This problem is handled by setting a route invalidation timer for each entry in the route table. For example, when router B first hears about 10.1.5.0 and enters the information into its route table, B sets a timer for that route. At every regularly scheduled update from router A, B discards the update's already-known information about 10.1.5.0 as described in "Routing by Rumor." But as B does so, it resets the timer on that route.

If router A goes down, B will no longer hear updates about 10.1.5.0. The timer will expire, B will flag the route as unreachable and will pass the information along in the next update.

Typical periods for route timeouts range from three to six update periods. A router would not want to invalidate a route after a single update has been missed, because this event may be

the result of a corrupted or lost packet or some sort of network delay. At the same time, if the period is too long, reconvergence will be excessively slow.

Split Horizon

According to the distance vector algorithm as it has been described so far, at every update period each router broadcasts its entire route table to every neighbour. But is this really necessary? Every network known by router A in with a hop count higher than 0, has been learned from router B. Common sense suggests that for router A to broadcast the networks it has learned from router B back to router B is a waste of resources. Obviously, B already knows about those networks.

A route pointing back to the router from which packets were received is called a reverse route. Split horizon is a technique for preventing reverse routes between two routers.

So the routing protocol knows which interface a network route was learned on and will not advertise the route back out to same interface.

Now a packet with a destination address of 10.1.5.3 arrives at router B. B consults its route table and forwards the packet to A. A consults its route table and forwards the packet to B, B forwards it back to A, ad infinitum. A routing loop has occurred.

Implementing split horizon prevents the possibility of such a routing loop. There are two categories of split horizon: simple split horizon and split horizon with poisoned reverse.

Simple split horizon does not advertise routes back to the neighbours from which the routes were learned.

The routers in implement simple split horizon. Router B sends an update to router A for networks 10.1.1.0, 10.1.2.0, and 10.1.3.0. Networks 10.1.4.0 and 10.1.5.0 are not included because they were learned from router A. Likewise, updates to router C include 10.1.4.0 and 10.1.5.0 with no mention of 10.1.1.0, 10.1.2.0, and 10.1.3.0.

10.1.1.0 10.1.2.0 10.1.3.0 10.1.4.0 10.1.5.0

10.1.4.0 10.1.1.010.1.5.0 10.1.2.0

10.1.3.0

Simple split horizon works by suppressing information. Split horizon with poisoned reverse is a modification that provides more information.

Split Horizon with poisoned reverse

10.1.1.0 10.1.2.0 10.1.3.0 10.1.4.0 10.1.5.0

hops hops 10.1.1.0 inf 10.1.1.0 2 10.1.2.0 inf 10.1.2.0 1 10.1.3.0 inf 10.1.3.0 0 10.1.4.0 0 10.1.4.0 inf 10.1.5.0 1 10.1.5.0 inf

In the scenario router B would in fact advertise 10.1.4.0 and 10.1.5.0 to router A, but the network would be marked as unreachable. Above shows what the route tables from B to C and A would look like. Notice that a route is marked as unreachable by setting the metric to infinity;

Split horizon with poisoned reverse advertises reverse routes but with an unreachable (infinite) metric.

Split horizon with poisoned reverse is considered safer and stronger than simple split horizon—a sort of "bad news is better than no news at all" approach. For example, suppose that router C in receives corrupted information causing it to believe that subnet 10.1.1.0 is reachable via router B. Simple split horizon would do nothing to correct this misperception, whereas a poisoned reverse update from router B would immediately stop the potential loop. For this reason, most modern distance vector implementations use split horizon with poisoned reverse.

Split horizon will not prevent routing loops here.

10.1.4.0 10.1.5.0

Link failure

10.1.1.0 10.1.3.0

10.1.2.0 I can reach 10.1.5.0 via D one hop or via B

3 hops away.

Question

The graphic shows a network that is configured to use RIP routing protocol. Router2 detects that the link to Router1 has gone down. It then advertises the network for this link with a hop count metric of 16. Which routing loop prevention mechanism is in effect?

A. split horizon B. error condition C. hold-down timer D. route poisoning E. count to infinity

Answer D

route poisoning

Counting to Infinity

Split horizon will break loops between neighbours, but it will not stop loops in a network such as the one in above. Again, 10.1.5.0 has failed. Router D sends the appropriate updates to its neighbours router A (the dashed arrows) and router C (the solid arrows). Router C marks the route via D as unreachable, but router B is advertising a next-best path to 10.1.5.0, which is 3 hops away. C posts that route in its route table.

C now informs D that it has an alternative route to 10.1.5.0. D posts this information and updates A, saying that it has a 4-hop route to the network. A tells B that 10.1.5.0 is 5 hops away. B tells C that the network is now 6 hops away.

"Ah," router C thinks, "router B's path to 10.1.5.0 has increased in length. Nonetheless, it's the only route I've got, so I'll use it!"

C changes the hop count to 7, updates D, and around it goes again. This situation is the counting-to-infinity problem because the hop count to 10.1.5.0 will continue to increase to infinity. All routers are implementing split horizon, but it doesn't help.

Setting a maximum hop count of 15 helps solve the counting-to-infinity problem, but convergence will still be very slow. Given an update period of 30 seconds, a network could take up to 7.5 minutes to reconverge and is susceptible to routing errors during this time. The two methods for speeding up reconvergence are triggered updates and holddown timers.

Triggered Updates

Triggered updates, also known as flash updates, are very simple: If a metric changes for better or for worse, a router will immediately send out an update without waiting for its update timer to expire. Reconvergence will occur far more quickly than if every router had to wait for regularly scheduled updates, and the problem of counting to infinity is greatly reduced, although not completely eliminated. Regular updates may still occur along with triggered updates. Thus a router might receive bad information about a route from a not-yet-

reconverged router after having received correct information from a triggered update. Such a situation shows that confusion and routing errors may still occur while an internetwork is reconverging, but triggered updates will help to iron things out more quickly.

