Chapter 13
Routing Protocols (RIP, OSPF, and BGP)
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Outlineo INTERIOR AND EXTERIOR ROUTINGo RIPo OSPFo BGP
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Introductiono An internet is a combination of networks connected
by routers
o How to pass a packet from source to destination ?n Which of the available pathways is the optimum pathway ?
o Depends on the metric: n Metric: a cost assigned for passing through a networkn A router should choose the route with the smallest metric
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Introduction (Cont.)o The metric assigned to each network depends on the
type of protocoln RIP (Routing Information Protocol)
o Treat each network as equalso The cost of passing through each network is the same: one hop
countn Open Shortest Path First (OSPF)
o Allow administrator to assign a cost for passing through a network based on the type of serviced requiredn For example, maximum throughput or minimum delay
n Border Gateway Protocol (BGP)o The criterion is the policy, which can be set by the administrator
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Introduction (Cont.)o Routing table can be static or dynamicn An internet needs dynamic routing tables
o Dynamic routing table is achieved by the routing prococols
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INTERIORAND
EXTERIORROUTING
13.113.1
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Interior and Exterior Routingo An internet can be so large that one routing
protocol cannot handle the task of updating routing table of all routers
o Thus, an internet is divided into autonomous systems (AS)n AS is a group of networks and routers under the
authority of a single administration
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Interior and Exterior Routingo Interior routingn Routing inside an autonomous systemn Each AS can chose its own interior routing
protocoln Examples: RIP and OSPF
o Exterior routingn Routing between autonomous systemsn Only one exterior routing protocol is usually used
for exterior routingn Examples: BGP
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Figure 13-1
Popular Routing Protocols
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Exampleo R1, R2, R3 and R4 use an interior and an
exterior routing protocol
o Solid thin linesn Interior routing protocol
o Broken thick linesn Exterior routing protocol
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Figure 13-2
Autonomous Systems
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RIP:Routing
InformationProtocol
13.213.2
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RIPo RIP: Routing Information Protocoln Based on distance vector routing
n Use the Bellman-Ford algorithm for calculating the routing tables
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Distance Vector Routingo Each router periodically shares its knowledge
about the entire internet with its neighborsn Sharing knowledge about the entire AS
o At the start, a router’s knowledge may be sparseo But, how much it knows is unimportant, it sends
whatever it has
n Sharing only with neighborso Sends its knowledge only to neighbors
n Sharing at regular intervals
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RIP Updating Informationo Routing table is updated on receipt of a RIP response messageo Receipt: a response RIP messageo Add one hop to the hop count for each advertised destinationo Repeat the following steps for each advertised destination
n If (destination not in the routing table)o Add the advertised information to the table
n Elseo If (next-hop field is the same)
n Replace retry in the table with the advertised oneo Else
n If (advertised hop count smaller than one in the table)o Replace entry in the routing table
o Return
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Example of Updating a Routing Table
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Initializing the Routing Tableo When a router is added to a networkn It initializes a routing table for itself using its
configuration file
n The table contains only the directly attached networks and the hop count (= 1)
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Updating the Routing Tableo Each routing table is updated upon receipt of
RIP message n Using the RIP updating message algorithm shown
above
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Initial Routing Tables in a Small Autonomous System
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Final Routing Tables for the Previous Figure
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RIP Message Formato Command: 8-bit
n The type of message: request (1) or response (2)o Version: 8-bit
n Define the RIP versiono Family: 16-bit
n Define the family of the protocol usedn TCP/IP: value is 2
o Address: 14 bytes n Defines the address of the destination networkn 14 bytes for this field to be applicable to any protocoln However, IP currently uses only 4 bytes, the rest are all 0s
o Distance: 32-bitn The hop count from the advertising router to the destination network
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Figure 13-6
RIP Message Format
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Requests and Responseo RIP uses two type of messagesn Request and response
o Requestn Sent by a router that has just come up or has some
time-out entriesn Can ask specific entries or all entries
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Figure 13-7
Request Messages
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Requests and Response (Cont.)o Response: solicited or unsolicitedn A solicited response: sent only in answer to a
requesto Contain information about the destination specified in
the corresponding requestn An unsolicited response: sent periodically
o Every 30so Contains information about the entire routing tableo Also called update packet
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o What is the periodic response sent by router R1 in the next slide?
o Assume R1 knows about the whole autonomous system.
