Sunday, October 10, 2010

Who's the boss? (OSPF edition)

So, a router interface that's participating in OSPF floods the network with a HELO packet -- this is sent to a multicast address, so if you've got acl's in play make sure they allow the OSPF multicast.  One of the things the HELO traffic does, besides just announcing "hey, I'm here" to any likely devices that might also be looking for neighbors, is share out the information used for election of the "Designated Router" (DR) and "Backup Designated Router" (BDR). 

The DR and BDR serve as points of contact for exchange of routing information -- instead of each router updated every other OSPF neighbor with link state announcements, all the devices update the DR and BDR and they send out link state updates to all of the OSPF neighbors.  This reduces the complexity of the exchange of routing information.

DR and BDR election is done by OSPF priority.  If two interfaces on a given network segment have the same priority, the higher Router ID is used as a tie-breaker.  For any given OSPF device, the "Router ID" is the highest IP address on the box (including loopback interfaces).  Specific router interfaces may have an OSPF priority set explicitly as well to adjust whether or not they become DR; a priority of 0 means that the interface should never be the DR or BDR; this is described as the state "DROTHER"

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Regis has worked as a network engineer since 1994 for small companies and for large companies.