A further refinement is to include in the update only the networks that actually triggered it, rather than the entire route table. This technique reduces the processing time and the impact on network bandwidth.

Holddown Timers

A holdown prevents regular update messages from reinstating a route that is going up and down (flapping). Typically this happens on a serial link that’s losing connectivity and then coming back up. Holdowns prevent routes from changing rapidly by allowing time for either the downed route to come back up or the network to stablise before it tries to reconverge. Holddown timers introduce a certain amount of skepticism to reduce the acceptance of bad routing information.

If the distance to a destination increases (for example, the hop count increases from 2 to 4), the router sets a holddown timer for that route. Until the timer expires, the router will not accept any new updates for the route.

Obviously, a trade-off is involved here. The likelihood of bad routing information getting into a table is reduced but at the expense of the reconvergence time. Like other timers, holddown timers must be set with care. If the holddown period is too short, it will be ineffective, and if it is too long, normal routing will be adversely affected.

Asynchronous Updates

Routers should not broadcast their updates at the same time; if they do, the update packets will collide. Yet this situation is exactly what can happen when a several routers share a broadcast network. System delays related to the processing of updates in the routers tend to cause the update timers to become synchronized. As a few routers become synchronized, collisions will begin to occur, further contributing to system delays, and eventually all routers sharing the broadcast network may become synchronized.

If update timers become synchronized, collisions may occur.

Asynchronous updates may be maintained by one of two methods:

Each router's update timer is independent of the routing process and is, therefore, not affected by processing loads on the router.

A small random time, or timing jitter, is added to each update period as an offset.

Link State Routing Protocols also called shortest path first or distributed database protocols

A link state router cannot be fooled as easily into making bad routing decisions, because it has a complete picture of the network. The reason is that unlike the routing-by-rumor approach of distance vector, link state routers have firsthand information from all their peer routers. (all routers speaking the same routing protocol.) Each router originates information about itself, its directly connected links, and the state of those links (hence the name). This information is passed around from router to router, each router making a copy of it, but never changing it. The ultimate objective is that every router has identical information about the internetwork, and each router will independently calculate its own best paths.

Link-state routing uses:

Link-state advertisements (LSAs) – A link-state advertisement (LSA) is a small packet of routing information that is sent between routers.

Topological database – A topological database is a collection of information gathered from LSAs.

SPF algorithm – The shortest path first (SPF) algorithm is a calculation performed on the database resulting in the SPF tree.

Routing tables – A list of the known paths and interfaces.

Link state routing protocols only send out routing updates when a change in the internetwork forces them to change their routing table, these updates only contain the changes that have occurred in the internetwork they don’t send out the whole routing table. These link states updates are called Link State Advertisements (LSA) they are much less bandwidth intensive and flood throughout the network. All the routers note the change, and recompute their routes accordingly.

When the router receives an LSA, the database is updated with the most recent information and computes a map of the internetwork using the accumulated data and calculates the

shortest path to other networks using the SPF algorithm. Each time an LSA packet causes a change to the link-state database, SPF recalculates the best paths and updates the routing table.

Link-state concerns

Processor overhead Memory requirements Bandwidth Consumption

Routers running link-state protocols require more memory and perform more processing than distance vector routing protocols. Routers must have sufficient memory to be able to hold all the information from the various databases, the topology tree, and the routing table. Initial link-state packet flooding consumes bandwidth. During the initial discovery process, all routers using link-state routing protocols send LSA packets to all other routers. This action floods the internetwork and temporarily reduces bandwidth available for routed traffic carrying user data. After this initial flooding, link-state routing protocols generally require only minimal bandwidth to send infrequent or event triggered LSA packets reflecting topology changes.

Link State protocols only send updates when a change occurs, which makes them more efficient for larger networks. Bandwidth and delay are the most widely used metrics when using Link-State protocols. Eg: OSPF NLSP and IS-IS.

Benefits of Link State protocols:  

1. Allows for a larger scalable network 2. Reduces convergence time

3. Allows “supernetting”

QuestionWhy is it difficult for routing loops to occur in networks that use link-state routing?

A. Each router builds a simple view of the network based on hop count.B. Routers flood the network with LSAs to discover routing loops.C. Each router builds a complete and synchronized view of the network.E. Routers use hold-down timers to prevent routing loops.

Answer C

Each router builds a complete and synchronized view of the network.

Exam Essentials

To configure RIP routing

enableconfig t 1st enter global configuration mode.Router(config)#router rip type router ripThen add all directly connected networks using a classful address ie if network 10.1.1.0 is directly connected the command would be network 10.0.0.0

Router(config-router)#network 10.0.0.0Router(config-router)# ˆZRouter#

Configuring RIPv2

config t Router(config)#router ripRouter(config-router)#network 10.0.0.0Router(config-router)#version 2

Just add version 2

Troubleshooting Routing Protocols

Its important to verify your configuration of routing protocols the same commands used to verify are also used to troubleshoot.

The show ip route command displays the current contents of the routing table.

RouterA#sh ip routeCodes: C – connected etc… , D –EIGRPGateway of last resort is not setD 192.168.30.0/24 [90/2172416] via 192.168.20.2, 00:04:36, serial0/0 C 192.168.10.0/24 is directly connected, FastEthernet0/0D 192.168.40.0/24 [90/2681856] via 192.168.20.2, 00:04:36, serial0/0C 192.168.20.0/24 is directly connected, serial0/0D 192.168.50.0/24 [90/2707456] via 192.168.20.2, 00:04:35, serial0/0RouterA#

QuestionAfter typing the command show ip route, you see a list of IP addresses with [120/8] next to them. What does the [120/8] mean?