Example 1Example 1
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Figure 13-8
Example 1
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o R1 can advertise three networks 144.2.7.0, 144.2.9.0, and 144.2.12.0.
o The periodic response (update packet) is shown in the next slide.
SolutionSolution
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Figure 13-9
Solution to Example 1
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Timers in RIPo RIP uses three timersn Periodic timer
n Expiration timer
n Garbage collection timer
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Figure 13-10
RIP Timers
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Periodic Timero Periodic timern Control the advertising of regular update message
n Although protocol specifies 30 s, the working model uses a random number between 25 and 35 so Prevent routers update simultaneously
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Expiration Timero Govern the validity of a routeo Set to 180 s for a route when a router receives update
information for a routen If a new update for the route is received, the timer is resetn In normal operation, this occurs every 30 s
o If timer goes off, the route is considered expiredn The hop count of the route is set to 16, which means
destination is unreachable
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Garbage Collection Timero When a route becomes invalid, the router does not
immediately purge that route from its tableo It continues advertise the route with a metric value of
16o A garbage collection timer is set to 120 s for that
routeo When the count reaches zero, the route is purged from
the tableo Allow neighbors to become aware of the invalidity of
a route prior to purging
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o A routing table has 20 entries. o It does not receive information about five
routes for 200 seconds. o How many timers are running at this time?
Example 2Example 2
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o The timers are listed below:n Periodic timer: 1n Expiration timer: 20 - 5 = 15n Garbage collection timer: 5
SolutionSolution
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Problems in RIPo Slow Convergence
o Instability
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Slow Convergenceo A change somewhere in the internet
propagates very slowly through the rest of the internet
o For examplen A change is Net1, R1 updates itself immediatelyn R1->R2: an average of 15 sn R2->R3: an average of 15 sn …n Thus, R1-Rn: an average of 15 x n s
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Figure 13-11
Slow Convergence
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Solutiono Limit the hop count to 15n Prevent data packet from wandering around
forever
o Thus, an autonomous system using RIP is limited to a diameter of 15n The number 16 is considered infinity and
designates an unreachable network
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Figure 13-12
Hop count
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Instabilityo An internet running RIP can become unstablen A packet could go from one router to another in a
loop
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Instability
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Solutionso Triggered Update
o Split Horizons
o Poison Reverses
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Triggered Updateo If there is no change, updates are sent at the
30-s intervals
o If there is a change, the routers sends out its new table immediately
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Split Horizonso A router must distinguish between different interfaceo If a router received route updating message from an
interfacen This same updated information must not be sent back
through this interfaceo Example
n B receives information about Net1 and Net2 through its left interface
n This information is updated and passed on through the right interface but not to the left
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Figure 13-14
Split Horizon
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Poison Reverseo A variation of split horizonso Information received is used to update routing table
and then passed out to all interfaceo However, a table entry that has come through one
interface is set to a metric of 16 as it goes out through the same interface
o For examplen Router B has received information about Net1 and Net2
through its left interfacen Thus, it sends information out about Net1 and Net2 with a
metric of 16
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Figure 13-15
Poison Reverse
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RIP Version 2o Does not augment the length of the message
of each entry
o Only replace those fields in version 1 that were filled with 0s with some new fields
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RIP Version 2o New fieldsn Routing Tag: carries information such as the
autonomous system numbero Enable RIP to receive information from an exterior
routing tablen Subnet mask: carries the subnet mask (or prefix)
o RIP2 support classless addressing and CIDRn Next-hop address: show the address of the next
hop
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Figure 13-16
RIP-v2 Format
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RIP version 2 supports RIP version 2 supports CIDR.CIDR.
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Authenticationo Protect the message against unauthorized
advertisement
o The first entry of the message is set aside for authentication informationn Family field = FFFF16
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Figure 13-17
Authentication
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Multicastingo Version 1 of RIP uses broadcasting to send
RIP message to every neighborn All the router and the hosts receive the packets
o RIP version 2n Uses the multicast address 224.0.0.9 to multicast
RIP message only to RIP routers in the network
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Encapsulationo RIP message are encapsulated in UDP user
datagram
o The well-known port assigned to RIP in UDP is port 520
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RIP uses the services of UDP RIP uses the services of UDP on wellon well--known port 520.known port 520.
