BGP Transit AS


Tillywood is one of the major movie studios located in Tilburg, The Netherlands. You are working there as a junior network engineer and assigned at the BGP team. One of the trainees has configured part of the network which has a transit AS but he has trouble sending packets between AS 1 and AS 3. Can you show him how it’s done?


  • All IP addresses have been preconfigured for you as specified in the topology picture.
  • Configure EBGP between AS 1 and AS 2.
  • Configure EBGP between AS 2 and AS 3.
  • Configure IBGP within AS 2.
  • Ensure AS 1 and AS 3 can reach each others loopback interfaces.

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BGP Transit AS

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Written by René Molenaar - CCIE #41726

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About the Author: Rene Molenaar

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 Rene,
    On the final part of this task, you redistributed OSPF….and explained the precaution against it.

    I actually made a static route for the unknown routes On either side.
    (192.168.12.x and 192.168.56.x)

    I am wondering if this was okay for the task. It worked.

    1. Hi Mark,

      Using static routes will work and most of the times using a static route will fix any problem. The downside of this solution is that it’s not "dynamic". If anything happens to your topology (router or link failure) a dynamic routing protocol can recover from this.

      Static routes will stay in your config with the risk of "blackholing" traffic.

      So yes it’ll work, but it’s not the best solution.

  2. I also applied static on Bullock and Roberts, that resolved the issue. I also tested the other method of advertising the only the needed routes with bgp and that fixed the issue. So there are multiple approaches to fixing the problem.

    I hope this helps.

    Best regards,

  3. Actresses surnames! nice naming convention. Off topic but Reece witherspoon gets my vote

  4. Used a default route for Bollock adn Roberts routers pointing to AS 2. Then added redistribute connnected subnets on OSPF in Parker and Witherspoon. Gives AS 1 and AS 3 dynamic capabilities to adveritse or withdrawl what they want without any further action on AS 2’s part. If they go down. The BGP Prefixes will stop advertising out from affected BGP Session.

    1. Could also just send a default-originate to the customer network (AS 1,2) via BGP.

  5. Hi, Rene.
    Why do you use redistribution, instead of announcing the subnet 12 and 56 in a BGP configuration Bullock and Roberts? In your tutorial, you do not recommend use of redistribution.

    1. You can use redistribution or the network command to get something into BGP. You just need to be aware that redistribution (especially between IGPs) can be problematic if you redistribute on more than 1 router.

  6. Its a good job their isn’t a Jolie as getting it to peer with Aniston might have caused some problems 🙂

  7. Good lab. 😉

    Can you just indulge me for a while as im fairly new to BGP.
    Wouldnt blackholing be a problem for most companies that use static routes to access ISPs

    Could you use an IGP on the Bullock and Roberts interfaces instead

    Can BGP work without the reliance of an IGP?


    1. Think about it, if you use BGP in an IGP environment, advertise routes the way you would advertise in OSPF or EIGRP

      ie network,

      the local preference, the weight, the origin code, the AS path, the metric would be the same. How does your router’s then choose the best path? Lowest IP address?

      Unless you are going to use several AS in your LAN envinronment.

      You do realize too that IBGP has a metric that is worse than any other routing protocol, so that if you have EIGRP / OSPF / RIP running in your network, your routers would choose those routes first because they take into account bandwidth, delay, hop counts, and prove to be better routes.

  8. Rene,

    Isn’t it by using redistribution ospf 1 in bgp2, will cause roberts and bullock get full routing table from AS 2? How to prevent this?

    And if I use the network command as shown in your “How to master CCNP route”, both roberts and bullock will get rib-failure. Did I miss any step?

    1. I just added the ethernet segment between the AS’ into BGP on Witherspon & Parker and that did the trick!

  9. Hi there. I have a question.

    What if the objective said you can’t have full mesh iBGP relationship and still manage to get the eBGP peers talking to each other? This would mean the Witherspoon and Parker won’t be able to see each other via BGP. Could use OSPF to learn the eBGP links and then get the eBGP speakers to peer with each other using the ebgp-multihop command. Will this work?

  10. Rene hello,

    Bullock still has no route to network (Witherspoon hasn’t as well) and vice versa for network. I belive it is because split horizon in BGP and probably i can solve it with route reflector way but in your final configs i don’t see any configuration in iBGP routers for route reflector. Do i miss something?

