OSPF Exchange State

[Steph1963]

Hi Everybody,

The best definition that I have found for OSPF exchange state is the following:

In this state, the routers exchange DBD packets, which describe their entire link-state database. The routers also send link-state request packets, which request more recent link-state advertisements (LSA) from neighbors.

I am not sure about this definition because I do not see any information about DBD packets when I am looking at OSPF DB Descr.  packets with WireShark.

Can somebody also confirm me that this exchange state will be done with every neighbor in a multi-access segment.

Thanks for your help

Stéphane

[Richard Burts]

Stephane

Actually a router does not do the full exchange of database with every neighbor on a multiaccess network. It will do the full exchange with the DR of the segment and with the BDR ( if present ) but not with every neighbor.

HTH

Rick

Sent from Cisco Technical Support iPhone App

[Laurent Aubert · Accepted solution]

Hi Stephane,

If you focus between two routers only, you will see that the first DD packets exchanged are used to establish master/slave relationship. Then the following ones will contain the LSA header. I noticed there is a last empty DD exchange but not sure about the reason of this one. I saw the M bit is set during the 2nd exchange which explains the 3rd exchange but don't know why.

For DR/BDR election to occur, all routers on the segment must have reach 2-ways or higher state so yes you can expect DD exchange between everyone until DR/BDR election is done.

Please check RFC 2328 for all the dirty details ;-)

HTH

Laurent.