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OSPF:Open Shortest
Path First
13.313.3
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OSPFo OSFP: Open Shortest Path Firsto Another interior routing protocolo OSPF divides an autonomous system into
areasn To handle routing efficiently and in a timely
manner
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Areaso A collection of networks, hosts, and routers
all contained within an autonomous system
o Thus, an autonomous system can be divided into many different areas
o All networks inside an area must be connected
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Areas (Cont.)o Routers inside an area flood the area with
routing information
o Each area has a special router called area border routersn Summarize the information about the area and
sent it to other areas
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Areas (Cont.)o Among the area inside an autonomous system
is a special area called backbonen All of the areas inside an AS must be connected
to the backboneo The routers inside the backbone are called the
backbone routersn A backbone router can also be an area border
router
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Areas (Cont.)o If the connectivity between a backbone and an
area is brokenn A virtual link must be created by the
administration
o Each area has an area identificationn The area identification of the backbone is zero
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Figure 13-18
Areas in an Autonomous System
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Metricso OSPF allows the administrator to assign a
cost, called the metric, to each routeo Metric can be based on a type of servicen Minimum delayn Maximum throughput
o A router can have multiple routing tablesn Each based on a different type of service
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Link State Routingo OSPF uses link state routing to update the
routing table in an area
o Link state routing: a process by which each router shares its knowledge about its neighbor with every router in the area
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Link State Routing (Cont.)o Three keysn Sharing knowledge about the neighborhood
o Each router sends the state of its neighborhood to every other router in the area
n Sharing with every other routero Each router sends the state of its neighborhood to
every other router in the area by floodingn Sharing when there is a change
o Each router shares the state of its neighborhood only when there is a change
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Link State Routing (Cont.)o The idea behind link state routingn Each router should have the exact topology of the
internet at every momento Thus, every router should have the whole
picture of the internetn A router can calculate the shortest path between
itself and each networko But, how to represent an internet by graph?
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Types of Linkso In OSPF, a connection is called a link
o Four types of linksn Point-to-pointn Transientn Stubn Virtual
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Figure 13-19
Types of Links
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Point-to-Point Linko Connect two routers without any other host or router
in betweeno Example
n Telephone linen T-line
o Graphically representationn The routers are represented by nodesn The link is represented by a bidirectional edge
o The metricn Usually the same at the two ends
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Figure 13-20
Point-to-Point Link
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Transient Linko A network with several routers attached to itn Data can enter through any of the routers and
leave through any router
o Examplen All LANs and some WANs with two or more
routers
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Transient Link (Cont.)o Graphically representation
n Figure b in the next slide. However, it iso Not efficient: each router need to advertise the neighborhood of
four other routersn For a total of 20 advertisement
o Not realistic: there is no single network (link) between each pair of routersn There should be only one network that serves as a crossroad
between all five routers
o Solution: one of the routers acts as a single networkn This router has a dual purpose: a true router and a
designated routero The link is represented as a bidirectional edgeo Figure c in the next slide
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Figure 13-21
Transient Link
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Stub Linko A network that is connected to only one routern Data packet enter and leave through this only one
routero A special case of transient networko Graphically representationn The router as a noden The designed router as the networkn Note, the link is only one-directional
o From the router to the network
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Figure 13-22
Stub Link
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Virtual Linko When the link between two routers is brokenn The administrator may create a virtual path
between them using a longer path and may go through several routers
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Figure 13-23
Example of an internet
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Figure 13-24
Graphical Representation of an internet
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Link State Advertisementso Each entity distributes Link State Advertisement to
share information about their neighborso An LSA announces the states of entity linkso Five LSAs, depend on the type of entity
n Router linkn Network linkn Summary link to networkn Summary link to AS boundary routern External link
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Figure 13-25
Types of LSAs
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Router Linko Define the links of a true router
o A true router uses this advertisement to announce information about n All of its links n What is at the other side of the links (neighbors)
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Figure 13-26
Router Link
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Network Linko Defines the links of a network
o A designated router, on behalf of the transient network, distributes this type of LSA packet
o Announce the existence of all of the routersconnected to the network
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Figure 13-27
Network Link
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Summary Link to Networko Router link and network link advertisements
n Flood the area with information inside an areao But a router must also know about the networks
outside its arean The area border routers provide this information
o An area border is active in more than one arean Receive router link and network link advertisementsn Create a router table for each arean Provide one area’s information to other areas by the
summary link to network advertisement
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Exampleo R1 is an area border routern Two routing table: one for area 1 and one for area
0o R1 will flood area 1 with information about
how to reach a network located in area 0
o R2 plays the same role
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Figure 13-28
Summary Link to Network
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Summary Link to AS Boundary Routero Previous advertisement lets every router know the
cost to reach all of the networks inside the ASo But, how to reach a network outside an AS?o A router must know how to reach the autonomous
boundary router firsto The summary link to AS boundary router provides
this informationn The area border routers flood their area with this
information
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Figure 13-29
Summary Link to AS Boundary Router
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External Linko How a router inside an AS know which
networks are available outside the AS ?