A. Cost/MetricB. Administrative Distance/MetricC. Metric/DistanceD. Cost/Hop Count

Answer B

The routing table information shown gives the administrative distance (AD) with the metric. A is incorrect, as the [/] shows the AD and metric. Cost is a metric used by OSPF. C is incorrect, as the AD is shown first, then the metric, not the other way around. D is incorrect, as the [AD/Metric] is not shown as cost and hop count.

The show protocols command

The show protocols command displays all the routed protocols and the interface upon which the protocol is enabled.

RouterB#show ip protocolGlobal values:Internet Protocol routing is enabledFastEthernet0 is up, line protocol is upInternet address is 192.168.30.1/24Serial0/0 is up, line protocol is upInternet address is 192.168.20.2/24Serial0/1 is up, line protocol is upInternet address is 192.168.40.1/24RouterB#

This output shows the IP address of the FastEthernet 0/0, Serial 0/0, and Serial 0/1 interfaces of RouterB.

The show ip protocols command. Displays the routing protocols configured on your router. The following output shows that both RIP and IGRP are running on the router. Only the IGRP shows on the routing table because it has a lower administrative distance AD.

RouterB#show ip protocolsRouting Protocol is “rip”Sending updates every 30 seconds next due in 6 secondsInvalid after 180 seconds, hold down 180, flushed after 240Outgoing update filter list for all interfaces isIncoming update filter list for all interfaces isRedistributing: ripDefault version control: send version1 receive any versionInterface Send Recv Key-chainFastEthernet 1 1 2Serial0/0 1 1 2Serial0/1 1 1 2Routing for networks192.168.10.0192.168.20.0192.168.30.0Routing Information Sources:Gateway Distance Last Update192.168.40.2 120 00:00:21192.168.20.1 120 00:00:23Distance: (default is 120)

Routing Protocol is “igrp 10”Sending updates every 90 seconds next due in 42 secondsInvalid after 270 seconds, hold down 280, flushed after 630Outgoing update filter list for all interfaces isIncoming update filter list for all interfaces isDefault networks flagged in outgoing updatesDefault networks accepted from incoming updatesIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0IGRP maximum hopcount 100IGRP maximum metric variance 1Redistributing: eigrp 10, igrp 10Routing for networks192.168.10.0192.168.20.0192.168.30.0Routing Information Sources:Gateway Distance Last Update

192.168.40.2 100 00:00:47192.168.20.1 100 00:01:18Distance: (default is 100)

The show ip protocols command includes the autonomous system (AS), routing timers, networks advertised, gateways and AD (100)

These commands can be used on all IP routing protocols

Exam Questions & Answers

QuestionThe full command to see all debugs running on the router is R3#show debug (IP packet debugging is on)

and the full command to turn off all running debugs is R3#undebug all (All possible debugging has been turned off) QuestionThe default behaviour of a distance vector protocol that does not allow a router to send a route advertisement out the same interface upon it was originally learned about is?

Answer: Split horizonSplit horizon prevents a router interface from advertising a route out of the same interface upon which it was learned in the first place.

Question You issue the command show ip route which of the following correctly describes the codes displayed in your route table after you issue this command? (Choose two.) A. I-Indicates a route was learned through an internal protocol. B. S-Indicates a route was learned through static command. C. R-Indicates a route was learned through RIP. D. S-Indicates a route was learned through a serial port. E. R-Indicates a route was learned through a reliable port.

Answer B, C

Question The network 131.107.4.0/24 was advertised by a neighbour router from RIP and IGRP. You also added a static route to 131.107.4.0/24 manually. Which route would be used to forward traffic?

A. The IGRP route. B. The static route. C. The RIP route. D. All three will load balance.

Answer B 

QuestionYour boss asks you to explain the difference between a routed protocol and a routing protocol. Which of the following statements best describe the difference? (Choose two.)

A. Routed protocols cannot cross routers.B. Routing protocols can identify data.C. Routed protocols help transport data between network segments.D. Routing protocols are used by routers to communicate routing information.

Answers C and D

Routed protocols such as IP or IPX are used to help communicate data across network segments by using logical addressing, which identifies both the network and host that need to communicate. D is a correct answer, as routers use routing protocols such as RIP and OSPF to communicate changes in their routing tables. A is incorrect because routed protocols can cross routers. B is incorrect because routing protocols populate only routing tables; they cannot identify data

QuestionYour boss is concerned about routing loops with the use of distance vector routing protocols such as RIP and IGRP in your network you would like to ensure him that there are mechanisms used to prevent the possibility of a routing loop. Which of the following are examples of this mechanism? (Choose two.) A. Link-state advertisement (LSA) B. Spanning Tree Protocol. C. Shortest path first tree. D. Split horizon. E. Hold-down timers. Answer D, E

ExplanationRIP and IGRP are distance vector routing protocols. RIP and IGRP use holddown counters and split horizon to prevent route looping.

Note: Hold-downs are used to prevent regular update messages from inappropriately reinstating a route that might have gone bad. Split horizons derive from the premise that it is never useful to send information about a route back in the direction from which it came.

Incorrect Answers A, Link-state routing protocols use LSAs. However, LSAs are not used by distance vector routing protocols like RIP or IGRP. B, C: The spanning tree protocol and the shortest path first tree is used by Layer 3 routing protocol such as Enhanced IGRP or OSPF, not by RIP or IGRP.

QuestionWhat one of the following protocols is an example of a link state routing protocol that uses the TCP/IP protocol stack?

