Chapter_10_Link-State Routing Protocols

Link-State Routing Protocols
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Routing Protocols and Concepts – Chapter 10
Objectives
Describe the basic features & concepts of link-state routing protocols.
List the benefits and requirements of link-state routing protocols.
Introduction
Link-State Routing
Link state routing protocols
Also known as shortest path first algorithms
These protocols built around Dijkstra’s SPF
Link-State Routing
Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm
Link-State Routing
The shortest path to a destination is not necessarily the path with the least number of hops
Link-State Routing
Link-State Routing Process
How routers using Link State Routing Protocols reach convergence
Each routers learns about its own directly connected networks
Link state routers exchange hello packet to “meet” other directly
Connected link state routers
Each router builds its own Link State Packet (LSP) which includes information about neighbors such as neighbor ID, link type, & bandwidth
After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information
Once all the routers have received all the LSPs, the routers then construct a topological map of the network which is used to determine the best routes to a destination
Link-State Routing
Directly Connected Networks
Link
This is an interface on a router
Link state
This is the information about the state of the links
Link-State Routing
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
Link-State Routing
Sending Hello Packets to Neighbors
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
Building the Link State Packet
Each router builds its own Link State Packet (LSP)
Contents of LSP:
State of each directly connected link
Includes information about neighbors such as neighbor ID, link type, & bandwidth
Link-State Routing
Flooding LSPs to Neighbors
Once LSP are created they are forwarded out to neighbors
After receiving the LSP the neighbor continues to forward it throughout routing area
Link-State Routing
LSPs are sent out under the following conditions:
Initial router start up or routing process
When there is a change in topology
Link-State Routing
Constructing a link state data base
Routers use a database to construct a topology map of the network
Link-State Routing
Link-State Routing
Shortest Path First (SPF) Tree
Building a portion of the SPF tree
Process begins by examining R2’s LSP information
R1 ignores 1st LSP
Reason: R1 already knows it’s connected to R2

Link-State Routing
Building a portion of the SPF tree
R1 uses 2nd LSP
Reason: R1 can create a link from R2 to R5 - this information is added to R1’s SPF tree

Link-State Routing
Building a portion of the SPF tree
R1 uses 3rd LSP
Reason: R1 learns that R2 is connected to 10.5.0.0/16
This link is added to R1’s SPF tree
Link-State Routing
Determining the shortest path
The shortest path to a destination determined by adding the costs & finding the lowest cost
Link-State Routing
Once the SPF algorithm has determined the shortest path routes, these routes are placed in the routing table
Link-State Routing Protocols
Advantages of a Link-State Routing Protocol
Link-State Routing Protocols
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
Link-State Routing Protocols
2 link state routing protocols used for routing IP
Open Shortest Path First (OSPF)
Intermediate System-Intermediate System (IS-IS)
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
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
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