1 © 2003, Cisco Systems, Inc. All rights reserved. CCNA 3 v3.0 Module 2 Single-Area OSPF

222 © 2003, Cisco Systems, Inc. All rights reserved. Objectives Link-state routing protocol Single-area OSPF concepts Single-area OSPF configuration

333 © 2003, Cisco Systems, Inc. All rights reserved. Overview of Link-State and Distance Vector Routing

444 © 2003, Cisco Systems, Inc. All rights reserved. Link-State Routing Features Link-state routing protocols collect route information from all other routers in the network or within a defined area of the network and calculates the best paths to all destinations in the network. Respond quickly to network changes Send triggered updates only when a network change has occurred Send periodic updates known as link-state refreshes Use a hello mechanism to determine the reachability of neighbors Features: Use the hello information and LSAs received from other routers to build a database about the network Use the SPF algorithm to calculate the shortest route to each network Store the route information in the routing table

555 © 2003, Cisco Systems, Inc. All rights reserved. How Routing Information Is Maintained A link is the same as an interface on a router. The state of the link is a description of an interface and the relationship to the neighbor routers. For example, a description of the interface would include the IP address of the interface, the subnet mask, the type of network that it is connected to, the routers connected to that network, and so on. The collection of link-states form a link-state database which is sometimes called a topological database. The link-state database is used to calculate the best paths through the network. Link-state routers apply the Dijkstra shortest path first algorithm against the link-state database. This builds the SPF tree with the local router as the root. The best paths are then selected from the SPF tree and placed in the routing table.

666 © 2003, Cisco Systems, Inc. All rights reserved. Link-State Routing Protocol Algorithms LSA exchange is triggered by an event in the network instead of periodic updates. This speeds up the convergence process. A B C D

777 © 2003, Cisco Systems, Inc. All rights reserved. Advantages and Disadvantages of Link- State Routing

888 © 2003, Cisco Systems, Inc. All rights reserved. Comparing Distance Vector and Link-State Routing

999 © 2003, Cisco Systems, Inc. All rights reserved. OSPF Overview OSPF is becoming the preferred IGP protocol when compared with RIPv1 and RIPv2 because it is scalable.

10 © 2003, Cisco Systems, Inc. All rights reserved. OSPF Terminology

11 © 2003, Cisco Systems, Inc. All rights reserved. More OSPF Terminology

12 © 2003, Cisco Systems, Inc. All rights reserved. Comparing OSPF Link State with Distance Vector Routing Protocols

13 © 2003, Cisco Systems, Inc. All rights reserved. Shortest Path Algorithm The best path is the lowest-cost path.

14 © 2003, Cisco Systems, Inc. All rights reserved. OSPF Network Types

15 © 2003, Cisco Systems, Inc. All rights reserved. OSPF Hello Protocol Hello packets are addressed to multicast address to all OSPF routers, sent every 10 seconds by default. = 1

16 © 2003, Cisco Systems, Inc. All rights reserved. Steps in the Operation of OSPF Discover neighbors

17 © 2003, Cisco Systems, Inc. All rights reserved. Steps in the Operation of OSPF Elect DR and BDR on Multi Access Network On multi-access networks, the Hello protocol elects a designated router (DR) and a backup designated router (BDR). The Hello carries information about which all neighbors must agree to form an adjacency and exchange link-state information. On multi-access networks the DR and BDR maintain adjacencies with all other OSPF routers on the network.

18 © 2003, Cisco Systems, Inc. All rights reserved. Steps in the Operation of OSPF Selecting the Best Route

19 © 2003, Cisco Systems, Inc. All rights reserved. Basic OSPF Configuration Process-id: a number used to identify an OSPF process on the router Network addresses are configured with a wildcard mask. Wildcard mask represents the links or host addresses that can be present in this segment Area 0 is called backbone area

20 © 2003, Cisco Systems, Inc. All rights reserved. Basic OSPF Configuration

21 © 2003, Cisco Systems, Inc. All rights reserved. Configuring OSPF Loopback Address and Router Priority When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF router ID. If there is no active interface, the OSPF process will not start. If the active interface goes down, the OSPF process has no router ID and therefore ceases to function until the interface comes up again. To ensure OSPF stability there should be an active interface for the OSPF process at all times. A loopback interface, which is a logical interface, can be configured for this purpose. When a loopback interface is configured, OSPF uses this address as the router ID, regardless of the value. On a router that has more than one loopback interface, OSPF takes the highest loopback IP address as its router ID.

22 © 2003, Cisco Systems, Inc. All rights reserved. Setting OSPF Priority The priorities can be set to any value from 0 to 255. A value of 0 prevents that router from being elected. A router with the highest OSPF priority will win the election for DR. The default OSPF priority is 1.

23 © 2003, Cisco Systems, Inc. All rights reserved. Modifying OSPF Cost Metric

24 © 2003, Cisco Systems, Inc. All rights reserved. Configuring OSPF Authentication

25 © 2003, Cisco Systems, Inc. All rights reserved. Configuring OSPF Timers

26 © 2003, Cisco Systems, Inc. All rights reserved. OSPF - Propagating a Default Route Router(config)#ip route [interface | next- hop address ] Router(config-router)#default-information originate This command will propagate the route to all routers in a normal OSPF area.

27 © 2003, Cisco Systems, Inc. All rights reserved. Common OSPF Configuration Issues

28 © 2003, Cisco Systems, Inc. All rights reserved. Verifying OSPF Configuration show ip protocol show ip route show ip ospf interface shop ip ospf show ip ospf neighbor detail show ip ospf database

29 © 2003, Cisco Systems, Inc. All rights reserved. The debug and clear Commands for OSPF Verification