BGP Synchronization


You are a freelance network consultant specialized in solving BGP problems. One of your customers, a small ISP has trouble with customers that are unable to reach each other. They use BGP synchronization on some of their routers.


  • All IP addresses have been preconfigured as specified in the topology picture.
  • Each router has a loopback0 interface.
  • Configure OSPF within AS 2 to advertise the loopback0 interfaces. Don’t advertise or run OSPF on the links interconnecting AS 1 and AS 3.
  • Configure IBGP between router Foxx and Green. Source the BGP updates from the loopback0 interfaces.
  • Enable BGP synchronization.
  • Configure EBGP between router Foxx and Baldwin.
  • Configure EBGP between router Green and Rollins.
  • Advertise the loopback0 interfaces on router Baldwin and Rollins.
  • Ensure AS 1 and AS 3 can communicate with each other without removing the BGP synchronization command.

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BGP Synchronization

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Written by René Molenaar - CCIE #41726

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René - CCIE #41726 is the creator of where he shares CCNA, CCNP and CCIE R&S labs. He also blogs about networking on


  1. Hi Talal,

    Static and default routes will work…but they almost work anytime/everywhere ;D You should use the BGP solution to fix this. If you read a little on “BGP Synchronization” you can fix this!


  2. Hi Rene
    Is it ok to redistribute all our internet router into IGP (in this case OSPF).
    dont we loose all BGP attribute while redistributing into IGP.
    If router Mitchel is not able to handle such a huge routing table then how we make this work

    I have disable synchronization on every router running BGP in this lab. still not able to see communicate between AS 1 and AS 3.

    what if router Mitchel is also BGP enables and it has a neighborship with Foxx and Green. Foxx and Green are not BGP neighbor (not in a full mesh environment ). Then is it possible for Mitchel to advertise routes coming from Foxx to green and vice verse. if yes then how and if no then how ?

    I am really confuse in this.

    1. Hi,

      The problem is that router Mitchell will drop traffic because it doesn’t know about the destination networks. Normally if you are a transit AS like AS 2 you need to run IBGP on [b]*all*[/b] your routers.

      Another option would be to redistribute BGP into OSPF but yes you will lose your BGP attributes and OSPF isn’t capable of having so many prefixes in it’s LSDB. A full internet routing table has >300.000 prefixes and there’s no way you can redistribute these into OSPF.

      So the short answer is… IBGP on all your routers in the Transit AS or redistribute "some" prefixes into OSPF so you have connectivity.

      You could also create a GRE tunnel between Foxx and Green but that’s more a solution you would use in a lab.


  3. Bgp Black holing will be good solution or we can use specific prefix to be redistributed in ospf.if i am wrong please let me know.

  4. This scenerio says Enable BGP synchronization. Isn’t synchronization is enable by default ?

  5. good lab.

    @ Rizwan – i can’t even find “bgp synchronization” in the Cisco IOS Master Command List All Releases.
    but after doing an Internet web search, i found a reference saying BGP synchronization was set to “disabled by default” in IOS 12.2(8)T.

    1. silly me… i looked for “bgp synchronization”.
      i found it in the Cisco IOS Master Command List All Releases right where it is supposed to be… under just “synchronization”… DOH! 🙂

      anywho, confirmed new behavior in IOS 12.2(8)T:
      12.2(8)T Command default behavior changed to disabled.

  6. Excellent lab.

    So, does this mean that if synchronization was turned off, that and would have been successfully advertised to Baldwin and Rollins (without redistributing BGP into OSPF), because the rule of “don’t advertise unless IGP is advertising it also” would be unused?

    I’m still a little fuzzy on how synchronization on or off changes the routing behavior.

  7. Had fun with synchronization, ip ospf network point-to-point and redistribute static into OSPF.

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