A. IP B. IS-IS

C. NLSP D. OSPF E. RIP ver 2 Answer D

ExplanationOSPF is a link-state protocol that can be used in routing that is part of the TCP/IP protocol stack. Incorrect AnswersA. IP is a protocol, and a routed protocol, but NOT a routing protocol. B. IS-IS is a routing algorithm used by the ISO protocol stack. C. NLSP is part of the Novell protocol stack. E. RIP (and RIP V2) are distance vector protocols.

QuestionThere are three major groups of routing protocols: distance-vector protocols, link-state protocols, and hybrid protocols. Select two valid statements regarding routing protocols? (Choose two) A. Distance vector protocols send the entire routing table to directly connected neighbors. B. Link state protocols send the entire routing table to all routers in the network. C. Distance vector protocols send updates about directory connected neighbors to all networks listed in the routing table. D. Link state protocols send updates containing the state of their own links to all other routers on the network. Answer A, D

ExplanationDistance vector protocols send their entire routing table to adjacent routers. Link-state protocols only send link-state updates to all routers on their network (or autonomous system). Incorrect AnswersB. Link state protocols do not send their entire routing table rather only updates on their own links. C. Distance vector protocols in fact send their entire routing tables.

QuestionWhen Layer 2 devices were first created there was no way of detecting bridging loops. A protocol was created by DEC to stop these loops. What protocol did DEC create? A. Virtual LANs. B. Frame filtering. C. Cut through switching. D. Spanning tree protocol. Answer D

Explanation Spanning Tree protocol builds ONE path through all the nodes, and eliminates any loops. Anything sent along the tree will not encounter any loops because the protocol will eliminate any loops.Incorrect AnswersA. VLANs is not a loop resolution technology. B. Frame Filtering is not a loop resolution technology.C. Cut through switching is not a loop resolution technology. Rather it is a form of forwarding.

QuestionWhich of the following statements regarding routed and routing protocols are true?(Choose two) A. A routed protocol is assigned to an interface and determines the method of packet delivery. B. A routing protocol determines the path of a packet through a network. C. A routed protocol determines the path of a packet through a network. D. A routing protocol operates at the transport layer of the OSI model. E. A routed protocol updates the routing table of a router.

Answer A, B

ExplanationA. Routed Protocol delivers data to an interface or host.B. Routing Protocol Routes data. Incorrect answers:C, E. Routed protocol only delivers data, Routed Protocol will not update any routing tables. D. A Routing Protocol operates on Layer 3 of OSI Model. ie Network layer.

QuestionYou are configuring a network at a main site in Toronto. You use a distance vector routing protocol.What could you use to prevent routing loops in the network? (Choose two)

A. Link-state advertisements (LSA)B. Spanning Tree ProtocolC. Shortest path first treeD. Split horizonE. Hold-down timers

Answer D, E

ExplanationSplit horizon – the routing protocol advertises routes out an interface only if they were not learned from updates entering that interface.Hold-down timer – After finding out that a router to a subnet has failed, a router waits a certain period of time before believing any other routing information about that subnet.

QuestionWhich statement describes the rule of split horizon?

A. Only routers can split boundaries (horizons) between concentric networks.B. All distance vector protocols require fall back routers that may cause momentary loops as the topology changes.C. Networks can only remain fully converged if all information about routers is sent out all active interfaces.D. Information about a route should not be sent back in the direction from which the original update come.E. Each AS must keep routing tables converged to prevent dead routes from being advertised across the AS boundary.

Answer D

ExplanationSplit horizon includes two related concepts that affect what routes are included in a routing updateAn update does not include the subnet of the interface out which the update is sentAll routes with outgoing interface of interface x are not included in updates sent out that same interface x.

QuestionWhich of the following technologies can be used in distance vector routing protocols to prevent routing loops? (Select all valid answer choices)

A. Spanning Tree Protocol B. Shortest path first tree C. Link-state advertisements (LSA) D. Hold-down timers E. Split horizon F. VRRP

Answer D, E

Explanation Distance vector routing protocols use the rule of split horizons and hold down timers to prevent routing loops after a topology change. * Splithorizon - the routing protocol advertises routes out an interface only if they were not learned from updates entering that

interface. * Hold-down timer - After finding out that a router to a subnet has failed, a router waits a certain period of time before believing any other routing information about that subnet.

Incorrect AnswersA. STP is used in bridged LANs to prevent bridging loops. It is a means for preventing loops at layer two, not layer 3. B, C. These are two of the mechanisms of Link State Protocols, not distance vector protocols. F. VRRP is the Virtual Router Redundancy Protocol, which is a standards based method similar to Cisco's proprietary HSRP. Neither of these two methods deal with distance vector routing protocols.

QuestionWhat is the reason for configuring a passive interface on a router? (Select only one answer)

A. Allows interfaces to share common IP addresses. B. Allows an interface to remain up without the aid of keepalives. C. Allows a router to send routing and not receive updates via that interface. D. Allows a routing protocol to forward updates that is missing its IP address. E. Allows a router to receive routing updates on an interface but not send updates via that interface.

Answer E

Explanation: The passive-interface command is used to control the advertisement of routing information. The command enables the suppression of routing updates over some interfaces while allowing updates to be exchanged normally over other interfaces. For any interface specified as passive, no routing information will be sent. Routing information received on that interface will be accepted and processed by the router. This is often useful for DDR links such as ISDN.

QuestionWhich one of the following statements best explains the split horizon rule?

A. Only routers can split boundaries (horizons) between networks in separate AS numbers. B. Each AS must keep routing tables converged to prevent dead routes from being advertised across boundaries. C. Once a route is received on an interface, advertise that route as unreachable back out the same interface. D. Information about a route should never be sent back in the direction from which the original update came.

