1. Link State Routing Protocol W.lilakiatsakun

2. Introduction (1) Link-state routing protocols are also known as shortest path first protocols and built around Edsger Dijkstra's shortest path first (SPF) algorithm. Link-state routing protocols are also known as shortest path first protocols and built around Edsger Dijkstra's shortest path first (SPF) algorithm.

3. Introduction (2) Dijkstra's algorithm is commonly referred to as the shortest path first (SPF) algorithm. Dijkstra's algorithm is commonly referred to as the shortest path first (SPF) algorithm. This algorithm accumulates costs along each path, from source to destination. This algorithm accumulates costs along each path, from source to destination.

4. Introduction (3)

5. Introduction (4) R1 ‘s RouteR5 ‘s Route

6. Link state Routing Process (1) 1. Each router learns about its own links, its own directly connected networks. 1. Each router learns about its own links, its own directly connected networks. –This is done by detecting that an interface is in the up state. 2. Each router is responsible for meeting its neighbors on directly connected networks. 2. Each router is responsible for meeting its neighbors on directly connected networks. –Similar to EIGRP, link state routers do this by exchanging Hello packets with other link-state routers on directly connected networks. 3. Each router builds a Link-State Packet (LSP) containing the state of each directly connected link. 3. Each router builds a Link-State Packet (LSP) containing the state of each directly connected link. –This is done by recording all the pertinent information about each neighbor, including neighbor ID, link type, and bandwidth.

7. Link state Routing Process (2) 4. Each router floods the LSP to all neighbors, who then store all LSPs received in a database. 4. Each router floods the LSP to all neighbors, who then store all LSPs received in a database. –Neighbors then flood the LSPs to their neighbors until all routers in the area have received the LSPs. –Each router stores a copy of each LSP received from its neighbors in a local database. 5. Each router uses the database to construct a complete map of the topology and computes the best path to each destination network. 5. Each router uses the database to construct a complete map of the topology and computes the best path to each destination network. –Like having a road map, the router now has a complete map of all destinations in the topology and the routes to reach them. –The SPF algorithm is used to construct the map of the topology and to determine the best path to each network.

8. Learning about directly connected network (1) When you correctly configure and activate the interfaces, the router learns about its own directly connected networks. Regardless of the routing protocols used, these directly connected networks are now part of the routing table.

9. Learning about directly connected network (2)

10. Learning about directly connected network (3) Link-State: Information about the state of those links. Link-State: Information about the state of those links. This information includes: This information includes: –The interface's IP address and subnet mask. –The type of network, such as Ethernet (broadcast) or Serial point-to-point link. –The cost of that link. –Any neighbor routers on that link.

11. Sending Hello Packet to neighbors (1) R1 sends Hello packets out its links (interfaces) to discover if there are any neighbors

12. Sending Hello Packet to neighbors (2) R2, R3, and R4 reply to the Hello packet with their own Hello packets because these routers are configured with the same link-state routing protocol. There are no neighbors out the FastEthernet 0/0 interface. Because R1 does not receive a Hello on this interface, it will not continue with the link-state routing process steps for the FastEthernet 0/0 link.

13. Sending Hello Packet to neighbors (3) When two link-state routers learn that they are neighbors, they form an adjacency. When two link-state routers learn that they are neighbors, they form an adjacency. These small Hello packets continue to be exchanged between two adjacent neighbors which serve as a "keepalive" function to monitor the state of the neighbor. These small Hello packets continue to be exchanged between two adjacent neighbors which serve as a "keepalive" function to monitor the state of the neighbor. If a router stops receiving Hello packets from a neighbor, that neighbor is considered unreachable and the adjacency is broken. If a router stops receiving Hello packets from a neighbor, that neighbor is considered unreachable and the adjacency is broken. In the figure, R1 forms an adjacency with all three routers. In the figure, R1 forms an adjacency with all three routers.

14. Building the link state packet (1) Once a router has established its adjacencies, it can build its link-state packets (LSPs) that contain the link- state information about its links. Once a router has established its adjacencies, it can build its link-state packets (LSPs) that contain the link- state information about its links. A simplified version of the LSPs from R1 is: 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 –3. R1 -> R3; Serial point-to-point network; 10.3.0.0/16; Cost 5 –4. R1 -> R4; Serial point-to-point network; 10.4.0.0/16; Cost 20

15. Building the link state packet (2)

16. Flooding Link state Packet to neighbors (1) Each router floods its link-state information to all other link-state routers in the routing area. 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. 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. This process creates a flooding effect of LSPs from all routers throughout the routing area.

17. Flooding Link state Packet to neighbors (2)

18. Flooding Link state Packet to neighbors (3)

19. Flooding Link state Packet to neighbors (4) LSPs are flooded almost immediately after being received, without any intermediate calculations. LSPs are flooded almost immediately after being received, without any intermediate calculations. Unlike distance vector routing protocols that must first run the Bellman-Ford algorithm to process routing updates before sending them to other routers, link-state routing protocols calculate the SPF algorithm after the flooding is complete. Unlike distance vector routing protocols that must first run the Bellman-Ford algorithm to process routing updates before sending them to other routers, link-state routing protocols calculate the SPF algorithm after the flooding is complete. As a result, link-state routing protocols reach convergence much faster than distance vector routing protocols. As a result, link-state routing protocols reach convergence much faster than distance vector routing protocols.

20. Flooding Link state Packet to neighbors (5) Remember that LSPs do not need to be sent periodically. Remember that LSPs do not need to be sent periodically. An LSP only needs to be sent: An LSP only needs to be sent: –During initial startup of the router or of the routing protocol process on that router –Whenever there is a change in the topology, including a link going down or coming up, or a neighbor adjacency being established or broken

21. Constructing a Link-state Database (1) R1 has learned the link-state information for each router in its routing area. R1 also includes its own link-state information in the link-state database.

