IP Routing - Fundamentals

7/23/2019 IP Routing - Fundamentals

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7-Mar-97 Version 1.1 Chapter 1 Page 1Robert Wright

IP Routing Fundamentals

Table of contents: Chapter One

Introduction. Topology and router configurations. What does a router do? How does a router know what to do? Forwarding:

How are forwading decisions made? How are forwading decisions made for multipoint interfaces?

How does an end system send send packets to other subnets?

Convergence: What is convergence? Primary activities involved. Update / invalid / holddown / flush defined. With Parallel paths. View multiple paths in a routing table. View invalid timer in routing table. Invalid timer expired. Invalid timer expired but route is still used. Invalid and holddown in action. Holddown. Converged state. Invalid, holddown& flush. Debug messages and reality. Invalid, holddown& flush, continued

Split horizon.

Convergence:.Routing advertisements with split horizon. enabled. Disable split horizon. Routing advertisements with split horizon. disabled. Routing advertisements with split horizon. enabled, continued.

Routing advertisements with split horizon. disabled, continued. Split horizon disabled and poison reverse. Split horizon and multi-point interfaces. Split horizon enabled and subinterfaces. Loss of a conncected route vs. a dynamic route. Poison reverse in action. Summary:

Invalid. Holddown. Flush. Split horizon. Poison reverse.

:Parallel Paths: Load balancing & switching paths, fast switching.


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Table of contents: Chapter One

Parallel Paths: Load balancing & switching paths, process switching. Load balancing & switching paths, fast switching, continued. Load balancing & switching paths, process switching, continued. Summary:

Load balancing and switching paths.


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Introduction

This course covers the generic behaviour of IP routing and packet forwarding using Cisco routers. It will go intodetailed analysis of several scenarios to provide insight into the fundamentals of IP that everybody supportingIP in a network should know.

This course is not intended to be an in depth analysis of the individual routing protocols presented.

This course is not intended to be a design and implementation guide for building IP networks.

Although this course was created with stand up teaching in mind it will work best if the students also use it aslab reference material to experiment with the concepts it presents.

Many of the functions & concepts covered in this course have been documented in the multitude of booksavailable on IP with various levels of success. This document specifically focuses on how Cisco routersimplement these functions and how users can use Cisco routers to learn more about IP.

This course will not cover EIGRP, OSPF, Integrated ISIS, Gateway Protocols such as BGP or RIP V2.


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The appendix includes information on where to find out more about specific routing protocols.

The routers in the topology and configuration section may appear in different topologies throughout this course.With the exception of removing some links in some examples the actual configurations will not change fromthose shown unless specifically noted. Links that have been disconnected or removed in some examples areshutdown in the related configuration.

Some of the fundamentals such as split-horizon, poison reverse, holddown and invalid also apply to IPX RIPand Appletalk RIP.

Introduction


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Topology and router configurations

Router A as used in this documentation has a connection to the real Cisco network via its secondary address.Although the secondary address space may not be mentioned again look for references to it in the sections onsummarisation, default routes and other issues that require access to networks that are not known explicitly bythe routers.

Major Net 168.71.0.0Mask 255.255.255.0

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1 168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

171.68.207.164Secondary


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Version 1.1 Chapter 1 Page7-Mar-97 Version 1.1 Chapter 1 Page 6Robert Wright

RouterA

!

interface ethernet0

ip address 168.71.5.1 255.255.255.0

ip address 171.68.207.164 255.255.128 secondary

!

interface serial0ip address 168.71.6.1 255.255.255.0

bandwidth 128

!

interface serial1

ip address 168.71.9.1 255.255.255.0

bandwidth 128

!

router rip

network 168.71.0.0

passive-interface Ethernet0

!



168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S0

168.71.7.2

168.71.7.1

168.71.6.2 S0

171.68.207.164

Secondary


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RouterB

!

interface serial0

ip address 168.71.6.2 255.255.255.0bandwidth 128

!

interface serial1

ip address 168.71.7.2 255.255.255.0

bandwidth 64

!

router rip

network 168.71.0.0

!

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S0

168.71.7.2

168.71.7.1

168.71.6.2 S0


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RouterC

!

interface tokenring0

ip address 168.71.8.1 255.255.255.0!

interface serial0

ip address 168.71.7.1 255.255.255.0

bandwidth 64

!

interface serial1

ip address 168.71.9.2 255.0.0.0

bandwidth 128

!

router rip

network 168.71.0.0

!

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S0

168.71.7.2

168.71.7.1

168.71.6.2 S0


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What does a router do?

168.71.8.0

Router A

Router C

Router B

168.71.5.0

S0

S1S0 168.71.6.1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Routers forward IP packets towards their destination networks on a hop-by-hop basis.

In most instances the forwading router does not know whether the next hop router is the final router in thechain or if it is merely another hop in a longer chain.

In the example above if Router A received a packet for 168.71.8.2 (a host on the token ring) Router A wouldsimply forward the packet to Router B. Router A would neither know nor care that Router B is not actuallyconnected to 168.71.8.0 and in fact has to hand the packet off to yet another router.

As far as Router A is concerned the route to 168.71.8.0 is known via 168.71.6.2 over serial 0. Known viameans that this is the IP address of the advertising router. On a point to point link the advertising router isalways the only other router on the link. On a multipoint link such as Frame Relay there may be several routersadvertising routes that arrive over a serial interface. In this case it is imperitive to store the next hop address toidentify which router is advertising the routes.

The multipoint example will be covered later in this course.

It is important to note that the destination address of an IP packet never changes at it flows throughthe network.


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How does a router know what to do?

Routers send each other information about networks that they know about. They do this with different types ofprotocols that are generically referred to as routing protocols. Routers use this information to build a routingtable that consists of the available networks, the costs associated with reaching the available networks and thepath to the next hop router.

The two primary classes of routing protocols in use today are link state and distance vector.

A router running a Link State protocol sends updates that describe the state of the links attached the router andthe costs associated with using those links. Other routers listen to these updates and build a picture of thetopology of the network based on what networks the other routers have indicated that they are connected to.The metric for Link State protocols are typically cost based. A low speed link costs more than a high speed linkin terms of performance. Paths that run over lower cost, higher speed links are preferred.

A router running a Distance Vector protocol sends updates that contain all of the networks that the routerknows about, not just the networks it is connected to. The advertising router increases the metric (typically hopcount) of the routes that it has heard about and advertises them to the next routers in the path.