Answer D

Explanation The split horizon rule states: * Never advertise a route out of the interface through which you learned it. For instance, in Figure 4a below, if Router One is connected to Routers Two and Three through a single multipoint interface (such as Frame Relay), and Router One learned about Network A from Router Two, it will not advertise the route to Network A back out the same interface to Router Three. Router one assumes that Router Three would learn about Network A directly from Router Two.

Incorrect AnswersA. There is no such requirement B. Distance vector protocols updates routing table at regular intervals instead of Topology changes C. This is the definition of the poison reverse rule, not the split horizon rule.

QuestionThe statements below compare and contrast link state and distance vector routing protocols. Which of these are true? (Choose two)

A. Distance vector protocols send the entire routing table to directly connected neighbors. B. Distance vector protocols are responsible for sending updates to all networks listed in the routing table. C. Link state protocols are responsible for sending the entire routing table to the whole network.

D. Link state protocols send updates regarding their own links status to all other routers on the network.

Answer A, D

ExplanationDistance Vector Protocols advertise routing information by sending messages, called routing updates, out the interfaces on a router. These updates contain a series of entries, with each entry representing a subnet and a metric. Link-State Protocols: Send partial updates when link status changes and floods full routing table updates every 30 minutes. The flooding, however, does not happen all at once, so the overhead is minimal.

Incorrect AnswersB. Distance Vector protocols only send information to adjacent neighbors. C. Only partial routing updates and sent to neighbors on a regular basis. The entire table is not sent to all neighbors. This would obviously create far too much overhead traffic.

QuestionWhat are the characteristic of link state routing protocols? (Choose all that apply.)

A. The exchange of advertisement is triggered by a change in the network. B. All routers exchange routing tables with each other in a multipoint network. C. Packets are routed based upon the shortest path to the destination. D. Paths are chosen depending on the cost efficiency factor. E. Every router in an OSPF area is capable of representing the entire network topology. F. Only the designated router in an OSPF area can represent the entire network topology.

Answer A, C, E

Explanation: Open Shortest Path First

* Each router discovers its neighbors on each interface. The list of neighbors is kept in a neighbor table. * Each router uses a reliable protocol to exchange topology information with its neighbors. * Each router places the learned topology information into its topology database. * Each router runs the SPF algorithm against its own topology database. * Each router runs the SPF algorithm against its own topology database to calculate the best routes to each subnet in the database. * Each router places the best roué to each subnet into the IP routing table. The following list points out some of the key features of OSPF: * Converges very quickly - from the point of recognizing a failure, it often can converge in less than 10 seconds. * Supports VLSM. * Uses short Hello messages on a short regular interval, with the absence of hello messages indicating that a neighbor is no longer reachable. * Sends partial updates when link status changes, and floods full updates every 30 minutes. The flooding, however, does not happened all at once, so the overhead s minimal. * Uses cost for the metric.

QuestionWhen dealing with distance-vector routing protocols, you encounter the term split-horizon. Which of the following statements describes this feature?

A. It allows routers to split up networks.B. All distance-vector protocols require fallback routers that might cause momentary loops as the topology changes.C. Convergence is achieved if all information about routers is sent out on all active interfaces.D. Information about a route should not be sent back in the direction from which the original update came.

Answer D

With all distance-vector routing protocols, split-horizon states that it is not useful to send information back the way it came.

A is incorrect, as routers themselves are the devices that split networks, not split-horizon. B is incorrect, as split-horizon is used to solve routing loop issues. C is incorrect because distance-vector routing protocols are notoriously slow in updating, as a result of their periodic nature. This can cause convergence issues.

QuestionWhen a router sets the metric for a network that has gone down to the maximum value, what is it doing? A. Applying split-horizonB. Putting the route in hold-downC. Poisoning the routeD. Sending a triggered update

Answer C

The distance-vector mechanism to set the metric to the maximum value is called route poisoning. A is incorrect, even though route poisoning is part of the split-horizon family. Split-horizon, by definition, does not allow updates to travel back out the interface on which they arrived. B is incorrect, as hold-down timers are not activated by route poisoning. D is incorrect, as triggered updates occur when a change occurs on a network and require an update that is earlier than the default timer.

QuestionWhich of these statements is true regarding distance-vector routing protocols?

A. They send the entire routing table to directly connected neighbors.B. They send the entire routing table to every router in the network.C. They send the changes to the routing table to directly connected neighbors.D. They send the changes to the routing table to every router in the network.

Answer A

Distance-vector routing protocols send the entire routing table to directly connected neighbors. B is incorrect, as the routers send the tables only to their directly connected neighbors. If a router learns of another route from its neighbor, it then passes that information on to another neighbor. C is incorrect, as distance-vector routing protocols send the entire table, not just the changes. D is incorrect, as the routers send the entire table, and they send it only to directly connected neighbors.

QuestionTo prevent routing loops, distance-vector routing protocols use a maximum metric. What is the maximum reachable metric for RIP?

A. 10B. 15C. 16D. 60E. 100F. 255

Answer B

The maximum hop count that RIP allows is 15.

C is incorrect because a hop count of 16 is the point that is considered unreachable by the RIP protocol. All other answers are incorrect because they either overshoot or underestimate the maximum hop count.

QuestionWhat routing loop solution prevents a router from sending information back to the neighbor that originally sent the information?

A. Split-horizonB. Hold-down timerC. Maximum hop countD. Route poisoningE. Counting to infinity

Answer A

Split-horizon is a mechanism in distance-vector routing protocols that prevents routers from sending updates back the way they came. B is incorrect, as hold-down timers are used to prevent routing tables from responding too quickly to sudden changes. C is incorrect, as the Maximum Hop count refers to the maximum metric that RIP can use. D is incorrect, as route poisoning is the process of taking a route and making it the maximum distance, thus "poisoning" the route. E is incorrect, as "counting to infinity" is a problem if a routing update loop occurs.

