Routing With a Link-State Protocol

1. Routing With a Link-State Protocol Introducing Routing and Switching in the Enterprise– Chapter 6

2. Objectives • Describe and plan a network using OSPF. • Design and configure a network using single-area OSPF. • Work with multi-protocol environments.

3. 6.1.1Describe/Plan a Network Using OSPF • Link-state routing protocol • Open standard developed by IETF • Classless interior gateway protocol • Divides the network into different sections called areas for scalability • All routers using Link-State database have the same status information

4. 6.1.1 LINK STATE ROUTING PROTOCOLS • Do not send frequent updates of the entire routing table • Sends updates only when a change in the topology occurs • Such as a link going down • Will send full update every 30 minutes • Works for larger networks because of faster convergence • When all routers have identical link-state databases, they have CONVERGED • Requires more network planning • Requires increased router resources, more memory, more CPU and processing power for complex routing calculations in beginning • After initial LSA flooding, requires LESS bandwidth • Link-State protocols INDEPENDENTLY calculate full routes • Distance vector protocols take existing routes from their neighbors and add to them • Routers have direct knowledge of all links in the network and how they are connected

5. 6.1.1 OSPF ADVANTAGE OVER RIP • Routers using RIP receive updates from immediate neighbors, but no details about the network as a whole • Routers using OSPF have a complete map of the network from their own viewpoint • Which allows them to determine loop-free alternate paths in case of network link failure • OSPF uses bandwidth to determine the cost of a link • Routers trust a metric based on bandwidth more than hop count • OSPF administrative distance is 110 (lower than RIP)

6. 6.1.2 OSPF Metrics and Convergence • Bandwidth determines the cost of a link • Faster links are considered to be a lower cost, so a T1 link will be preferred over a 56kbs • Higher bandwidth = lower cost = desirable route • Cost = 100,000,000 / bandwidth of link in bps • Manual cost configuration at an interface: ip ospf cost

7. 6.1.2 Describe and Plan a Network Using OSPF • SPF algorithm (Diijkstra’s Algorithm) • Used to calculate the shortest path for each destination • Each router has a topological tree • SPF tree information stored in topology database

8. 6.1.3 OSPF Neighbors and Adjacencies • Adjacencies: neighbor relationships is required to share routing information • Hello protocol • DR sends to other routers as multicast 224.0.0.5 • Router ID • Role of loopback interfaces

9. 6.1.3 OSPF Neighbors and Adjacencies • Designated router (DR) • Backup designated router (BDR) • DROther

10. 6.1.3 OSPF NEIGHBORS AND ADJACENNCIES • DR—used to pass updates • Routers only accept updated from the DR • When a link fails, info is sent by router with the information to the DR as a multicast 224.0.0.6 • DR distributes the changes to all other OSPF routers using multicast 224.0.0.5 • Everyone gets the information from the DR, so they all agree • BDR is the backup, which will take over if the DR goes down • All other routers are known ad DROther • The DR and BDR do not change until the next election...even if another router is added

11. 6.1.3 OSPF NEIGHBORS AND ADJACENNCIESHOW ARE DR and BDR DECIDED? • DR IS ROUTER WITH THE HIGHEST ROUTER ID • BDR IS ROUTER WITH THE SECOND HIGHEST ROUTER ID • ID is based on IP ADDRESS • Loopback addresses are also included in this election process • EXCEPTION!!!! • IF you want to FORCE a router to be the DR, you can set the OSPF priority • OSPF priority default is 1 • Highest Priority will win election OVER IP ADDRESS

12. 6.1.3 OSPF NEIGHBORS AND ADJACENCIES • Which would be the DR and which would be the BDR?? • R1 will be the DR • R3 will be the BDR

13. 6.1.3 OSPF NEIGHBORS AND ADJACENCIES SO, WHEN ARE DR AND BDR ELECTIONS NECESSARY? • Elections are required in Broadcast Multi-Access networks such as ETHERNET • Elections are SOMETIMES required in Non-Broadcast Multi-Access Networks (NBMA) • Elections are NOT required in Point-to-Point networks (PPP) • Such as FRAME RELAY • Point-to-multipoint environment,which requires neighboring routers to be statically defined.

14. 6.1.4 OSPF Areas • Area 0—backbone area • Roles of routers in an OSPF Autonomous System: • Area Border Router (ABR) • router that connects an area to the backbone area • Autonomous System Border Router (ASBR) • a router that connects to a different routing protocol or redistributes static routes into the OSPF area

15. 6.2.1 CONFIGURING OSPF (SINGLE AREA) No how to enter the network statements for the test OSPF configuration steps: • Enable OSPF + process ID • Advertise networks + wildcard mask and area ID

16. 6.2.1 CONFIGURING OSPF (SINGLE AREA)

17. What would the network statement be on Router 2 to advertise the 172.16.32.0 network to Router 1?? Router2(config-router)# network 172.16.32.0 0.0.15.255 6.2.1 CONFIGURING OSPF (SINGLE AREA)

18. 6.2.2 Design and Configure a Network Using Single-Area OSPF • Authentication protects integrity of routing information • Simple password authentication • Message Digest 5 (MD5)

19. 6.2.3 Tuning OSPF Parameters • Interface priority—to force election • Router ID • Loopback and interface addresses (to override IP addresses) • Bandwidth (cost )—modifies the bandwidth value used to calculate the OSPF cost metric. Command is ip ospf cost

20. 6.2.4 verifying OSPF Operation Verification and troubleshooting commands: • show ip ospf neighbor • show ip protocols, show ip route • show ip ospf, show ip ospf interface

21. 6.2.4 verifying OSPF Operation Two routers may not form an OSPF adjacency if: • The subnet masks do not match, causing the routers to be on separate networks • OSPF hello or dead timer intervals do not match • OSPF network types do not match • There is a missing or incorrect OSPF network command

22. 6.2.4 Verifying OSPF Operation • What does the 110 represent? Administrative distance of OSPF • What does the 1562 represent? OSPF cost metric

23. Work with Multi-Protocol Environments • Default route configured on ASBR • ASBR distributes the route into the OSPF network

24. Work with Multi-Protocol Environments Benefits of OSPF summarization: • Reduces number of networks advertised • Reduces memory requirements • Reduces number of entries in router updates • Isolates flapping and other problems to their location

25. Work with Multi-Protocol Environments OSPF limitations: • Increased router memory and processing demands • Strict design requirements • Knowledgable administrator required • Initial discovery process takes up network bandwidth

26. Work with Multi-Protocol Environments • Administrative distance (AD) determines routes that appear in routing table

27. Summary • OSPF is a classless interior link-state routing protocol used in enterprise networks • OSPF uses bandwidth to generate the cost metric • OSPF routers elect a DR and BDR on multi-access networks • Router ID or router priority can be used to dictate the selection of DR and BDR • The OSPF network command uses a wildcard mask • Default route distribution and inter-area route summarization are used in OSPF networks