Routers typically do not advertise networks back out the interface they are known via.

Router A Router B 168.71.8.0168.71.5.0

168.71.6.0


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Forwarding: How are forwarding decisions made?

IP routing is normally done on a next hop basis. In this example Router A may have told Router B about subnet168.71.5.0. Router B doesnt really care if Router A is actually attached to 168.71.5.0. If Router A says it canreach 168.71.5.0 that is sufficient for Router B to make a forwarding decision to send packets destined for168.71.5.0 to Router A.

Routers take an incoming packet and compare it to entries in their routing table. A longest match lookup isperformed to determine if a route is available for forwarding the packet. If no explicit route is available therouter may chose to forward the packet to a gateway of last resort. This will be covered in a later section.

In this example Router B receives a packet destined for 168.71.5.1. The longest match in the routing table

indicates that that 168.71.5.0 is known via serial 0 with a next hop of 168.71.6.1. In this case the next hopaddress is redundant because the serial link is point-to-point.

Router A Router B 168.71.8.0168.71.5.0 168.71.6.2

168.71.5.1 168.71.8.1

168.71.5.1 168.71.8.1

168.71.6.1

Protocol Network Interface Next HopIGRP 168.71.5.0 Serial0 168.71.6.1Connected 168.71.6.0 Serial0Connected 168.71.8.0 Token0

Router B

168.71.5.1 168.71.8.1

168.71.5.1 168.71.8.1


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Forwarding: How are forwarding decisions made for multipoint interfaces?

Router B 168.71.8.0

Router A168.71.5.0

168.71.5.1

168.71.8.1

168.71.5.1 168.71.8.1

168.71.5.1 168.71.8.1

168.71.5.1 168.71.8.1

Frame Relay168.71.6.0

Router C

168.71.9.0

.1

.2 .3

Protocol Network Interface Next HopIGRP 168.71.5.0 Serial0 168.71.6.1Connected 168.71.6.0 Serial0Connected 168.71.8.0 Token0IGRP 168.71.9.0 Serial0 168.71.6.2

Router B

RouterB#show frame-relay mapSerial0 (up): ip 168.71.6.1 dlci 100(0x64,0x1840), static, broadcast, CISCORouterB#

In this example the next hop interface of serial 0 is not sufficient for forwarding the packet to 168.71.5.1accurately. The further distiction of a next hop IP address eliminates the ambiguity of only pointing to the serialinterface. In this case Router B knows that the next hop is out serial 0 to the next hop address of 168.71.6.1.

Router B will have a frame relay map entry on serial 0 that indicates the appropriate DLCI (100) to send thepacket to.

The frame-relay map for IP address 168.71.61. to dlci 100 is essentially an ARP entry for the layer two andlayer three protocol addresses. This is similar to an ethernet MAC address to IP address ARP entry.


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168.71.8.2Router B168.71.6.2

T0S0

168.71.8.2


Forwarding: How Does An End System Send Packets To Other Subnets?

0000.0f00.8674

A B

0000.0c01.87930000.0c00.1234

168.71.5.1

0000.0d00.5463

168.71.8.1

Layer Two

Layer Three

Before a router can even get involved in forwarding a packet from an end station the end station must figurehow to get the packet to the router in the first place.

When an end system wants to send an IP packet to another end system it compares the destination IPaddress with its own address. If the destination address is within the same subnet (on the same local cable) theoriginating end station will ARP for the destination end system. If the originating end system determines thatthe destination end system is on a different cable it will send the packet to the MAC (cable address) of its

gateway and the IP address of the destination end sysytem.

In this example end system A wants to send a PING to end station B. End system A determines that endsystem B is on a different cable segment because the destination IP address is on a different subnet of168.71.0.0.

End station A has 168.71.5.2 configured as its gateway address. End system A will ARP for the MAC addressof 168.71.5.2 so that it can use this address as the MAC address for delivering packets to 168.71.8.1 The layertwo (MAC) addresses are point-to-point. The layer three (IP addresses) are end to end. The layer two adresses

for the serial links have been ommitted for clarity. Gateways are covered in a later section of this course.

168.71.5.1 168.71.8.1Router A 168.71.6.1

E0S0

168.71.5.2


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Convergence: What is convergence?

Convergence -is the process that routers go through when a route (network) or group of routes (networks)have become unavailable either due to a link going down in the network or extreme packet loss on a link. Thiswill result in the routers flushing the lost routes and listening to see if there are other routes available. Routersusually only store the best route to a network in their routing table. Other, higher cost routes may exist but therouter will ignore them while they believe that the better route still exists.

In the example above Router A has had to converge on a new path to 168.71.8.0 because the link betweenRouter A and Router C has gone down.

168.71.7.2

168.71.8.1Router C

Router B

Router A168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0


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Convergence: Primary activities involved

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Here you can see the default timers used by RIP

RouterA# show ip protocol

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 27 seconds

Invalid after 180 seconds, hold down 180, flushed after 240

.

.

Stuff deleted

.

RouterA#

Convergence involves four primary activities; Update, Invalid, Holddown and Flush. This example shows thedefault timers used by RIP. These activities will be explained in this section.


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Convergence - Update / Invalid / Holddown / Flush; defined

Invalid -This is both a term used to describe a particular state that a route may be in and a timer that is usedto monitor the status of a route. The term invalidis used to refer to routes that have not been heard from for theperiod of time that the invalidtimer is set for. EG: If the invalidtimer is 60 seconds and an advertisement for aroute from the router it is learned from has not been received for 61 seconds the invalidtimer will expire andthe route will be considered invalid.

Update-This is the time between between routing updates sent by a router.

Holddown -This is both a term used to describe a particular state that a route may be in and a timer that is

used to monitor the status of a route. The term is used to refer to routes that have been marked as

Holddown -This is both a term used to describe a particular state that a route may be in and a timer that is

used to monitor the status of a route. The term holddownis used to refer to routes that have been marked asinvalid (expired) but are not yet capable of being replaced with a new route of a higer metric. The holddowntimer determines how long a route will remain in holddown. (Unless the flush timer fires before the holddowntimer finishes running.)

Flush -This is both a timer and a term that describes removing a route from the routing table. The flushtimerrestarts everytime an update is received for a route from the router that it is learned from. It is important to notethat the flushtimer and the invalidtimer both restart at the same time and run concurrently. When the flush

timer has expired for a route the route is removed from the routing table. For RIP the flushtimer expires beforethe holddowntimer so holddownnever runs for its complete cycle.