QuestionYou type show ip route on Router A to find out what entries are in your RIP routing table. Which of the following routes would not be found on a separate router receiving an RIP update from Router A?

A. R 172.16.0.0/16 [120/4]B. R 192.168.9.0/24 [120/2]C. C 192.168.4.0/24D. R 192.168.7.0/24 [120/15]E. R 192.168.8.0/24 [120/8]

Answer D

The metric shown on this route is 15. If it is passed to a neighboring router it increments by one equaling 16, which is unreachable. A is incorrect, as this route could be passed to neighbors. B is incorrect, as this route could be passed to a neighboring router. C is incorrect, as a directly connected network could be advertised to a neighboring router. E is incorrect, as this route is also within the 15-hop limit.

QuestionYou need to know what IP routing protocol is in use on your router. Which command should you use?

A. show protocolB. show routing protocolC. show running-configD. show ip protocol

Answer D

The show ip protocol command lists all routing protocols that are running on your router.

A is incorrect because it shows you only the routed protocols, not the routing protocols, on your router. B is incorrect, because this is not a valid command. C is incorrect, as the show running-config command shows you what routing protocols are running, but it also gives you all the other configuration information currently operating in RAM. This is too much information, when all you wanted was to find out which routing protocol was running on the machine.

QuestionYou are configuring a network at your headquarters in Phoenix, Arizona. You decide to use a distance-vector routing protocol. What command do you need to type to activate the mechanisms that stop routing loops? (Choose two.)

A. PhoenixRTR1(config-router)#split-horizon rip

B. PhoenixRTR1(config-router)#split-horizon distance vector

C. PhoenixRTR1(config-router)#distance vector

D. PhoenixRTR1(config-router)#router igrp 100

E. PhoenixRTR1(config-router)#no routing-loops

F. PhoenixRTR1(config-router)#router rip

Answers D and F

You merely need to activate the routing protocols to turn on the routing loop-prevention mechanisms; they are activated by default. A and B are incorrect, as you actually use the no split-horizon command to turn it off and just the split-horizon command to turn it back on. Regardless, it is on by default. C and E are incorrect, as there are no such commands.

QuestionYou are training some new network engineers when one of them asks you the difference between classful and classless routing protocols. What do you tell him?

A. They are pretty much the same, except classful protocols have more options.B. Classless supports VLSM, whereas classful supports FLSM.C. Classful can be used only between autonomous systems.D. RIPv1 and IGRP are the standard classless protocols, as they were some of the first protocols in existence.

Answer B One of the main features of classless protocols is to include the subnet mask information in their updates, whereas classful does not. This means that you can use only fixed-length subnet masks (FLSMs), not variable-length subnet masks (VLSMs) in a classful routing environment. A is incorrect, as classless protocols actually have more options. C is incorrect, as both classful and classless protocols can cross autonomous systems. D is incorrect, as RIPv1 and IGRP are actually classful protocols.

QuestionWhich of the following are valid routing protocol troubleshooting commands? (Choose two.)

A. R1#show ip routeB. R1(config-if)#show ip protocols

C. R1>debug ip ripD. R1#show ip protocols

Answers A and D

The show ip route and show ip protocols commands give you information on the routing protocols and what routes they have placed into the routing table. B is incorrect, as you cannot issue the show ip protocols command from Interface Configuration mode. C is incorrect, as you cannot use debug commands from the User EXEC mode. You must be in Privileged EXEC mode.

QuestionWhich of the following are considered characteristics of a link-state routing protocol?(Choose three.) A. Provides common view of entire topologyB. Exchanges routing tables periodically with neighborsC. Calculates shortest pathD. Utilizes event-triggered updatesE. Uses routing loop prevention with split-horizonF. Utilizes only periodic updates

Answers A, C, and D

Link-state routing protocols have the entire network mapped in their topology database. From this database, it calculates the shortest path using Dijkstra's algorithm. They also send small update announcements called LSAs when there is a change in the topology. B is incorrect because link-state routing protocols exchange their routing information with their neighbors once and then send only changes. E is incorrect, as link-state routing protocols do not use split-horizon. F is incorrect, as link-state protocols exchange a periodic update, but only if no LSAs have been sent over a long period of time. QuestionWhat is the advantage of using VLSM in a routed environment? A. It allows for more hosts than FLSM.B. It allows you to use different subnet masks throughout the network.C. It allows for a standard subnet mask to be used throughout the network.D. It prevents routing loops.

Answer B

The use of VLSM in a routed network allows you to use different subnet masks throughout the network. You can also use route aggregation (also called route summarization), which shrinks the size of routing tables. A is incorrect, as you actually lose host addresses when you use subnetting. C is incorrect, as you can use different size subnet masks such as /30 and /12 and /26 masks in the same network. D is incorrect, as VLSM is not a loop-routing preventive mechanism.

QuestionWhich of the following routing protocols are considered IGPs? (Choose four.)

A. RIPv1B. RIPv2C. IGRPD. BGPE. OSPF

Answers A, B, C, and E

A is correct, as RIPv1 is an Interior Gateway Protocol (IGP), meaning it exchanges routing information with routers in the same autonomous system (AS). B is correct, as RIPv2 is also an IGP. C is correct, as IGRP actually gets its name from being an IGP. E is correct, as OSPF is an industry standard IGP. D is incorrect, as Border Gateway Protocol is an External Gateway Protocol (EGP), which means it exchanges routing information between autonomous systems.