  11. Hi Rene,
    I completed this lab in one time as i used ospf as IGP and redistributed the bgp in ospf, also i made full mesh ibgp and also used bgp next hop self for all rest three IBGP routers on Witherspoon and parker.
    Advertising all network of AS2 on IBGP, also advertising EBGP networks i.e and for ebgp peers.
    Please let me know if any wrong configuration done by me.

    1. It isn’t considered best practice most times to redistribute your BGP routes into IGP. In this scenario it isn’t a lot of routes..but the current internet routing table is >500k routes.

      To complete this, you should have to accomplish the following:

      1. Why am I not learning AS 1 routes on Parker/Roberts? Why am I not learning AS 3 routes on WItherspoon/Bullock? Since iBGP does not send iBGP learned routes to another iBGP peer, Diaz and Aniston are not forwarding routes on to Witherspoon/Parker. The solution to this is either full mesh, route-reflectors, or confederation. I solved this by using WItherspoon as a route-reflector. To accomplish this, used EIGRP as my IGP and advertised the transit/loopbacks into EIGRP, and used the loopback for neighbor peering. Now, Bullock and Robers will now learn each otheres routes.

      2. I see the routes…but why can’t I ping them? If you do a show ip bgp, you will see that your next-hop is inaccessible. As a result, the router does not take what is in the BGP table and put it into the RIB. To correct can do a number of things: Advertise the transit to the eBGP neighbors into OSPF (not recommended) or we can configure next-hop-self on both Parker and WItherspoon.

      Let me know if you have any questions.

  12. IMHO There is one more solution of this LAB.
    As the requirements of the lab don’t explicitly state that you HAVE TO USE any of IGP protocols
    we could use only IBGP in Transit area.
    We would not use Loopbacks in transit area instead we would need to configure 6 neighbour statements on DIAZ and Aniston and 7 (1 eBgp and 6 iBgp ) on Witherspoon and Parker. (iGP neighbours with next hop self)

    In real life we would never want to do it this way but still we should be able to accomplish LAB Goals.

    Please correct me if I’m wrong.

  13. in my version of this, AS2 (Witherspoon, Diaz, Aniston and Parker) will be transit for AS1 (Bullock) and AS3 (Roberts); however, AS1 and AS3 do not need to know any details about the AS2 network topology.

    to that end, AS2 will permit ICMP only between AS1 and AS3… no ICMP will be permitted for AS2 internal network addresses.

    Type escape sequence to abort.
    Tracing the route to
    VRF info: (vrf in name/id, vrf out name/id)
    1 36 msec 68 msec 60 msec
    2 * * *
    3 * * *
    4 [AS 3] 112 msec * 72 msec

    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
    Success rate is 100 percent (5/5), round-trip min/avg/max = 60/84/112 ms

    Bullock#sh ip route | b Ga
    Gateway of last resort is not set is variably subnetted, 2 subnets, 2 masks
    C is directly connected, Loopback0
    L is directly connected, Loopback0 is subnetted, 1 subnets
    B [20/0] via, 00:03:49 is variably subnetted, 2 subnets, 2 masks
    C is directly connected, FastEthernet0/0
    L is directly connected, FastEthernet0/0
    B [20/0] via, 00:03:49

    as the benevolent network dictator of AS2, i like my privacy LOL 🙂

    another excellent lab.

  14. Advertising networks 12 and 56 on BGP, save a lot of space in routing table of the routers bullocks and Roberts, instead of doing redistribution.

    #Router Bullock
    Bullock#sh ip route | be Gate
    Gateway of last resort is not set

    C is directly connected, FastEthernet0/0 is subnetted, 1 subnets
    C is directly connected, Loopback0 is subnetted, 1 subnets
    B [20/0] via, 00:27:57
    B [20/0] via, 00:26:10


    #Router Roberts
    Roberts#sh ip route | be Gate
    Gateway of last resort is not set

    B [20/0] via, 00:26:41 is subnetted, 1 subnets
    B [20/0] via, 00:28:47 is subnetted, 1 subnets
    C is directly connected, Loopback0
    C is directly connected, FastEthernet0/0

    Great Lab Rene, learned a lot! xD

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