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Convergence: With parallel paths

If a router had two or more equal cost paths to a network it may use them concurrently. If it loses one or moreof them the router will continue to use the paths that are still available. Router A above has two equal costpaths to 168.71.7.0. If it loses the route via serial 0 it will continue to use the route via serial 1.

168.71.7.0

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0


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7-Mar-97 Version 1.1 Chapter 1 Page7-Mar-97 Version 1.1 Chapter 1 Page 18Robert Wright

Convergence: View multiple paths in a routing table

Here you can see the two routes to 168.71.7.0 in Router As routing table

RouterA# show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 171.68.207.129 to network 0.0.0.0

168.71.0.0 255.255.255.0 is subnetted, 5 subnetsC 168.71.9.0 is directly connected, Serial1

R 168.71.8.0 [120/1] via 168.71.9.2, 00:00:15, Serial1

R 168.71.7.0 [120/1] via 168.71.6.2, 00:00:00, Serial0

[120/1] via 168.71.9.2, 00:00:15, Serial1

C 168.71.6.0 is directly connected, Serial0

C 168.71.5.0 is directly connected, Ethernet0

171.68.0.0 is variably subnetted, 2 subnets, 2 masks

C 171.68.207.128 255.255.255.128 is directly connected, Ethernet0

S 171.68.0.0 255.255.0.0 [1/0] via 171.68.207.129

S* 0.0.0.0 0.0.0.0 [1/0] via 171.68.207.129

RouterA#

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

168.71.7.0


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Convergence: View invalid timer in routing table

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

In this example the timer invalid timer for 168.71.9.0 restarts everytime168.71.8.0 is advertised by 168.71.9.2 (RouterC)






U - per-user static route


168.71.0.0/16 is subnetted, 5 subnets


R 168.71.8.0 [120/1] via 168.71.9.2, 00:00:39, Serial1

R 168.71.7.0 [120/1] via 168.71.6.2, 00:00:11, Serial0

[120/1] via 168.71.9.2, 00:00:39, Serial1



S* 0.0.0.0/0 is directly connected, Ethernet0

RouterA#


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Convergence: Invalid timer expired

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

In this example the invalid timer for 168.71.8.0 has fired.










R 168.71.8.0/24 is possibly down,

routing via 168.71.9.2, Serial1

R 168.71.7.0 [120/1] via 168.71.6.2, 00:00:24, Serial0




RouterA#

Here you can that Router As invalid timer has expired for 168.71.8.0 and the route is marked possibly down


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Convergence: Invalid timer expired but the route is still used

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RoutwrA# deb ip packet

RouterA#ping 168.71.8.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 168.71.8.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 20/23/24 ms

RouterA#IP: s=168.71.9.1 (local), d=168.71.8.1 (Serial1), len 100, sending

IP: s=168.71.8.1 (Serial1), d=168.71.9.1 (Serial1), len 104, rcvd 3

IP: s=168.71.9.1 (local), d=168.71.8.1 (Serial1), len 100, sending








RouterA

Here you can that Router A is still using the route via serial 1

Pings


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RouterA# debug ip routing

RouterA# deb ip rip

Aug 23 22:20:05: RT: flushed route to 168.71.8.0 via 168.71.9.2 (Serial1)

Aug 23 22:20:05: RT: no routes to 168.71.8.0, entering holddown

Aug 23 22:20:33: RIP: sending v1 update to 255.255.255.255 via Serial0 (168.71.6.1)

Aug 23 22:20:33: subnet 168.71.8.0, metric 16

RouterA#


Convergence: Invalid and holddown in action

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

This example shows what Router A will do when the invalid timer for 168.71.8.0 has fired.

The flushed route message below is referring tothe process of advertising the routes asunreachable to other routers so that they removethe route. Router A puts the route into holddown

Router A advertises the route with a metric 16(unreachable).

Here is the debug showing RouterA putting 168.71.8.0 into holddown and advertising it as unreachable

(1)

(2)

(3)


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Convergence: Holddown

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

The following slides show what happens when a router stops advertising a network over one of two possiblepaths. In this case Router C will stop advertising 168.71.8.0 and 168.71.7.0 over its serial 1 interface.

A router will put a route into holddown when one of the following happens:

The router that was advertising the route stops advertising it for a period of time. This period of time is usually referred to as the invalid period.

The router that is advertising the route send a new advertisement for the route with a metric that is greater

than the stored metric. This usually indicates a routing loop. This causes the route to be deleted immediately and put into holddown immediately instead of waiting for the invalid timer to fire.


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Convergence: converged state

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

This is Router As routing table in a converged state prior to Router C ceasing to advertise routes over serial 1










R 168.71.8.0 [120/1] via 168.71.9.2, 00:00:39, Serial1

R 168.71.7.0 [120/1] via 168.71.6.2, 00:00:11, Serial0

[120/1] via 168.71.9.2, 00:00:39, Serial1




RouterA#


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Unfortunately the debug messages below are misleading. The next slide shows what is really happening.


Convergence: Invalid, Holddown & Flush

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RouterA# sh clock

20:33:30.246 UTC Fri Aug 2 1996





Aug 2 20:37:41: RT: garbage collecting entry for 168.71.8.0

Aug 2 20:37:50: RT: add 168.71.8.0/24 via 168.71.6.2, rip metric [120/2]

RouterA#

Here you can see the debug from Router A as it loses the old route via serial 1

Router C is no longer sending updates over its serial 1. However the phsical link is still active.


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Convergence: Debug messages and reality

RouterA# sh clock

20:33:30.246 UTC Fri Aug 2 1996





Aug 2 20:37:41: RT: garbage collecting entry for 168.71.8.0

Aug 2 20:37:50: RT: add 168.71.8.0/24 via 168.71.6.2, rip metric [120/2]

RouterA#

What the debug says

RouterA# sh clock

20:33:30.246 UTC Fri Aug 2 1996


Aug 2 20:36:45: RT: invalid timer expired no routes to 168.71.8.0, entering holddown

Aug 2 20:36:45: RT: advertising 168.71.8.0 via 168.71.9.2 (Serial1) as unreachable

Aug 2 20:37:41: RT: flush timer expired terminating holddown for 168.71.8.0

Aug 2 20:37:50: RT: add route 168.71.8.0/24 via 168.71.6.2, rip metric [120/2]

RouterA#

What is really happening

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Question: Why doesnt Router A add a new route for 168.71.7.0?