QuestionSplit-horizon states that no advertisements will be sent back through the interface on which they were received. What mechanism overrides that behaviour?

A. Triggered updatesB. Hold-down timersC. Poison reverseD. Nothing overrides split-horizon.

Answer C

Poison reverse, or "reverse poisoning," overrides split-horizon and sends a route poison broadcast back out the interface on which the poisoned route came in. A is incorrect, as triggered updates override the periodic update behavior of distance-vector protocols. B is incorrect, as hold-down timers maintain stability of the routing table by waiting before making any changes. D is incorrect, as poison reverse overrides split-horizon.

QuestionYour boss asks you what "count to infinity" means on a router. Which of the following explanations is the most accurate?

A. It is when a router continues to send out packets to an unknown destination.B. It is when a routing table entry continues to increment its metric without stopping.C. It is when a router calculates an unknown destination.D. It is when a router receives a poisoned route.

Answer B

A router without active default mechanisms could theoretically increment its metric forever if a routing update loop occurs, thus the phrase "counting to infinity." All distance-vector routing protocols now have built-in mechanisms to stop this behavior. A is incorrect, as a router, if it does not know where to send a packet, either discards the packet or sends it to a "default route." C is incorrect, as a router does not run a calculation on a route it does not know. D is incorrect, as a router marks a route as "possibly down" when it receives a poisoned route update. It then activates a hold-down timer until it either receives an update that the route is back up, or it hears nothing and flushes the route from its table.

QuestionThere are some differences between routed and routing protocols. Which of the following are examples of those differences? (Choose two.)

A. A routed protocol is assigned to an interface and determines the method of packet delivery. B. A routing protocol determines the path of a packet through a network. C. A routed protocol determines the path of a packet through a network.

D. A routing protocol operates at the transport layer of the OSI model. E. A routed protocol updates the routing table of a router. Answer: A, B

Explanation: A: A routed protocol delivers data. B: A routing protocol routes data. Incorrect AnswersC, E: A routed protocol only delivers data, it does not route data or update any routing tables. D: A routing protocol does not work at transport layer, layer 4. Many routing protocols work at layer 3, the network layer.

QuestionWhat is the total bandwidth of an ISDN BRI circuit? A. 54 kps B. 64 kps C. 112 kps D. 128 kps E. 144 kps Answer E

Explanation: A BRI Circuit is 2 x 64K data channels plus one 16K control channel = 144K A PRI Circuit is 23 64K data channels plus one 64K control channel = 1.544M = T1 (for USA) Incorrect Answers:A: One B channel is 64 and the one class D is 16 kbps. 54 kbps is impossible. B: A BRI contains 2 B channels 2 x 64 = 128 plus a 16 class D channel for a total of 144 kbps. C: There is no mathematical way to obtain 112 kpbs with BRI. D: 128 kbps only accounts for the class B channels and not for the class D channel.

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QuestionYour company has decided to pay for one ISDN B channel to your to house so that you can do some technical support from home. What is the bandwidth capacity of a single ISDN B channel? A. 16 Kbps B. 64 Kbps C. 128 Kbps D. 512 Kbps E. 1.54 Mbps Answer B

ExplanationThe Bearer (B) channel transfer rate is 64 kbps. Incorrect Answers:A. Is the Delta (D) channel that has a transfer rate of 16 kbps. C, D and E: The ISDN B channel only operates at a maximum of 64 kbps.

QuestionYou are troubleshooting a router that you believe is configured with an incorrect IP address. You issue the show configuration command from privileged mode. Which part of the output shows the specific configured IP addresses and subnet masks?

A. The IP host table.

B. The interfaces output. C. Each section of the output. D. Each section of the output. E. The global configuration statements. F. The section under the autonomous system number. Answer B

Explanation: Each interface that has the IP protocol, will show all IP addresses that were configured on that interface, with subnet masks. Incorrect AnswersA. This only shows the IP to host mappings, like the mapping of a HOSTS table.C, D, E, and F: These options do not provide this information.

QuestionWhich characteristics are representative of a link-state routing protocol? (Choose three)A. Provide common view of entire topologyB. Exchange routing tables with neighbors.C. Calculates shortest path.D. Utilizes event triggered updates.E. Utilizes frequent periodic updates.

Answer A, C, D

ExplanationThese are unique features of Link-State protocols.

Incorrect AnswersB. Link-state routing protocol (OSPF) does not exchange routing tables but instead exchanges routing updates (changes).E. This statement is correct but not a unique feature of Link-state routing protocol.

QuestionYour trainee is curious about characteristics of link-state routing protocols.What should you tell him? (Choose three)

A. Packets are routed based upon the shortest path to the destination.B. Paths are chosen based upon the cost factor to the destination.C. The exchange of advertisement is triggered by a change in the network.D. In a multipoint network, all routers exchange routing tables directly with all other routers.E. Every router in an OSPF area is capable of representing the entire network topology.F. Only the designated router in an OSPF area is capable of representing the entire network topology.

Answer A, C, E

ExplanationOpen Shortest Path FirstEach router discovers its neighbors on each interface. The list of neighbors is kept in a neighbor table.Each router uses a reliable protocol to exchange topology information with its neighbors.Each router places the learned topology information into its topology database.Each router runs the SPF algorithm against its own topology database.Each router runs the SPF algorithm against its own topology database to calculate the best routes to each subnet in the database.Each router places the best route to each subnet into the IP routing table.

The following list points out some of the key features of OSPF:Converges very quickly – from the point of recognizing a failure, it often can converge in less than 10 seconds.Supports VLSM.Uses short Hello messages on a short regular interval, with the absence of hello messages indicating that a neighbor is no longer reachable.Sends partial updates when link status changes, and floods full updates every 30 minutes. The flooding, however, does not happened all at once, so the overhead s minimal.

