BGP Notes – Synchronization

  • With synchronization on, route must be synchronized to an IGP in order for that routes to be able to be voted ‘best” by BGP.
    • That means the exact route must already be in the routing table via an IGP.
    • Static routes don’t count.
    • This is traditionally accomplished by redistributing BGP routes into an IGP.
    • With today’s Internet prefix count over 350k, this may not be such a good idea in some situations.
    • Synchronization is off by default.
  • Synchronization prevents black hole routes from being advertised via iBGP.
    • Unless every router is participating in iBGP, there’s no guarantee that any one router will have a route to NEXT_HOP.
  • Synchronization also prevents a router from advertising the black hole to an eBGP neighbor.
    • You don’t want to tell the world you have a path to a prefix when you really have a !N.
  • Synchronization can be safely disabled with the use of route reflectors or confederations.


These are just some notes I’ve been taking. ┬áComment with corrections, please.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages.

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2 comments for “BGP Notes – Synchronization

  1. Jay
    June 10, 2011 at 11:36 pm

    remember that sync is disabled by default now. (yay)

  2. June 11, 2011 at 3:33 pm

    Thanks, Jay. Updated with that info.

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