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Convergence: Invalid, holddown & flush

This is Router As routing table after it converged










R 168.71.8.0 [120/2] via 168.71.6.2, 00:00:05, Serial0

R 168.71.7.0 [120/1] via 168.71.6.2, 00:00:06, Serial0




RouterA#

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0


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Router A was initially ignoring Router Bs updates for 168.71.8.0 because the metric was greater than themetric from Router C.

Eventually Router A realised that Router C was no longer advertising 168.71.8.0. At this point Router A markedthe route invalid and went into holddown for other potential routes to 168.71.8.0

When the flush timer fired at 240 seconds 168.71.8.0 came out of holddown and Router A accepted the verynext update from Router B and installed Router Bs route to 168.71.8.0 in its table.

Notice the different metrics in the route. When the route changed from Router C to Router B the metric wentfrom 1 to 2.



RouterA# show ip route 168.71.8.0(edited)

R 168.71.8.0 [120/ 1] via 168.71.9.2, 00:00:39, Serial1

Old route is lost, Router A shows the new route:

RouterA# show ip route 168.71.8.0(edited)

R 168.71.8.0 [120/ 2] via 168.71.6.2, 00:00:05, Serial0

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0


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a - Router C stops advertising 168.71.8.0 at 0 seconds.

b - Router A marks 168.71.8.0 as invalid and puts it into holddown at 180 seconds.

c - Router A exits holddown when flush timer expires and accepts new route via Router B at 240 seconds.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Router B advertises 168.71.8.0 every 30 seconds

0 . 1 . 2 . 3 . 4 minutes

Router Astops receiving

updates for 168.71.8.0from Router C

Router Amarks route invalid

and entersholddown

Router Afires flush timer,exits holddown

and accepts newroute

Start invalid timerStart flush timer

Invalid timer expiresStart holddown timer

Flush timer expiresTerminate holddown


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Convergence: Split horizon

The role of Split Horizon is to prevent a router from advertising routes in its routing table that are known via aparticular interface back out that same interface.

The known via rule applies to connected routes as well. A connected route is a route that the router has a

direct physical attachment to. In the example above Router A has a connected route to 168.71.6.0 via interfaceserial 0. In otherwords as far as Router A is concerned 168.71.6.0 is known via serial 0. Router A will notadvertise 168.71.6.0 out serial 0 if split horizon is enabled.

In this example Router As preferred route (the route it has stored) to network 168.71.8.0 is out Serial 1. WhenRouter A is due to send an update to Router C Router A will build an update that contains all of the routes in itsrouting table except for the routes that are known via Serial 1. In this case it will not contain 168.71.8.0 or168.71.7.0.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0


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Router A will not advertise a route to 168.71.8.0 out serial 1 to Router C because that is the path the route isknow via.

Router A will not advertise 168.71.7.0 out either serial0 or serial 1 because it has equal cost paths out bothinterfaces to 168.71.7.0. In this case 168.71.7.0 is known via serial 0 and serial 1.

Router A will not advertise 168.71.6.0 out serial 0 and 168.71.9.0 out serial 1 because these are connectedroutes as far as these interfaces are concerned.

Router A will both receive and advertise a route for 168.71.8.0 over serial 0. It will receive Router Bs route to168.71.8.0 that Router B received from Router C and it will advertise its own route for 168.71.8.0 that it hasstored as originating over serial 1 from Router C.


Convergence: Split horizon

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RouterA# show ip route(edited for clarity)


R 168.71.8.0 [120/1] via 168.71.9.2, 00:00:24, Serial1

R 168.71.7.0 [120/1] via 168.71.9.2, 00:00:24, Serial1

[120/1] via 168.71.6.2, 00:00:19, Serial0




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Convergence: Routing advertisements with split horizon enabled

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Here you can see Router A both receiving and advertising 168.71.8.0 out serial 0.You can also see that Router A does not advertise 168.71.8.0 out serial 1 to Router C.

RouterA# debug ip rip

Aug 2 23:02:03: RIP: received v1 update from 168.71.9.2 on Serial1

Aug 2 23:02:03: 168.71.8.0 in 1 hops

Aug 2 23:02:03: 168.71.7.0 in 1 hops


Aug 2 23:02:05: 168.71.8.0 in 2 hopsAug 2 23:02:05: 168.71.7.0 in 1 hops





Aug 2 23:02:16: default, metric 1




Aug 2 23:02:16: default, metric 1RouterA#


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Convergence : Disable split horizon

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RouterA!interface ethernet0ip address 168.71.5.1 255.255.255.0!interface serial0ip address 168.71.6.1 255.255.255.0bandwidth 128no ip split-horizon!interface serial1ip address 168.71.9.1 255.255.255.0bandwidth 128no ip split-horizon!router ripnetwork 168.71.0.0

passive-interface Ethernet0!


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Convergence : Routing advertisements with split horizon disabled

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Here you can see Router A advertising all of its routes out both serial interfacesbecause split horizon has been disabled.





Aug 2 23:54:47: subnet 168.71.7.0, metric 2Aug 2 23:54:47: subnet 168.71.6.0, metric 1










RouterA#


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168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0








Aug 2 23:54:47: default, metric 1RouterA#

Here you can see Router A sending a route for 168.71.6.0 out serial 0.

On the previous slide you saw that Router A advertised every route that it knew about including routes that it isdirectly connected to back out the connected interface.

To illustrate this point the debug below shows Router A sending a route for 168.71.6.0 out serial 0.


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In this example split horizon is enabled. Notice that Router A does not advertise the connected route168.71.6.0 out of serial 0.


Convergence: Routing advertisements with split horizon enabled

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0







RouterA#

Here you can see that Router A is not sending a route for 168.71.6.0 out serial 0.

Question: How do you enable split horizon?


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168.71.8.1

Router A

Router C

Router B

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

In this section the the ip address is removed from ethernet 0 on Router A. This has the effect of making RouterA think it has lost its route to 168.71.5.0.

Router A initially accepts Router Cs advertisement for 168.71.5.0 even though Router Cs stored route to

168.71.5.0 actually points to Router A. Advertised routes do not contain the router of origin. This makes itimpossible for Router A to know that the advertisement it is receiveing from Router C is actually for a route thatRouter A itself originated.

The following slides show the process from the intial loss of the connected route to the poisoning of the routeand its placement into holddown.

