OSPF for IPv4 Notes

Uses of the Router ID: hello packets, neighbor routers use opsf rids to ID each other, many LSAs list the RID of the router

Steps for choosing the RID

  1. Use the value in the router-rid ospf subcommand
  2. Chose the highest value used on a sub interface (IP address)
  3. Chose the highest non loop back interface IP address

If no RID value can be determined then no OSPF routes will work

LSDB update by maintaining neighbor states by sending hello messages flood any changes to each neighbor and reflood unchanged LSAs as their lifetime expires.

Dedicated routers are the device that updates all other OSPF routers

LSA types

Router LSA type1 are descriptions of the routers in an ospf area and contain rid ip addresses subnet masks and current interface status

Network type 2 LSA describes a network that has a DR and BDR, ip subnet id and subnet mask.

Summary Type 3 LSA describes a subnet in an other area contains ip address, rid of the ABR (area boarder router) and advertises the LSAs.

Key Terms

OSPF RID, LSDB, DR, BDR, neighbor states, 2-way, full, OSPF areas, LSA types, ABR


OSPF is a link state routing protocol that uses LSDBs to store information on routes. OSPF has neighbor that exchange information. There are three types of lsa messages determine the ospf area and communicate network changes.

OSPF Configuration Overview

  1. Enter the Router OSPF “Process-ID” in configuration mode
  2. Configure Router ID (optional)
    1. Use router-id cmd
    2. configure a loopback with the highest IP value of all loopbacks
  3. Configure one or more network ip addresses and masks with area cmds
  4. configure any interface as passive if a neighbor does not exist. (optional)

To configure an ABR router assign multiple areas to its interfaces converting the OSPF router to an ABR.


router ospf area
network network ID
router-id ip to id the router
interface loopback

OSPF configuration takes only a few commands. IT can be a single or multi area config.

Verifying a Multi-area Configuration

List OSPF interfaces and area numbers with the following commands

Show ip ospf interface brief

Commands to show DR and BDR

show ip ospf interface brief
show ip ospf interface interface
show ip ospf neighbor

Verify the number and types of LSAs

show ip ospf database

Show which routes use ospf

show ip route

configure cost and bandwidth on interfaces with faster than 100 mbps interfaces to ensure the fastest path is taken

Speed config

ip ospf cost cost
bandwidth speed
auto-cost reference-bandwidth

ospf load balancing

maximum-path 6 under router ospf config

OSPF Neighbor Troubleshooting

Requirements Verify Commands
Must be in the same subnet show interfaces, debug ip ospf hello
Must pass any neighbor authentication show ip ospf interface, debug ip ospf ads
Hello and Dead timers must match show ip ospf interface, debug ip ospf hello
Must be in the same area show ip ospf interface brief, debug ip ospf ads
RIDS must be unique show ip ospf
Mismatched MTU Show run, show ip interfaces

Mismatch network types are caused when one opsf router thinks the link is a PPP link and the other believes it is a multi point link.

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