o The AS boundary routers floods the autonomous system with the cost of each network outside the ASn Using a routing table created by an exterior
routing protocol
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External Link (Cont.)o Notably, each advertisement announces one
single networkn Separate announcements are made if more than
one network exists
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Figure 13-30
External Link
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o In Figure 13.31 (next slide), which router(s) sends out router link LSAs?
Example 3Example 3
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Figure 13-31
Example 3 and Example 4
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SolutionSolution
All routers advertise router link LSAs.
R1 has two links, Net1 and Net2.
R2 has one link, Net2 in this AS.
R3 has two links, Net2 and Net3.
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Example 4Example 4
In Figure 13.31, which router(s) sends out the network link LSAs?
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SolutionSolution
All three network must advertise network links:
Advertisement for Net1 is done by R1 because it is the only router and therefore the designated router.
Advertisement for Net2 can be done by either R1, R2, or R3, depending on which one is chosen as the designated router.
Advertisement for Net3 is done by R3 because it is the only router and therefore the designated router.
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Link State Databaseo Every router in an arean Receive the router link and network link LSA
from every other routern And form a link state database
o A tabular representation of the topology of the internet inside an area
o Notably, every router in the same area has the same link state database
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In OSPF, all routers have In OSPF, all routers have the same link state database.the same link state database.
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Dijkstra Algorithmo Each router applies the Dijkstra algorithm to
from its link state database
o Dijkstra algorithmn Calculate the shortest path between two points
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Figure 13-32-Part 1
Shortest Path Calculation
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Figure 13-32-Part 2
Shortest Path Calculation
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Figure 13-32 Part 3
Shortest Path Calculation
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Routing Tableo Each routers uses the shortest path tree method to
construct its routing tableo Routing table in OSPF includes the cost of reaching
each network in the areao To find the cost of reaching network outside of the
area, the routers use then Summary link to networkn Summary link to boundary routern External link advertisements
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Types of Packetso OSPF uses five different packets
n Hello packetn Database description packetn Link state request packetn Link state update packet
o Router linko Network linko Summary link to networko Summary link to AS boundary routero External link
n Link state acknowledgment packet
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Figure 13-33
Types of OSPF Packets
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Packet Formato All OSPF packets share the same headern Version: 8-bit
o The version of the OSPF protocol. Currently, it is 2n Type: 8-bit
o The type of the packetn Message length: 16-bit
o The length of the total message including the headern Source route IP address: 32-bit
o The IP address of the router that sends the packet
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Figure 13-34
OSPF Packet Header
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Packet Format (Cont.)n Area identification: 32-bit
o The area within which the routing take place
n Checksum: 16-bito Error detection on the entire packet excluding the authentication
type and authentication data field
n Authentication type: 16-bito Define the authentication method used in this areao 0: none, 1: password
n Authentication: 64-bito The actual value of the authentication datao Filled with 0 if type = 0; eight-character password if type = 1
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Hello Messageo OSPF uses the hello message ton Create neighborhood relationshipsn Test the reachability of neighbors
o First step in link state routingn It must first greet its neighbors
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Figure 13-35
Hello Packet
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Hello Packet Formato Network mask: 32-bit
n Define the network mask of the network over which the hello message is sent
o Hello interval: 16-bitn Define the number of seconds between hello message
o E flag: 1-bitn If it is set, the area is a stub area
o F flag: 1-bitn If it is set, the router supports multiple metrics
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Hello Packet Format (Cont.)o Priorityn The priority of the router. Used for the selection
of the designated routern The router with the highest priority is chosen as
the designated routern The router with the second highest priority is
chosen as the backup designated routern If it is 0, the router never wants to be a designated
or backup designated router
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Hello Packet Format (Cont.)o Dead interval: 32-bitn The number of seconds before a router assumes
that a neighbor is deado Designated router IP address: 32-bito Backup designated router IP address: 32-bito Neighbor IP address: a repeated 32-bit fieldn A current list of all the neighbors from which the
sending router has received the hello message