168.71.5.1


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168.71.8.1

Router A

Router C

Router B

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0



Aug 3 01:14:01: RT: del 168.71.5.0/24 via 0.0.0.0, connected metric [0/0]

Aug 3 01:14:01: RT: delete subnet route to 168.71.5.0/24


Aug 3 01:14:02: 168.71.5.0 in 2 hops

Aug 3 01:14:02: RT: add 168.71.5.0/24 via 168.71.9.2, rip metric [120/2]Aug 3 01:14:02: RIP: sending v1 update to 255.255.255.255 via Serial0 (168.71.6.1)





Aug 3 01:14:02: 168.71.5.0 in 16 hops (inaccessible)



Aug 3 01:14:02: RT: delete route to 168.71.5.0 via 168.71.9.2, rip metric [120/2]


RouterA#

Here you can see what happens when the address is removed from Router A

168.71.5.1

Notice the progression as 168.71.5.0 is advertised back and forth between the routers.


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168.71.8.1

Router A

Router C

Router B

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0






Aug 3 01:14:02: 168.71.5.0 in 2 hops











RouterA#

Here you can see that Router A receives an update for 168.71.5.0 with a metric of 2 from Router C and installs it

168.71.5.1


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168.71.8.1

Router A

Router C

Router B

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0






Aug 3 01:14:02: 168.71.5.0 in 2 hops











RouterA#

Here you can see that Router A increases the metric to 3 and advertises the route back to Routers B and C

168.71.5.1

7 M 97 V i 1 1 Ch t 1 P 41R b t W i ht


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Convergence : Split horizon disabled and poison reverse

168.71.8.1

Router A

Router C

Router B

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

This is an example of poison reverse in action. Poison reverse is explained later in this presentation.






Aug 3 01:14:02: 168.71.5.0 in 2 hops










Aug 3 01:14:02: RT: no routes to 168.71.5.0, entering holddownRouterA#

Here you can see that Routers B & C respond to Router As increased metric by poisoning the routes

168.71.5.1

7 M 97 V i 1 1 Ch t 1 P 42R b t W i ht


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Convergence : Split horizon disabled and poison reverse

168.71.8.1

Router A

Router C

Router B

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0






Aug 3 01:14:02: 168.71.5.0 in 2 hops










Aug 3 01:14:02: RT: no routes to 168.71.5.0, entering holddownRouterA#

Here you can see Route A reacting to the poison reverse by deleting the route to 168.71.5.0 and entering holddown

168.71.5.1

7 Mar 97 Version 1 1 Chapter 1 Page 43Robert Wright


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X


Convergence : Split horizon enabled and multi-point interfaces

168.71.7.1

168.71.8.1

Router A

Router C

Router BS0 168.71.6.1

168.71.6.3 S0

168.71.6.2 S0


Protocol Network Interface Next HopConnected 168.71.5.0 Ethernet0Connected 168.71.6.0 Serial0IGRP 168.71.7.0 Serial0 168.71.6.2IGRP 168.71.8.0 Serial0 168.71.6.3

Router A

Protocol Network Interface Next HopIGRP 168.71.5.0 Seial0 168.71.6.1Connected 168.71.6.0 Serial0Connected 168.71.7.0 TokenRing0

Router B

Protocol Network Interface Next Hop

IGRP 168.71.5.0 Seial0 168.71.6.1Connected 168.71.6.0 Serial0Connected 168.71.8.0 TokenRing0

Router C

168.71.7.0

168.71.8.0

In this example Router A has installed routes tosubnets 168.71.7.0 and 168.71.8.0 that are known via

serial 0. Due to split horizon Router A will not advertiseeither of these subnets back out serial 0. This preventsRouter B from learning about 168.71.8.0. It alsoprevents Router C from learning about 168.71.7.0.Router A is the only router with a complete routingtable. This is why Cisco defaults to disabling splithorizon on frame-relay interfaces. A more effectivesolution is to use sub interfaces as shown on the next

slide.

7 Mar 97 Version 1 1 Chapter 1 Page 44Robert Wright


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Convergence : Split horizon enabled and sub-interfaces

In this example Routers B & C have full connectivityeven though Router A is running split-horizon on serial0.1 and serial 0.2.

Protocol Network Interface Next HopConnected 168.71.5.0 Ethernet0Connected 168.71.6.0 Serial0.1IGRP 168.71.7.0 Serial0.1 168.71.6.2IGRP 168.71.8.0 Serial0.2 168.71.9.2Connected 168.71.9.0 Serial0.2

Router A

Protocol Network Interface Next HopIGRP 168.71.5.0 Seial0 168.71.6.1Connected 168.71.6.0 Serial0Connected 168.71.7.0 TokenRing0IGRP 168.71.8.0 Serial0 168.71.6.1IGRP 168.71.9.0 Serial0 168.71.6.1

Router B

Protocol Network Interface Next HopIGRP 168.71.5.0 Seial0 168.71.9.1IGRP 168.71.6.0 Serial0 168.71.9.1IGRP 168.71.7.0 Serial0 168.71.9.1Connected 168.71.8.0 TokenRing0Connected 168.71.9.0 Serial0

Router C

X

168.71.7.1

168.71.8.1Router C

Router BS0.1 168.71.6.1

168.71.9.2 S0

168.71.6.2 S0


168.71.8.0

S0.2 168.71.9.1

X168.71.7.0

7-Mar-97 Version 1 1 Chapter 1 Page 45Robert Wright


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Convergence : Loss of a connected route vs. a dynamic route

168.71.8.1

Router A

Router C

Router B

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RouterA#debug ip routing


Aug 5 23:11:04: RT: delete subnet route to 168.71.5.0/24Aug 5 23:11:15: RT: add 168.71.5.0/24 via 168.71.9.2, rip metric [120/2]

RouterA#

Here you can see Route A immediately accepting the new route

RouterC# debug ip routing



Aug 5 23:48:38: RT: add 168.71.5.0 255.255.255.0 via 168.71.7.2, rip metric [120/2]RouterC#

Here you can see Route C going into holddown and waiting another 58 seconds before adding the new route

Routers may react to losing a connected route differently than when they lose a dynamic route.

