Routing Protocols

© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
ITE PC v4.0
Chapter 1 1
Link-State Routing
Protocols
Routing Protocols and Concepts – Chapter 10
Modified by Tony Chen
04/01/2008

ITE PC v4.0
Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Notes:
 If you see any mistake on my PowerPoint slides or if
you have any questions about the materials, please
feel free to email me at chento@cod.edu.
Thanks!
Tony Chen
College of DuPage
Cisco Networking Academy

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Objectives
 Describe the basic features & concepts of link-state
routing protocols.
– Distance vector routing protocols are like road signs
because routers must make preferred path decisions based
on a distance or metric to a network.
– Link-state routing protocols are more like a road map
because they create a topological map of the network and
each router uses this map to determine the shortest path to
each network.
– The ultimate objective is that every router receives all of the
link-state information about all other routers in the routing
area. With this link-state information, each router can create
its own topological map of the network and independently
calculate the shortest path to every network.
 List the benefits and requirements of link-state routing
protocols.

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Link-State Routing
 Link state routing protocols
-Also known as shortest path first algorithms
-These protocols built around Dijkstra’s SPF
OSPF will be discussed in Chapter 11, and IS-IS will be discussed in CCNP.

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Link-State Routing
 Dikjstra’s algorithm also known as the shortest path first
(SPF) algorithm
–This algorithm accumulates costs along each path, from
source to destination.

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Link-State Routing
 The shortest path to a destination is not necessarily the
path with the least number of hops

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Link-State Routing Process
 How routers using Link State Routing Protocols reach convergence
1. Each routers learns about its own directly connected networks
– interface is in the up state
1. Each router is responsible for meeting its neighbors on directly
connected networks
– exchange hello packet to other directly connected link state routers.
1. Each router builds a Link-State Packet (LSP) containing the state of
each directly connected link
– recording all the pertinent information about each neighbor, including
neighbor ID, link type, and bandwidth.
1. Each router floods the LSP to all neighbors, who then store all LSPs
received in a database.
– Each router stores a copy of each LSP received from its neighbors in
a local database.
1. Each router uses the database to construct a complete map of the
topology and computes the best path to each destination network.
– The SPF algorithm is used to construct the map of the topology and
to determine the best path to each network.

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Link-State Routing:
Step 1 – Learn about directly connected Networks
 Link
This is an interface on a
router
 Link state
This is the information
about the state of the links

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Link-State Routing:
step 2 - Sending Hello Packets to Neighbors
 Link state routing protocols use a hello protocol
Purpose of a hello protocol:
-To discover neighbors (that use the same
link state routing protocol) on its link

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 Connected interfaces that are
using the same link state
routing protocols will exchange
hello packets.
 Once routers learn it has
neighbors they form an
adjacency
– 2 adjacent neighbors will
exchange hello packets
– These packets will serve as a
keep alive function
Link-State Routing:
step 2 - Sending Hello Packets to Neighbors

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 Contents of LSP:
– State of each directly connected link
– Includes information about
neighbors such as neighbor ID, link
type, & bandwidth.
 A simplified version of the LSPs from
R1 is:
1. R1; Ethernet network 10.1.0.0/16;
Cost 2
2. R1 -> R2; Serial point-to-point
network; 10.2.0.0/16; Cost 20
network; 10.3.0.0/16; Cost 5
network; 10.4.0.0/16; Cost 20
Link-State Routing:
step 3 - Building the Link State Packet (LSP)

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 Once LSP are created they are
forwarded out to neighbors.
–Each router floods its link-state
information to all other link-state
routers in the routing area.
–Whenever a router receives an LSP
from a neighboring router, it
immediately sends that LSP out all
other interfaces except the interface
that received the LSP.
–This process creates a flooding
effect of LSPs from all routers
throughout the routing area.
Link-State Routing:
step 4 - Flooding LSPs to Neighbors

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 LSPs are sent out under the following conditions
– Initial router start up or routing process
– When there is a change in topology
• including a link going down or coming up, or a neighbor
adjacency being established or broken
Link-State Routing:
step 4 - Flooding LSPs to Neighbors

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 Routers use a database to
construct a topology map of the
network
–After each router has propagated
its own LSPs using the link-state
flooding process, each router will
then have an LSP from every link-
state router in the routing area.
–These LSPs are stored in the link-
state database.
–Each router in the routing area can
now use the SPF algorithm to
construct the SPF trees that you saw
earlier.
Link-State Routing:
step 5 - Constructing a link state data base

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Link-State Routing:
step 5 - Constructing a link state data base
router R1 has learned the link-state
information for each router in its
routing area.
With a complete link-state database, R1
can now use the database and the
shortest path first (SPF) algorithm to
calculate the preferred path or shortest
path to each network.

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 Process begins by examining R2’s LSP information
–R1 can ignore the first LSP, because R1 already knows that it is
connected to R2 on network 10.2.0.0/16 with a cost of 20.
–R1 can use the second LSP and create a link from R2 to another
router, R5, with the network 10.9.0.0/16 and a cost of 10. This
information is added to the SPF tree.
–Using the third LSP, R1 has learned that R2 has a network
10.5.0.0/16 with a cost of 2 and with no neighbors. This link is
added to R1's SPF tree.
Link-State Routing:
Example - How R1 constructs its SPF tree.