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Database Description Messageo When a router is connected to the system for
the first time or after a failuren It needs the complete link state database
immediatelyo Thus, it sends hello packets to greet its
neighborso If this is the first time that the neighbors hear
from the routern They send a database description packet
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Database Description Message (Cont.)o The database description message does not contain
complete database informationn It only gives an outline, the title of each line in the
databaseo The newly router examines the outline and find out
which lines it does not haven Send one or more link state request packets to get full
information about that particular linkn The content of the database may be divided into several
message
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Database Description Message (Cont.)o When two routers want to exchange database
description packetsn One of them acts as matern The other is the slave
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Figure 13-36
Database Description Packet
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Database Description Message Formato E flag: 1-bit
n Set to 1 if the advertising router is an autonomous boundary router
o B flag: 1-bitn Set to 1 if the advertising router is an area border router
o I flag: 1-bit, the initialization flagn Set to 1 if the message is the first message
o M flag: 1-bit, more flagn Set to 1 if this is not the last message
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Database Description Message Format (Cont.)o M/S flag: 1-bit, master/slave flag
n Indicate the origin of the packet. Master = 1, Slave = 0o Message sequence number: 32-bit
n Contain the sequence number of the messageo LSA header: 20-bit
n Used in each LSAn The format of this header is discussed in the link state
update messageo Only give the outline of each link
n It is repeated for each link in the link state database
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Link State Request Packeto Sent by a router that needs information about
a specific route or routesn Answered with a link state update packet
o Used by a newly connected router to request more information after receiving the database description packet
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Figure 13-37
Link State Request Packet
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Link State Update Packeto Used by a router to advertise the state of its
linkso Each update packet may contain several
different LSAso Packet formatn Number of link state advertisements: 32-bitn Link state advertisement
o There are five different LSAs, as discussed beforeo All have the same header format, but different bodies
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Figure 13-38
Link State Update Packet
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LSA Formato Link state age: the number of seconds elapsed
since this message was first generatedn LSA goes from router to router, i.e., floodingn When a router create a message, age = 0n When each successive router forwards this message
o Estimate the transmit time and add it to the age field
o E flag: if 1, the area is a stub arean i.e., an area that is connected to the backbone area
by only one path
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LSA Format (Cont.)o T flag: if 1, the router can handle multiple
types of serviceo Link state typen 1: router linkn 2: network linkn 3: summary link to networkn 4: summary link to AS boundary routern 5: external link
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LSA Format (Cont.)o Link state ID: depend on the type of linkn Router link: IP address of the routern Network link: IP address of the designed routern Summary link to network: address of the networkn Summary link to AS boundary router: IP address
of the AS boundary routern External link: address of the external network
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LSA Format (Cont.)o Advertisement router:
n IP address of the router advertising this messageo Link state sequence number:
n Sequence number assigned to each link state update message
o Link state checksum: n A special checksum algorithm: Fletcher’s checksum
o Length: n Total packet length
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Figure 13-39
LSA Header
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Router Link LSAo Advertise all of the links of a router o Format
n Link ID: o Depend on the type of link, see Table 13.3
n Link data: o Give additional information about the link, also depend on the
type of link, see Table 13.3n Link type:
o Four different types of links are defined based on the type of network, see Table 13.3
n Number of types of services (TOS)o The number of type of services announced for each link
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Router Link LSA (Cont.)n Metric for TOS 0:
o Define the metrics for the default type of service (TOS 0)
n TOS: o Define the type of service
n Metric: o Define the metric for the corresponding TOS
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Figure 13-40
Router Link LSA
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Table 13.3
Router addressAddress of neighbor router
Type 4: virtual link
Network maskNetwork addressType 3: connection to stub network
Router addressAddress of designed router
Type 2: connection to any-to-any network
Interface numberAddress of neighbor router
Type 1: point-to-point connection to another network
Link DataLink IdentificationLink Type
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Example 5Example 5
Give the router link LSA sent by router 10.24.7.9 in Figure 13.41.