168.71.5.1

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7 Mar 97 Version 1.1 Chapter 1 Page 46Robert Wright


Convergence : Poison reverse in action

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Here you can see the output of debug ip ripfrom Router A when serial 0 is shutdown

RouterA#debug ip rip

RouterA#config termRouterA-config#interface serial 1

RouterA#-config-if#

Oct 3 17:02:25: RIP: sending v1 update to 255.255.255.255 via Serial0 (168.71.6.1)

Oct 3 17:02:25: subnet 168.71.9.0, metric 1


Oct 3 17:02:25: default, metric 1

RouterA-config-if#shutdown

Oct 3 17:02:33: RIP: sending v1 update to 255.255.255.255 via Serial0 (168.71.6.1)


Oct 3 17:02:33: subnet 168.71.8.0, metric 16RouterA#

Router As serial 1 interface is shut down. Router A can no longer reach 168.71.9.0 and 168.71.8.0 out of

serial 1. By poisoning the Routes out serial 0 Router A is informing any other routers downstream of serial 0that it can no longer reach the two subnets. Upon receiving these poison advertisements routers that wereusing this path will delete their references to the two subnets and put them into holddown. This saves thedownstream routers from having to wait until their invalid timers fired for these subnets prior to going intoholddown



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7 Mar 97 Version 1.1 Chapter 1 Page 47Robert Wright



Here you can see the output of debug ip ripfrom Router Bs point of view

RouterB#debug ip rip

Oct 3 17:31:05: RIP: received update from 168.71.6.1 on Serial0

Oct 3 17:31:05: 168.71.9.0 in 1 hops

Oct 3 17:31:05: 168.71.8.0 in 2 hops

Oct 3 17:31:05: 168.71.5.0 in 1 hops

Oct 3 17:31:05: 0.0.0.0 in 1 hopsOct 3 17:31:11: RIP: received update from 168.71.6.1 on Serial0

Oct 3 17:31:11: 168.71.9.0 in 16 hops (inaccessible)

Oct 3 17:31:11: 168.71.8.0 in 16 hops (inaccessible)

Oct 3 17:31:11: 168.71.5.0 in 1 hops

Oct 3 17:31:11: 0.0.0.0 in 1 hops

Oct 3 17:31:11: RIP: received request from 168.71.6.1 on Serial0

Oct 3 17:31:11: RIP: sending update to 168.71.6.1 via Serial0 (168.71.6.2)



Oct 3 17:31:11: subnet 168.71.7.0, metric 1Oct 3 17:31:11: subnet 168.71.6.0, metric 1

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0



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a 9 e s o C apte age 8obe t g t



Here you can see the output of debug ip routingfrom Router As point of view when Serial 1 is shut down

RouterA#deb ip routing

RouterA#conf t

RouterA(config)#int s 1

RouterA(config-if)#shutRouterA(config-if)#

Oct 3 17:41:22: RT: interface Serial1 removed from routing table

Oct 3 17:41:22: RT: del 168.71.9.0/24 via 0.0.0.0, connected metric [0/0]

Oct 3 17:41:22: RT: delete subnet route to 168.71.9.0/24

Oct 3 17:41:22: RT: delete route to 168.71.8.0 via 168.71.9.2, Serial1

Oct 3 17:41:22: RT: no routes to 168.71.8.0, flushing

Oct 3 17:41:22: RT: delete route to 168.71.7.0 via 168.71.9.2, Serial1

Oct 3 17:41:22: RT: add 168.71.9.0/24 via 168.71.6.2, rip metric [120/2]

Oct 3 17:41:22: RT: add 168.71.8.0/24 via 168.71.6.2, rip metric [120/2]

Oct 3 17:41:24: RT: delete route to 168.71.9.0 via 168.71.6.2, rip metric [120/2]Oct 3 17:41:24: RT: no routes to 168.71.9.0, entering holddown

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0



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p gg



Here is the debug ip routingoutput from Router Bs point of view

RouterB#debug ip routing

Oct 14 23:38:34: RT: delete route to 168.71.9.0 via 168.71.6.1, rip metric [120/1]

Oct 14 23:38:36: RT: delete route to 168.71.9.0 via 168.71.7.1, rip metric [120/1]

Oct 14 23:38:36: RT: no routes to 168.71.9.0, entering holddown

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0



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p gg


Convergence Summary: Invalid

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Invalid - This is one of the timers that is used by dynamic routing protocols. It is used to keep track of the lasttime an update was received for a route in the routing table from the router the route is known via. In theprevious examples this timer has been set to 180 seconds. If a new advertisement is not received within theperiod this timer is set for the route is considered to be suspect and should not be advertised to other routers

as reachable. The router will continue to use this route for packets it receives as if it was still viable until theflush timer fires.



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Convergence Summary: Holddown

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Holddown -This is one of the timers that is used by dynamic routing protocols. It is used to help preventrouting loops. It does this by watching for increasing metrics for a route from the router that a network is knownvia. It also watches for networks that have ceased to be advertised by the router they are known via. Whenone of the two previously mentioned events takes plase this timer fires. It will run its full course unless it is

terminated by another timer known as the flush timer. Whether holddown is exited by having run its course orby having been terminated by the flush timer the router will then accept a new route for the network inquestion.



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Convergence Summary: Flush

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Flush -This is one of the timers that is used by dynamic routing protocols. It is used to determine when a routeshould be removed entirely from the routing tables. It typically works in conjunction with holddown although itcould be configured to a slightly different time. When this timer fires the route or network in question isremoved in its entireity from the routing tables. Although it is appropriate to have the flush timer terminate the

holddown timer it is not appropriate for the holddown timer to expire prior to the flush timer. This would result inaccepting a new route while the old, expired route is still in the routing table.



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Convergence Summary: Split-Horizon

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Split-horizon -This is the function of not advertising routes out over the interface that the router is using toaccess the route. In the example above Router As route to 168.71.8.0 is out serial 1 via 168.71.9.2. Router Awill not advertise this route back to Router C. However Router A will advertise 168.71.8.0 out serial 0 eventhough Router B advertised 168.71.8.0 to Router A.

168.71.8.0

168.71.8.0



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Convergence Summary: Poison Reverse

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Poison Reverse -This is the function of a router informing other routers that routes it was once capable ofreaching via a particular interface are no longer reachable because the interface has gone down. In thisexample Router A is using serial 1 to reach the connected network of 168.71.9.0 and the remote network of168.71.8.0. When serial 1 is shutdown Router A tells the other routers it is connected to (Router B in this case)

that the routes that it used to be able to reach via serial 1 are no longer reachable. Router B reacts to thepoison reverse message and puts only the poisoned routes it was using Router A to reach into holddownimmediately instead of waiting for them to time out.