22. Constructing a Link-state Database (2) 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.

23. Shortest Path First (SPF) Tree (1) Determining the Shortest Path Determining the Shortest Path Because all LSPs have been processed using the SPF algorithm, R1 has now constructed the complete SPF tree. Because all LSPs have been processed using the SPF algorithm, R1 has now constructed the complete SPF tree. The 10.4.0.0/16 and 10.9.0.0/16 links are not used to reach other networks, because lower- cost or shorter paths exist. The 10.4.0.0/16 and 10.9.0.0/16 links are not used to reach other networks, because lower- cost or shorter paths exist. However these networks still exist as part of the SPF tree and are used to reach devices on those networks. However these networks still exist as part of the SPF tree and are used to reach devices on those networks.

24. Shortest Path First (SPF) Tree (2) R1 determines that the shortest path for each network is: R1 determines that the shortest path for each network is: –Network 10.5.0.0/16 via R2 serial 0/0/0 at a cost of 22 –Network 10.6.0.0/16 via R3 serial 0/0/1 at a cost of 7 –Network 10.7.0.0/16 via R3 serial 0/0/1 at a cost of 15 –Network 10.8.0.0/16 via R3 serial 0/0/1 at a cost of 17 –Network 10.9.0.0/16 via R2 serial 0/0/0 at a cost of 30 –Network 10.10.0.0/16 via R3 serial 0/0/1 at a cost of 25 –Network 10.11.0.0/16 via R3 serial 0/0/1 at a cost of 27

25. Shortest Path First (SPF) Tree (3)

26. Shortest Path First (SPF) Tree (4)

27. Advantages of Link state Protocol (1) Builds a Topological Map Builds a Topological Map –Link-state routing protocols create a topological map, or SPF tree of the network topology. –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. –Because link-state routing protocols exchange link- states, the SPF algorithm can build an SPF tree of the network. –Using the SPF tree, each router can independently determine the shortest path to every network.

28. Advantages of Link state Protocol (2) Fast Convergence 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. –Faster convergence is achieved for link-state routing protocols. – A notable exception is EIGRP.

29. Advantages of Link state Protocol (3) Event-driven 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. Note: OSPF routers do flood the own link-states every 30 minutes. This is known as a paranoid update. Also, not all distance vector routing protocols send periodic updates. RIP and IGRP send periodic updates; however, EIGRP does not. Note: OSPF routers do flood the own link-states every 30 minutes. This is known as a paranoid update. Also, not all distance vector routing protocols send periodic updates. RIP and IGRP send periodic updates; however, EIGRP does not.

30. Advantages of Link state Protocol (4) Hierarchical Design 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.

31. Requirements of a link state protocol (1) Modern link-state routing protocols are designed to minimize the effects on memory, CPU, and bandwidth. 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. 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. 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.

32. Requirements of a link state protocol (2)

33. Requirements of a link state protocol (3) When there is a change in the topology, only those routers in the affected area receive the LSP and run the SPF algorithm. 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. This can help isolate an unstable link to a specific area in the routing domain. In the figure, there are three separate routing domains: Area 1, Area 0, and Area 51. In the figure, there are three separate routing domains: Area 1, Area 0, and Area 51. –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. –Only routers in Area 51 will need to update their link-state databases, rerun the SPF algorithm, create a new SPF tree, and update their routing tables. –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. – Routers in other areas can update their routing tables directly.

34. Requirements of a link state protocol (4) Memory Requirements Memory Requirements –Link-state routing protocols typically require more memory, more CPU processing, and at times more bandwidth than distance vector routing protocols. –The memory requirements are due to the use of link-state databases and the creation of the SPF tree. Processing Requirements Processing Requirements –Link-state protocols can also require more CPU processing than distance vector routing protocols. Bandwidth Requirements Bandwidth Requirements –The flooding of link-state packets can adversely affect the available bandwidth on a network. –This should only occur during initial startup of routers, but can also be an issue on unstable networks.

35. Requirements of a link state protocol (5) There are two link-state routing protocols used for routing IP today: There are two link-state routing protocols used for routing IP today: –Open Shortest Path First (OSPF) –Intermediate System-to-Intermediate System (IS-IS) OSPF OSPF OSPF was designed by the IETF (Internet Engineering Task Force) OSPF Working Group, which still exists today. OSPF was designed by the IETF (Internet Engineering Task Force) OSPF Working Group, which still exists today. The development of OSPF began in 1987 and there are two current versions in use: The development of OSPF began in 1987 and there are two current versions in use: –OSPFv2: OSPF for IPv4 networks (RFC 1247 and RFC 2328) –OSPFv3: OSPF for IPv6 networks (RFC 2740) Most of the work on OSPF was done by John Moy, author of most of the RFCs regarding OSPF. Most of the work on OSPF was done by John Moy, author of most of the RFCs regarding OSPF.

36. Requirements of a link state protocol (6) IS-IS IS-IS IS-IS was designed by ISO (International Organization for Standardization) and is described in ISO 10589. IS-IS was designed by ISO (International Organization for Standardization) and is described in ISO 10589. –The first incarnation of this routing protocol was developed at DEC (Digital Equipment Corporation) and is known as DECnet Phase V. –Radia Perlman was the chief designer of the IS-IS routing protocol. IS-IS was originally designed for the OSI protocol suite and not the TCP/IP protocol suite. IS-IS was originally designed for the OSI protocol suite and not the TCP/IP protocol suite. –Later, Integrated IS-IS, or Dual IS-IS, included support for IP networks. –Although IS-IS has been known as the routing protocol used mainly by ISPs and carriers, more enterprise networks are beginning to use IS-IS.