OSPF network types, DR election and LSAs

The idea

OSPF network types: four OSPF routers (coreRouter, floorRouter, serverRouter, labRouter) share one campus segment and elect a DR, so they do not all sync with each other; coreRouter's point-to-point link to branchRouter skips the election.

Steps

1. Electing a DR and a BDR

   Four routers share campusLan. They elect a designated router (DR) that everyone syncs with, and a backup (BDR). Ask floorRouter who won. Every change on the segment passes through the DR, so it is the first thing to check.

   floorRouter:~# vtysh

2. Full with some, 2-Way with others

   Read labRouter's neighbours. Then floorRouter changes its cost on campusLan, and the change is recorded on the wire. Two routers that are not fully synced look broken at first sight, so learn which states are healthy.

   labRouter:~# vtysh
   floorRouter:~# vtysh -c 'configure terminal' -c 'interface campusLan' -c 'ip ospf cost 20'