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Parallel Paths: Load balancing & switching paths [fast-switching]

The end system is sending pings to 168.71.7.1 and 168.71.7.2. Router A is load sharing them on a destinationby destination basis.

Notice that only one packet to each destination shows up in the debug. This is because debugging only shows

the packets getting process switched. The later pings in sequence were fast switched and therefore invisible tothe debugging process.

RouterA#deb ip packet

RouterA#deb ip cache

IP: s=171.68.207.222 (Ethernet0), d=168.71.7.1 (Serial0), g=168.71.6.2, len 74, forward

IP: created cache entry for 168.71.7.1/32

IP: s=171.68.207.222 (Ethernet0), d=168.71.7.2 (Serial1), g=168.71.9.2, len 74, forwardIP: created cache entry for 168.71.7.2/32

Here you can see the IP cache getting populated when the packets are forwarded

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

171.68.207.222

171.68.207.164 secondary

Pings



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RouterA#sh ip cache

IP routing cache 3 entries, 444 bytes

Minimum invalidation interval 2 seconds, maximum interval 5 seconds,

quiet interval 3 seconds, threshold 0 requests

Invalidation rate 0 in last second, 0 in last 3 seconds

Last full cache invalidation occurred 0:00:25 ago

Prefix/Length Age Interface Next Hop

168.71.7.1/32 0:00:08 Serial1 168.71.9.2

168.71.7.2/32 0:00:03 Serial1 168.71.9.2

171.68.207.222/32 0:00:09 Ethernet0 171.68.207.222

RouterA#

Here you can see Router As IP cache

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

171.68.207.222

171.68.207.164 secondary

Pings



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RouterA#debug ip packet


IP: s=168.71.7.2 (Serial0), d=171.68.207.222 (Ethernet0), g=171.68.207.222, len 64, forward





IP: s=168.71.7.2 (Serial0), d=171.68.207.222 (Ethernet0), g=171.68.207.222, len 64, forwardRouterA#

Here you can see the debug from Router A as the pings from the end system are switched

IP cache entries are created as they packets leave the router, not as they enter. So it is possible to fast switchping packets from the end station to 168.71.7.2 and process switch the responses.

This example show what this looks like. The fist ping from the end system is process switched out serial 0. Theresponse is process switched out ethernet 0.

The following pings from the end system to 168.71.7.2 are fast switched out serial 0 and therefore hidden fromthe debug. However the following four responses are process switched and show in the debug.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

168.71.7.2

168.71.7.1

168.71.6.2 S0

171.68.207.222

171.68.207.164 secondary

Pings

Fast Switching

Process Switching



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It is possible to prove that IP cache entries are created as packets are switched out of an interface by pingingan address that does not exist. In the example Router C will attempt to ping 1.1.1.1. Router C will send these

pings to Router A because Router A is advertising a Gateway Of Last Resort. Router A will forward the pingsout its ethernet 0 because that is where Router As Gateway Of Last Resort exists. Router A will create a cacheentry for the prefix even though no system with 1.1.1.1 exists.

RouterC#ping 1.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

.....Success rate is 0 percent (0/5)

RouterC#

Here you can see Router C sending the pings to 1.1.1.1

168.71.8.1

Router A Router C168.71.5.1

S1 168.71.9.1 168.71.9.2 S1

Pings

RouterA#debug ip cache



IP: created cache entry for 1.0.0.0/8IP: ager ran for 0ms, 1 entries, 1 examined, 0 aged, 0 reaped

Here you can see Router A forwarding the ping and creating the cache entry



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Parallel Paths: Load balancing & switching paths [process-switching]

Here you can see the packets going out serial 0 and serial 1 because there are two equal cost routes to 168.71.7.0








IP: s=171.68.207.222 (Ethernet0), d=168.71.7.1 (Serial1), g=168.71.9.2, len 74, forwardIP: s=171.68.207.222 (Ethernet0), d=168.71.7.1 (Serial0), g=168.71.6.2, len 74, forward

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

171.68.207.222

171.68.207.164 secondary

Pings

In this example all four pings from the end system to each destination show up in the debug. This is becausethey are all being process switched.



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Router#-config#interface serial 1Router#-config-if#ip route-cache

Router#-config-if#^Z

RouterA#sh ip int s 1

Serial1 is up, line protocol is up

Internet address is 168.71.9.1/24

Broadcast address is 255.255.255.255

Address determined by non-volatile memory

MTU is 1500 bytes

Helper address is not set

Directed broadcast forwarding is enabled Multicast reserved groups joined: 224.0.0.9

Outgoing access list is not set

Inbound access list is not set

Proxy ARP is enabled

Security level is default

Split horizon is enabled

ICMP redirects are always sent

ICMP unreachables are always sent

ICMP mask replies are never sent

IP fast switching is enabled IP fast switching on the same interface is enabled

IP multicast fast switching is enabled

Router Discovery is disabled

IP output packet accounting is disabled

IP access violation accounting is disabled

TCP/IP header compression is disabled

Probe proxy name replies are disabled

Gateway Discovery is disabled

Policy routing is disabledRouterA#



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Parallel Paths: Load balancing & switching paths [process-switching]

Router#-config#interface serial 0Router#-config-if#no ip route-cache

Router#-config-if#^Z

RouterA#sh ip int s 0


Internet address is 168.71.6.1/24

Broadcast address is 255.255.255.255

Address determined by non-volatile memory

MTU is 1500 bytes

Helper address is not set

Directed broadcast forwarding is enabled Multicast reserved groups joined: 224.0.0.9

Outgoing access list is not set

Inbound access list is not set

Proxy ARP is enabled

Security level is default

Split horizon is enabled

ICMP redirects are always sent

ICMP unreachables are always sent

ICMP mask replies are never sent

IP fast switching is disabled

IP fast switching on the same interface is disabled

IP multicast fast switching is enabled

Router Discovery is disabled

IP output packet accounting is disabled

IP access violation accounting is disabled

TCP/IP header compression is disabled

Probe proxy name replies are disabled

Gateway Discovery is disabled

Policy routing is disabledRouterA#



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Parallel Paths summary : Load balancing & switching paths

Load Balancing & Switching Paths -Routers will load balance IP packets on a per destination basis whenfast switching and on a packet by packet basis when process switching. IP cache entries are created aspackets are forwarded out the outward bound interface.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0



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Table of contents: Chapter two

Metric vs. distance: Terms defined. RIP vs IGRP Summary:

RIP vs. IGRP. Metrics:

IGRP - Bandwidth vs. delay Summary:

IGRP - bandwidth vs. delay



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Metric vs. distance: Terms defined

Metric -This term is used to describe the overall desirability of a route to a remote (not locally attached)network. Metrics can be as simple as measuring the number of hops in a path. Or they can be as complex asIGRP which may use bandwidth, delay, reliability, load and interface mtu depending on how it is configured. Bydefault IGRP uses only bandwidth and delay.

