OSPF Key Concepts Part II/II

For configuring basic OSPF:

 Router(config)#router ospf<Process ID>
 Router(config)#router ospf 2
 Router(config-router)#network <Subnet ID/Network ID> <Wildcard Mask> Area<#>
 Router(config-router)#network 192.168.10.4 0.0.0.3 Area 0
 Router(config-router)#network 130.50.10.64 0.0.0.31 Area 0
 Router(config)#interface loopback0
 Router(config-if)#ip ospf network point-to-point
 (P2P, instead of w.x.y.z/32 you will see w.x.y.z/proper mask)

Useful show commands for OSPF:

Router#show ip ospf neighbor
Router#show ip ospf interface
Router#show ip route ospf

For configuring OSPF to broadcast itself as the default-gateway:

Router(config)#ip route 0.0.0.0 0.0.0.0 serial 0/0
Router(config)#router ospf <Process ID> 
Router(config)#router ospf 2 
Router(config-router)#default-information originate always 

By default, OSPF Metric is Cost (Cost = 100 Mbps/BW).  The reference bandwidth for OSPF Metric, Cost, can be changed by the following command.

Router(config-router)# auto-cost reference-bandwidth 1000 
(where the reference bandwidth is defined as 1Gbps)

The cost can be manually assigned under interface configuration mode, as follows.

Router(config-if)#ip ospf cost 27, where 27 is the cost

The following command shows the Link State Advertisement (LSA) age (in seconds) and sequence number of each router in the LSDB.

Router#show ip ospf database

OSPF LSA Types

Stubby Area: Blocks type 5 LSAs from entering

R1(config)#router ospf 1
R1(config-router)#area 1 stub

Totally Stubby Area: Blocks type 3, 4, and 5 LSAs from entering

R2(config)#router ospf 1 
R2(config-router)#area 1 stub no-summary

Not-So-Stubby Area: Passes external routes through via type 7 LSAs; these convert back to type 5 LSASsonce they reach the backbone.  It denies type 3, 4, and 5 LSAs from entering.

R1(config)#router ospf 1
 R1(config-router)#no area 1 stub
 R1(config-router)#area 1 nssa
R2(config)#router ospf 1
 R2(config-router)#no area 1 stub no-summary
 R2(config-router)#no area 1 stub
 R2(config-router)#area 1 nssa no-summary

OSPF Packets

OSPF uses several different types of packets to establish and maintain neighbour relationships, and to maintain correct routing information.

1. Helloidentifies neighbours and serves as a keepalive.  The following parameters within the OSPF Hellos must match for two routers to become neighbours
   • Hello/Dead Timers
     1. Default 10sec/40sec for LAN and point-to-point interfaces
     2. Default 30sec/120sec for NBMA interface
   • Area ID
   • Common Subnet
   • Authentication type and passwor
   • Stub Area Flag
2. Link State Request (LSR) requests a Link State Update (LSU).  It contains the type of LSU requested and the ID of router requesting it.
3. Database Description (DBD) consists of the summary of the LSDB, including the RID and  sequence number of each LSA in the LSDB
4. Link State Update (LSU) contains a full Link State Advertisement (LSA) entry.  An LSA includes topology information; for example, the RID of this router and RID and cost to each neighbour.  One LSU can contain multiple LSAs.
5. Link State Acknowledgment (LSAck) acknowledges all other OSPF packets (except Hellos and LSAcks)

Establishing Neighbours and Exchanging Routes

1. Down state – OSPF process has not yet started, so no Hellos sent
2. Init state – Router sends Hello packets out of all  OSPF interfaces
3. Two-way state B – Router receives a Hello from another   router that contains its own router ID in the neighbour list.  All other required elements match, so      routers can become neighbours
4. Exstart state – If routers become adjacent (exchange routes), they determine which one starts the exchange process
5. Exchange state – Routers exchange DBDs listing the LSAs in their LSD by RID and  sequence number
6. Loading state – Each router compares the DBD received      to the contents of its LS database.  It then sends a LSR for missing or outdated LSAs.  Each router responds to its neighbor’s  LSR with a Link State Update.  Each  LSU is acknowledged.
7. Full state – The LSDB has been synchronized with the  adjacent neighbour.