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–R1 can ignore the first LSP, because R1 already knows that it is
–R1 can use the second LSP and create a link from R3 to the
router R4, with the network 10.7.0.0/16 and a cost of 10. This
information is added to the SPF tree.
–Using the third LSP, R1 has learned that R3 has a network
added to R1's SPF tree.
Link-State Routing:

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–R1 can ignore the first LSP because R1 already knows that it is
–R1 can also ignore the second LSP because SPF has already learned
about the network 10.6.0.0/16 with a cost of 10 from R3.
–However, R1 can use the third LSP to create a link from R4 to the router
R5, with the network 10.10.0.0/16 and a cost of 10. This information is
added to the SPF tree.
–Using the fourth LSP, R1 learns that R4 has a network 10.8.0.0/16 with a
cost of 2 and with no neighbors. This link is added to R1's SPF tree.
Link-State Routing:

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–R1 can ignore the first two LSPs (for the networks 10.9.0.0/16 and
10.10.0.0/16), because SPF has already learned about these links
and added them to the SPF tree.
–R1 can process the third LSP learning that R5 has a network
added to the SPF tree for R1.
Link-State Routing:

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Link-State Routing
 Determining the shortest path
–The shortest path to a destination
determined by adding the costs & finding the
lowest cost
•Network 10.5.0.0/16 via R2 serial 0/0/0
at a cost of 22
at a cost of 7
at a cost of 15
at a cost of 17
at a cost of 30
at a cost of 25
at a cost of 27
Only the LANs are shown in
the table, but SPF can also be
used to determine the shortest
path to each WAN link
network.

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Link-State Routing
 Once the SPF algorithm has
determined the shortest path
routes, these routes are placed in
the routing table.
 The routing table will also include
all directly connected networks
and routes from any other
sources, such as static routes.
Packets will now be forwarded
according to these entries in the
routing table.

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Link-State Routing Protocols
Advantages of a Link-State Routing Protocol
Routing
protocol
Builds
Topological
map
Router can
independently
determine the
shortest path
to every
network.
Convergence
Event driven
routing
updates
Use
of
LSP
Distance
vector
No No Slow Generally No No
Link State Yes Yes Fast Generally Yes Yes

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 There are several advantages of link-state routing protocols compared to distance vector
routing protocols.
 Builds a Topological Map
• Link-state routing protocols create a topological map, or SPF tree of the network topology.
•Using the SPF tree, each router can independently determine the shortest path to every network.
• Distance vector routing protocols do not have a topological map of the network.
•Routers implementing a distance vector routing protocol only have a list of networks, which includes
the cost (distance) and next-hop routers (direction) to those networks.
 Fast Convergence
• When receiving a Link-state Packet (LSP), link-state routing protocols immediately flood the LSP out all
interfaces except for the interface from which the LSP was received.
• A router using a distance vector routing protocol needs to process each routing update and update its
routing table before flooding them out other interfaces, even with triggered updates.
 Event-driven Updates
• After the initial flooding of LSPs, link-state routing protocols only send out an LSP when there is a change
in the topology. The LSP contains only the information regarding the affected link.
• Unlike some distance vector routing protocols, link-state routing protocols do not send periodic updates.
 Hierarchical Design
• Link-state routing protocols such as OSPF and IS-IS use the concept of areas. Multiple areas create a
hierarchical design to networks, allowing for better route aggregation (summarization) and the isolation of
routing issues within an area.

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Requirements for using a link state routing protocol
 Memory requirements
– Typically link state routing protocols use more memory
 Processing Requirements
– More CPU processing is required of link state routing
protocols
 Bandwidth Requirements
– Initial startup of link state routing protocols can consume lots
of bandwidth
– This should only occur during initial startup of routers, but can
also be an issue on unstable networks.

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 Modern link-state routing protocols are designed to
minimize the effects on memory, CPU, and
bandwidth.
• The use and configuration of multiple areas can reduce
the size of the link-state databases. Multiple areas can
also limit the amount of link-state information flooding
in a routing domain and send LSPs only to those
routers that need them.
• For example, when there is a change in the topology,
only those routers in the affected area receive the LSP
and run the SPF algorithm.
• This can help isolate an unstable link to a specific area
in the routing domain.
 In the figure, If a network in Area 51 goes down, the
LSP with the information about this downed link is
only flooded to other routers in that area.
• Routers in other areas will learn that this route is down,
but this will be done with a type of link-state packet that
does not cause them to rerun their SPF algorithm.
Note: Multiple areas
with OSPF and IS-IS
are discussed in
CCNP

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 2 link state routing protocols used for routing IP
-Open Shortest Path First (OSPF)
-Intermediate System-Intermediate System (IS-IS)

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Summary
 Link State Routing protocols are also known as
Shortest Path First protocols
 Summarizing the link state process
-Routers 1ST learn of directly connected networks
-Routers then say “hello” to neighbors
-Routers then build link state packets
-Routers then flood LSPs to all neighbors
-Routers use LSP database to build a network topology
map & calculate the best path to each destination

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Summary
 Link
An interface on the router
 Link State
Information about an interface such as
-IP address
-Subnet mask
-Type of network
-Cost associated with link
-Neighboring routers on the link

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Summary
 Link State Packets
After initial flooding, additional LSP are sent out
when a change in topology occurs
 Examples of link state routing protocols
-Open shortest path first
-IS-IS

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