Distance -This term is used to describe the overall believability of a route. Each type of routing protocol that

Cisco routers support has a distance value associated with it. Connected routes and static routes also have abelievability factor associated with a distance value. A router may need to decide between using a static routeor a dynamic route to the same network. Static routes are usually more believable based on the assumptionthat someone went to some degree of effort to configure it therefore it has a higher probability of beingaccurate. RIP has a distance of 120. IGRP has a distance of 100. Connected routes are the most believableand cannot be overruled by a static or dynamic route.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RIP

IGRP

IGRP



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Metrics vs. distance: RIP & IGRP

The configs have been changed so that Router A & Router C are using RIP on the 168.71.5.0 subnet .

Router A, Router B and Router C are running IGRP on the 168.71.6.0 and 168.71.7.0 subnets.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RIP

IGRP

IGRP

RouterC#deb ip igrp transactions

Oct 7 22:35:59: IGRP: received update from 168.71.7.2 on Serial0

Oct 7 22:35:59: subnet 168.71.6.0, metric 82125 (neighbor 80125)

Oct 7 22:35:59: subnet 168.71.5.0, metric 10004001 (neighbor 10002001)

RouterC#debug ip rip

Oct 7 22:33:10: RIP: received update from 168.71.9.1 on Serial1

Oct 7 22:33:10: 168.71.6.0 in 1 hops

Oct 7 22:33:10: 168.71.5.0 in 1 hops

Here you can see the debug messages from both routing protocols as the arrive at router C. The nextslide shows the routing table from Router C.



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Metrics vs. distance: RIP & IGRP

This is the routing table from router C. Notice that the route to 168.71.5.0 is via serial 0 even though the

connection via serial 1 is only one hop. IGRP has a distance of 100 while RIP has a distance of 120. Thedistance value is more important than the metric value within a routers routing table.

RouterC#show ip route






I* 10.0.0.0 [100/180250] via 168.71.7.2, 00:00:47, Serial0

168.71.0.0 255.255.255.0 is subnetted, 5 subnets


C 168.71.8.0 is directly connected, TokenRing0


I 168.71.6.0 [100/82125] via 168.71.7.2, 00:00:47, Serial0

I 168.71.5.0 [100/10004001] via 168.71.7.2, 00:00:47, Serial0

R* 0.0.0.0 0.0.0.0 [120/1] via 168.71.9.1, 00:00:18, Serial1

RouterC#

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RIP

IGRP

IGRP



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Metrics vs. distance summary: RIP & IGRP

Metric vs. Distance -Some routing protocols are considered to be more believable than others. Cisco routersbelieve that IGRP is more trustworthy that RIP. Hence the higher distance assigned to RIP routes. This willlead to a longer path via an IGRP derived route overriding a shorter RIP derived route.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RIP

IGRP

IGRP



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Metrics: IGRP - bandwidth vs. delay

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

In this routing table Router C shows only one route to 168.71.5.0. Changing the delay on serial 1 on Router C

to equal the combined delay of the 168.71.7.0 link and the 168.71.6.0 link will cause Router C to believe thatthe two links are equal cost. The next slides explain this further.

RouterC#show ip route





Gateway of last resort is not set

I* 10.0.0.0 [100/180250] via 168.71.7.2, 00:00:42, Serial0

168.71.0.0 255.255.255.0 is subnetted, 5 subnets


C 168.71.8.0 is directly connected, TokenRing0


I 168.71.6.0 [100/82125] via 168.71.9.1, 00:00:21, Serial1

[100/82125] via 168.71.7.2, 00:00:42, Serial0

I 168.71.5.0 [100/10002001] via 168.71.9.1, 00:00:21, Serial1

RouterC#


IP R i F d l


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By manipulating the interface delay on serial 1 in Router C it is possible to make Router C believe that thepaths via serial 1 and serial 0 to 168.71.5.0 are equal cost. This is because delay is cumulative in a path.Bandwidth on the other hand just determines the minimum bandwidth in any path.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

hostname RouterC!interface serial1ip address 168.71.9.2 255.0.0.0bandwidth 128delay 4000

!

RouterC#show interface serial 1


Hardware is HD64570

Internet address is 168.71.9.2 255.255.255.0 MTU 1500 bytes, BW 128 Kbit, DLY 40000usec, rely 255/255, load 1/255

Delay 40,000 usecs

Delay 20,000 usecs

Delay 20,000 usecs


IP R i F d l


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168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

Router C now has two routes to 168.71.5.0. Via serial 0 and serial 1.

RouterC#show ip route 168.71.5.0

Routing entry for 168.71.5.0 255.255.255.0

Known via "igrp 109", distance 100, metric 10004001

Redistributing via igrp 109

Advertised by igrp 109 (self originated)

Last update from 168.71.7.2 on Serial0, 00:00:54 ago

Routing Descriptor Blocks:

* 168.71.9.1, from 168.71.9.1, 00:01:01 ago, via Serial1

Route metric is 10004001, traffic share count is 1

Total delay is 40010 microseconds, minimum bandwidth is 128 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 1/255, Hops 0

168.71.7.2, from 168.71.7.2, 00:00:54 ago, via Serial0

Route metric is 10004001, traffic share count is 1

Total delay is 40010 microseconds, minimum bandwidth is 128 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 1/255, Hops 1

RouterC#

Delay 40,000 usecs

Delay 20,000 usecs

Delay 20,000 usecs


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IP R ti F d t l


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It is not possible to have both Routers B and C load share their traffic across the links connected to Router A.This is because increasing the delay on serial 0 in Router B to get Router B to load share over serial 0 andserial 1 affects the metric that Router C receives for 168.71.5.0 from Router B. This prevents Router B andRouter C from load sharing.

168.71.8.1

Router A

Router C

Router B

168.71.5.1

S0

S1S0 168.71.6.1

S1 168.71.9.1168.71.9.2 S1

168.71.7.2

168.71.7.1

168.71.6.2 S0

RouterB#show interface serial 0


Hardware is HD64570

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IP Routing - Fundamentals

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