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Figure 13-41
Example 5
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o This router has three linksn Two of type 1 (point-to-point)n One of type 3 (stub network)
o Figure 13.42 shows the router link LSA
SolutionSolution
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Figure 13-42
Solution to Example 5
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Network Link LSAo Announce the links connected to a networko Formatn Network mask
o Define the network maskn Attached router
o Define the IP addresses of all attached routers
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Figure 13-43
Network Link Advertisement Format
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Example 6Example 6
Give the network link LSA in Figure 13.44.
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Figure 13-44
Example 6
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SolutionSolution
The network, for which the network link advertises, has three routers attached. The LSA shows the mask and the router addresses. See Figure 13.45.
Note that only one of the routers, the designated router, advertises the network link.
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Figure 13-45
Solution to Example 6
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Summary Link to Network LSAo Used by the area router to announce the existence of
other networks outside the areao Each advertisement announces only one network
n If more than one network, a separate advertisement must be issued for each
o Formatn Network maskn TOS:
o Type of servicen Metric:
o Metric for the type of service defined in the TOS field
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Figure 13-46
Summary Link to Network LSA
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Summary Link to AS Boundary Router LSAo Announce the route to an AS boundary routern Define the network to which the AS boundary
router is attached
o Formatn The same as the summary link to network LSA
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Figure 13-47
Summary Link to AS Boundary LSA
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External Link LSAo Announce all the networks outside the ASo Format: similar to the summary link to AS
boundary router LSA but add two fieldsn Forwarding address
o Define a forward router than can provide a better route to the destination
n External route tago Used by other protocol, but not by OSPF
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Figure 13-48
External Link LSA
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Link State Acknowledgment Packeto OSPF forces every router to acknowledge the
receipt of every link state update packetn Make routing more reliable
o Formatn Common headern Link state header
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Figure 13-49
Link State Acknowledgment Packet
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Encapsulationo OSPF packets are encapsulated in IP
datagramn OSPF contains the acknowledgment mechanism
for flow and error control
n Doe not need a transport layer protocol to provide these services
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OSPF packets are OSPF packets are encapsulated in IP encapsulated in IP datagramsdatagrams..
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BGP:Border Gateway
Protocol
13.413.4
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BGPo BGP: Border Gateway Protocol
o An inter-autonomous system routing protocol
o Based on the path vector routing method
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BGP (Cont.)o Why the distance vector routing and link state
routing are not good candidates for inter-autonomous system routing?
o Distance vector routingn There are occasions in which the route with the smallest
hop count is not the preferred routeo For example, we may not want a packet to pass through an AS
that is not secure even it is the shortest pathn Unstable as discussed before
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BGP (Cont.)o Link state routingn An internet is usually too big for this routing
method
n If used, each router must have a huge link state databaseo It would take a long time for each router to calculate
its routing table by Dijkstra algorithm
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Path Vector Routingo Each entry in the routing table contains n The destination network
n The next router
n The path to reach the destination, usually defined as ordered list of ASs
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Example of a Path Vector Routing Table
AS62, AS02, AS09R12NO4
AS67, AS89, AS09, AS34
RO6NO3
AS22, AS67, AS05, AS89
R05NO2
AS14, AS23, AS67R01NO1
PathNext RouterNetwork
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Path Vector Messageo Autonomous boundary routers advertise the
reachability of networks in their own AS to neighbor autonomous boundary routers
o Each router receives a path vector messagen Verify the advertised path is in agreement with its policyn If yes, update its routing table and modify the message
before sending it to the next routero Add its AS number to the path and replacing the next router entry
with its own identification
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Exampleo R1 send a path vector messagen Advertise the reachability of N1
o R2 receives the messagen Update it routing tablen Send the message to R3 with adding its AS to the
path and inserting itself as the next router
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Figure 13-50
Path Vector Packets
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Loop Preventiono Path vector routing can solve n The instability of distance vector routing
n The creation of loops o When a router receive a message, it check to see if its
AS is in the path list or notn If yes, looping is involved and the message is ignored
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Policy Routingo Policy routing can be easily implemented through
path vector routingn Once a router receives a message, it can check the path. n If one of the AS listed in the path is against its policy,
