OSPF-Hello (224.0.0.5) to all the network aficionados on the web. This will be my first article on Packet Pushers. Routing is my first love, so rest assured you are in for some piping hot protocol intricacies (read delicacies) from my writing oven :). I am hoping that whoever swings by my piece learns something from it. The past few days have been sheer insanity (courtesy CCIE-Service Provider lab preps and grad school grind), yet somehow I squeezed in some time to write this piece on OSPF neighborships and adjacency formation on a broadcast link. Show
I have seen many people confuse neighborship and adjacency, and they end up thinking they mean the same thing, which is akin to committing blasphemy in the world of OSPF. (There – the wrath of Dijkstra burnt a n00b :).) I have taken a sample topology (shown below) to expound on this concept. OSPF Adjacency Formation StatesDuring the adjacency formation process, two OSPF routers transition through several states, which include:
(There. I rode on the horse’s back and proclaimed, “All OSPF adjacent routers are neighbors, but all OSPF neighbors are not adjacent”. Say with me “FREEDOMMMMMMM from CONFUSIONNNNN”, the way Sir William Wallace (aka Mel Gibson) said to his comrades in the epic movie”Braveheart”, who complied.) I have taken a Wireshark capture for neighborship and adjacency formation between the 2 routers to illustrate this concept. (Note: You can click on the image to enlarge it for better clarity. I hope the IP addresses and message exchanges are readable.) The adjacency formation can be diagrammatically summarized as follows: Nailing the OSPF Adjacency GotchasNeighborship stuck in down stage : When an OSPF router first receives a hello packet, it verifies that the data in some fields matches its own locally configured information. Should any of the checked data be different, the hello packet is discarded and not processed. The data fields verified are the Area ID, Authentication, Network Mask (on broadcast networks), Hello Interval, Router Dead Interval, and Options fields. If this information doesn’t match, then neighborship is stuck in the down stage. OSPF adjacency gets stuck in the ExStart state : This occurs due to a final check the routers perform. In the DD packet, each router advertises the IP MTU of the interface it is using. Should the local and remote routers not agree on the MTU of the network link, the database synchronization process stops and both neighbors remain in the ExStart state. Comments and acknowledgements invited. References1. http://www.ietf.org/rfc/rfc2328.txt
Overview
Link State Protocol
OSPF Operations
Neighbor AdjacnciesHellos
Network Types
Designated Router and Backup Designated Router
Building Adjacencies
Flooding
Areas
Backbone
Virtual Links
Router Types
Link-State Database
Different LSA Types
Router LSA
Network LSA
Network Summary LSA
ASBR Summary LSA
AS External LSA
OSPF ExtensionsStub Areas
Totally Stubby Areas
Not-So Stubby Areas
OSPF On-Demand Circuits
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