Uses cost for the metric.

QuestionYour trainee is studying the basics of distance vector and link state routing protocols.As her mentor, what could you tell her? (Choose two)

A. Distance vector protocols send the entire routing table to directly connected neighbors.B. Link state protocols send the entire routing table to all routers in the network.C. Distance vector protocols send updates about directory connected neighbors to all networks listed in the routing table.D. Link state protocols send updates containing the state of their own links to all other routers on the network.

Answer A, D

Explanation

Distance Vector Protocols advertise routing information by sending messages, called routing updates, out the interfaces on a router. These updates contain a series of entries, with each entry representing a subnet and a metric.

Link-State ProtocolsSends partial updates when link status changes and floods full updates every 30 minutes. The flooding, however, does not happen all at once, so the overhead is minimal.

QuestionWhich of the following routing protocols are less likely prone routing loops and network reachability problems when used in discontiguous networks? (Select all valid responses)

A. IGRP B. CDP C. OSPF D. RIP v1 E. RIP v2 F. EIGRP

Answer C, E, F

Explanation: Only OSPF, RIP version 2, and EIGRP carry VLSM information. In a discontiguous network, subnet masks of different lengths can be used, but this information will need to be propagated via the routing protocol if all networks are to be reached.

Incorrect Answers: A, D. With RIP version one and IGRP, discontiguous networks can be problematic, as VLSM is not supported. B. CDP is the Cisco Discovery Protocol, which is used to exchange information between Cisco devices. It can only be used between Cisco routers and switches, and it is not a routing protocol.

QuestionA router learns about a remote network from EIGRP, OSPF, and a static route. Assuming all routing protocols are using their default administrative distance, which route will the router use to forward data to the remote network?

A. The router will use the static route. B. The router will use the OSPF route. C. The route will use the EIGRP route. D. The router will load balance and use all three routes.

Answer A

Explanation: When a router learns about the same network via multiple sources, the router will choose the source with the lowest administrative distance (AD). By default, the AD for these routing protocols are: Connected Interface has 0 AD Static Route: 1 EIGRP: 90OSPF: 110 So, the static route will be chosen since it has the lowest AD.

QuestionWhich of the following dynamic routing protocols are considered classless? (Choose all that apply.)

A. IGRP

B. EIGRP

C. RIP v1

D. OSPF

E. IS-IS

F. BGP

G. RIP v2

Answer B, D, E, F, G

RIPv1 and IGRP are classfulAll the rest are Classless

Exam CramRouted and Routing protocols:

Routing protocols job is to maintain routing tables and route packets appropriately. Examples of routing protocols are RIP, IGRP, EIGRP, OSPF. Routers can support multiple independent routing protocols and can update and maintain routing tables for each protocol independently.

Routed protocols are used to transport user traffic from source node to destination node. Examples of routed protocols are IP, IPX, AppleTalk.

There are three ways a router learns how to forward a packet:

Static Routes - Configured by the administrator manually. The administrator must also update the table manually every time a change to the network takes place. Static routes are commonly used when routing from a network to a stub (a network with a single route) network.

The command is

ip route network mask address/interface [distance]

ip route 165.44.34.0 255.255.255.0 165.44.56.5

Here, 165.44.34.0 is the destination network or subnet 255.255.255.0 is the subnet mask

165.44.56.5 is the default gateway.

Default Routes - The default route (gateway of last resort) is used when a route is not known or is infeasible. The command is

ip route 0.0.0.0 0.0.0.0 165.44.56.

The default gateway is set to 165.44.56.5

Dynamic Routes - In dynamic routing, the routing tables are automatically updated. Dynamic routing uses broadcasts and multicasts to communicate with other routers. 

The commands to enable rip are:

router rip

network <major network number>.  

Show IP protocol: This command will show information on RIP timers including routing update timer (30sec default), hold-down timer (default 180sec). It also displays the number of seconds due for next update (this is fraction of update timer). This command also gives the network number for which IP RIP is enabled, Gateway, and the default metric.

Show IP route: This command will display the IP routing table entries. In addition, it displays the Gateway of last resort (if one is assigned). It also displays the codes used for various types of routes. Some of the important codes are:

S stands for static.R stands for RIP. C - directly connectedS - staticI - IGRPR - RIPM - mobileB - BGPD - EIGRP

EX - EIGRP externalO - OSPFIA - OSPF inter areaN1 - OSPF NSSA external type 1N2 - OSPF NSSA external type 2E1 - OSPF external type 1E2 - OSPF external type 2E - EGPi - IS-ISL1 - IS-IS level-1L2 - IS-IS level-2* - candidate defaultU - per-user static routeo - ODR

Show IP interface: This command shows you interface-wise information such as IP address assigned to each interface, whether the interface is up, MTU etc.

Debug IP RIP: Debug IP RIP will turn the RIP debugging ON. This will display a continuous list of routing updates as they are sent and received. This leads to lot of overhead, which is the reason that you use "undebug ip rip" to turn-off debugging as soon as you finish with debugging.

Remember how to verify IGRP routing. The show ip route command shows you the routing table I =IGRP found route. The [100/12345] indicates the administrative distance 100 and the composite metric (determined by bandwidth and delay of the line)

Know how to verify EIGRP operation using show ip eigrp neighbor and show ip eigrp topology

The passive-interface commandThe impact of the passive-interface command depends upon the routing protocol On an interface running RIP the passive-interface command prohibits the sending of route updates but allows their receipt. Thus a RIP router with a passive interface will still learn about networks advertised by other routers.

This is different from EIGRP where a passive interface will neither send nor receive updates.