o It can ignore that path and that destinationo Does not update its routing table with this patho Does not send this message to its neighbors
o Thus, path vector routing are not based on the smallest hop count or the minimum metricn Based on the policy imposed on the router by the
administrator
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Path Attributeso In previous example, the path was presented
as a list of AS
o Actually, the path was presented as a list of attributesn The list of attributes help the receiving router
make a better decision when applying its policy
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Path Attributes (Cont.)o Attributes are divided into two categories: well-known and
optionalo Well-known: one that every BGP router should recognize
n Mandatoryo Must appear in the description of a routeo e.g., ORIGIN: the source of the routing information (RIP or OSPF)o e.g., AS_PATH: the list of AS through which the destination can be
reachedo e.g., NEXT_HOP: the next router to which data packet should be sent
n Discretionaryo Must be recognized by each routero But is not required to be included in every update message
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Path Attributes (Cont.)o Optional: one that need not be recognized by
every routern Transitive
o Must be passed to the next router by the router that has not implemented this attribute
n Nontransitiveo One that should be discarded if the receiving router
has not implemented it
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Types of Packetso BGP uses four different types of messagesn Open
n Update
n Keepalive
n Notification
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Figure 13-51
Types of BGP Messages
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Packet Formato All BGP packets share the same common
headero Header formatn Marker: 16-bit
o Reserved for authenticationn Length: 2-bytes
o Define the length of the total message, including the header
n Type: 1-byteo Define the type of the packet
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Figure 13-52
BGP Packet Header
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Open Messageo Used to create a neighborhood relationshipo A router running BGP opens a TCP
connection with a neighbor and sends an open message
o If the neighbor acceptsn It responses with a keepalive messagen The relationship then has been established
between the two router
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Figure 13-53
Open Message
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Open Message Packet Formato Version: 1-byte
n Define the version of BGP. The current version is 4o My autonomous system: 2-byte
n Define the autonomous system numbero Hold time: 2-byte
n Define the maximum number of seconds that can elapsed before one of the parties receives a keepalive or update message from the other
n If a router does not receive one of the messages during the hold period, it considers the other party dead
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Open Message Packet Format (Cont.)o BGP number: 4-byten Define the router that sends the open message
o Option parameter length: 1-byten Define the length of the total option parameters
o Since open message may also contain some option parameters
o Option parametersn The only option parameters defined so far is
authentication
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Update Messageo Used by a router to n Withdraw destination that have advertised
previouslyn Announce a route to a new destination
o Note, a router can withdraw several destinations in a single update messagen However, it can only advertise one new
destination in a single update message
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Update Message Formato Unfeasible routes length: 2-byte
n Define the length of the next fieldo Withdraw routes
n List all routes that should be deleted from the previously advertised list
o Path attributes length: 2-byten Define the length of the next field
o Path attributes: n Defines the attributes of the path (route) to the network
whose reachability is being announced in this msssage
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Figure 13-54
Update Message
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Update Message Formato Network Layer reachability information
(NLRI)n Define the network that is actually advertised by
this messagen Has two fields
o Length: define the number of bits in the prefixo IP address prefix:o Thus, BGP4 supports classless addressing and CIDR
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BGP supports classless BGP supports classless addressing and CIDR.addressing and CIDR.
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Keepalive Messageo The BGP routers exchange keepalive message
regularlyn Tell each other that they are alive
o Formatn Consist of only the common header
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Figure 13-55
Keepalive Message
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Notification Messageo Sent by a router
n Whenever an error condition is detectedn A router wants to close the connection
o Formatn Error code: 1-byte, define the category of the error
o Message header erroro Open message erroro Update message erroro Hold timer expiredo Finite state machine erroro Cease
n Error subcode: 1-byteo Furthermore define the type of error in each category
n Error datao Used to give more diagnostic information about the error
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Figure 13-56
Notification Message
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Encapsulationo BGP message are encapsulated in TCP
segments using the well-known port 179n No need for error control and flow control
o Thus, when a TCP connection is openedn The exchange of update, keepalive, and
notification message is continued n Until a notification message of type cease is sent
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BGP uses the BGP uses the services of TCP on port 179.services of TCP on port 179.
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