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BGP Synchronization

Written by Rene Molenaar on . Posted in BGP

Scenario:

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.

Goal:

  • 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.

IOS:

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Topology:

BGP Synchronization

Video Solution:

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Comments (8)

  • avatar
    talaltk
    2011-10-08 22:36:08

    i jsut used static or default routes

  • avatar
    ReneMolenaar
    2011-10-10 11:54:08

    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!

    Rene

  • avatar
    jlashari
    2011-10-30 23:35:11

    I solved this lab by redistribution technique


  • avatar
    ReneMolenaar
    2011-10-31 00:31:59

    Good job Jawed that's how to finish this lab. The prefixes have to be in our IGP in order for BGP to advertise them to another AS.

  • avatar
    aravind
    2012-04-17 20:50:17

    Very good and useful information. Keep going....

  • avatar
    ReneMolenaar
    2012-04-19 08:59:40

    Glad you liked it, thanks!

  • avatar
    easter0009
    2012-05-08 06:33:17

    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.

  • avatar
    ReneMolenaar
    2012-05-08 06:47:59

    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 *all* 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.....run 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.

    Rene