CCIE Pursuit Blog

April 24, 2009

Core Knowledge Question of the Day: 24 April 2009

To be in compliance with the BGP synchronization rule, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path to be considered a valid path.  What additional criteria must an OSPF route meet?

Highlight for answer: The OSPF router ID must match the BGP router ID of the iBGP neighbor.

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3 Comments »

  1. I’m really enjoying these questions, but this one threw me. Do you have a link on Cisco that gives more info for this one?

    Comment by TL — April 24, 2009 @ 9:52 pm | Reply

  2. @TL – Don’t worry. I wouldn’t have known this one either. I came across it while reviewing BGP best path selection:

    http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

    Paths that are marked as not synchronized in the show ip bgp longer-prefixes output

    If BGP synchronization is enabled, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path to be considered a valid path. BGP synchronization is enabled by default in Cisco IOS® Software. If the matching route is learned from an Open Shortest Path First (OSPF) neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor. Most users prefer to disable synchronization with use of the no synchronization BGP subcommand.

    Note: Synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later.

    Comment by cciepursuit — April 24, 2009 @ 10:26 pm | Reply

  3. This behavior is a leftover from the old days when BGP3 routes were routinely redistributed into OSPF and is required by RFC1403. BTW, this RFC is also a source of some other weird IOS behavior.

    Comment by Ivan Pepelnjak — April 25, 2009 @ 2:28 am | Reply


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