Common OSPF issues and their cause

Troubleshooting OSPF Commands
-----------------------------------------
Show IP OSPF
Show IP OSPF Database
Show IP OSPF Database Database-Summary
show ip ospf neighbor
show ip ospf neighbor detail
show ip ospf interface
show ip ospf virtual-links
sh ip ospf stat
show ip ospf borders-routers
show ip ospf database self-originate
show ip ospf database adv-router x.x.x.x
Adjacency Is Not Coming Up
---------------------------------
Layer 2 is down
OSPF not enabled on the interface
Mismatched subnet mask
Mismatched authentication key
Mismatched area ID
Mismatched transit/stub/NSSA option
Useful Commands for This Problem:
Show IP OSPF neighbor
Show IP OSPF interface
Debug IP OSPF adjacency

Neighbor Stuck in ATTEMPT state
-----------------------------------------
Our hellos are getting lost in NBMA cloud
Neighbor hellos are getting lost in NBMA cloud
We received neighbor’s hello but rejects it for some reason
Misconfigured neighbor statement
Broken Unicast
Neighbor Stuck in INIT state
----------------------------------
One side is blocking the hello packet with access-list
One side is translating (NAT) OSPF hello
One side multicast capabilities is broken (Layer 2)
Dialer map or Frame Relay map is missing keyword ‘broadcast’
Neighbor Stuck in 2-WAY state
-------------------------------------
This is normal in broadcast network types
This is to reduce the amount of flooding on the wire
Problem can happen if all the router are configured with priority equal to ‘0’
Take care which routers are configured with priority 0 so they don’t participate in DR election
Neighbor Stuck in EXSTART/EXCHANGE
-------------------------------------------------
MTU mismatch—EXCHANGE
Note: If Cisco IOS is < 12.0.3 neighbor will show stuck in EXCHANGE
Neighbor RID is same as ours—EXSTART
Note: If Cisco IOS is > 12.0.7, it displays msg: %OSPF-3-DUP_RTRID & OSPF neighbor list will be empty
Unicast is broken—EXCHANGE
a. Wrong VC/DLCi mapping in frame/ATM environment in highly redundant network
b. MTU problem, can’t ping across with more than certain length packet
c. Access-list blocking unicast; after two-way OSPF send unicast packet except p2p links
d. NAT is translating unicast packet
Between PRI and BRI/dialer and network type is p2p—EXCHANGE
Neighbor Stuck in LOADING state
----------------------------------------
LS request is being made and neighbor is sending bad packet or mem corrupt
a. Do show IP OSPF bad to see bad LSA
b. Show log will show OSPF-4-BADLSATYPE msg
LS request is being made and neighbor is ignoring the request
MTU mismatch problem (RFC 1583 and 2178 compatibility issue) . OSPF should detect if the neighbor MTU is smaller than ours. Currently available in latest 12.3 and 12.0S.

Information is in the Database, but Not in the Routing Table
-----------------------------------------------------------------------
Generally caused due to any filtering options enabled
Mismatched Network Types
Point-to-Point Numbered and Unnumbered Links
Different Mask or IP Subnet on P2P Links
Address Flipped on Dual Links
Forwarding Address Problem
Discontigous Backbone