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OSPF Exchange State

Steph1963
Level 1
Level 1

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.

Sans titre.JPG

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

1 Accepted Solution

Accepted Solutions

Laurent Aubert
Cisco Employee
Cisco Employee

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.

View solution in original post

2 Replies 2

Richard Burts
Hall of Fame
Hall of Fame

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

HTH

Rick

Laurent Aubert
Cisco Employee
Cisco Employee

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.