Tuesday, January 20, 2009

Understanding OSPF contd...

After about 20 hours lab and roughly 10 hours going through OSPF, I still can't say I have the confidence to go through OSPF without going down belly fast in the CCIE lab. More practice is definately called for to get at least to 70% confidence levels.

So i decided to just make some notes, go through BGP and start on Multicast as the information digested. Plus Im curious how much of all this material I'll have to cover again.

Apparently OSPF is the most widely used IGP. It brings in the concept of Areas. If you imagine a midsized no actually a midsized company in Europe is probably a LARGE company in Kenya, so if you imagine a LARGE:-) company, an area would probably be a building or a department.

Router ID's, Neighbors, Adjacencies, LSA's *,Hello protocol,Areas, election (DR,BDR) are terms you'll come across and if the books you're reading don't address them:-), google will sort you out.

OSPF sort of goes around the whole network and maps out what you ask it to giving you paths to destinations, its a link state protocol.

OSPF order of operation:
1:A router sends Hello packets, discovers neighbors and elects a Designated Router. Link-state information and a list of neighbors is included in the packet.

*Mar 1 00:15:08.803: OSPF: Send hello to area 1 on Serial1/0 from
*Mar 1 00:15:10.903: OSPF: Rcv hello from area 1 from Serial1/0
*Mar 1 00:15:10.911: OSPF: End of hello processing
*Mar 1 00:15:11.071: %OSPF-5-ADJCHG: Process 1, Nbr on Serial1/0 from LOADING to FULL, Loading Done
*Mar 1 00:15:18.807: OSPF: Send hello to area 1 on Serial1/0 from

*Mar 1 00:15:20.907: OSPF: Rcv hello from area 1 from Serial1/0
*Mar 1 00:15:20.911: OSPF: End of hello processing

This is an initial hello.

Debugging ip packet detail shows the messages a bit clearly:

*Mar 1 00:20:48.815: IP: s= (local), d= (Serial1/0), len 80, sending broad/multicast, proto=89
*Mar 1 00:20:50.899: IP: s= (Serial1/0), d=, len 80, rcvd 0, proto=89

Protocol 89 is OSPF, is the ALLSPFRouters multicast address, from th esource you can also tell whether we originated the hello or not. Since this are serial links, no DR/BDR election takes place.

Let me swap out the links with ethernet to see how that works out....on my next post. Seems the lab is inaccessible....this is a simple lab, I think I'll dynamip on the laptop for a while:...

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