We have Default routes till date with our ISP and now we are planning to run BGP with ISP, reason we want RTBH triggering.


  1. Do we need to run eBGP or iBGP?
  2. If we run iBGP then does our router will hold full BGP table ~512k?

Actually we just want to do RTBH triggering and don't want to do high level of BGPing. It's going to be very simple implementation of BGP.

  • 2
    You really need to discuss your options with your ISP, it may have a preference or requirement. If you run eBGP, you may need to get your own AS number, but, again, that depends on your ISP. You probably don't need to get the full BGP routing table, and you shouldn't, unless you have a reason to do that. You can receive a default route from your ISP, or you can filter incoming prefixes and just use your own default route.
    – Ron Maupin
    Apr 6 '16 at 2:50
  • Lets say ISP say iBGP not possible and lets go with eBGP in that case i will buy AS but you are saying even after running eBGP i can use my existing default route? How can i filter incoming prefixes? do you have example snippet?
    – Satish
    Apr 6 '16 at 2:55
  • 1
    Just about any routing protocol can filer incoming prefixes. Cisco has a document about doing this with BGP.
    – Ron Maupin
    Apr 6 '16 at 3:02
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it.
    – Ron Maupin
    Aug 6 '17 at 23:00

You will likely run eBGP, since your AS number and the ISPs AS number are different. If you do not have an AS number, you can use one of the private AS numbers (analogous to private IP addresses).

You also do not need full routes, as you only have one path to the Internet.

For the details of RTBH routing, you need to talk to your ISP and follow their requirements.

  • Thanks for reply, we already talked to ISP and they have community (64XXX:0) for RTBH but now we are talking about go with iBGP vs eBGP. You are saying we can use private BGP with ISP and we don't need to hold full routing table right, can you explain how? I am new to BGP and as fas as i know if i run BGP it will load routes prefix in my tables. Am i missing something?
    – Satish
    Apr 6 '16 at 2:57
  • 3
    Your ISP can configure their routers to send you only a default route. It's commonly done.
    – Ron Trunk
    Apr 6 '16 at 3:02
  • 1
    Strictly speaking you wouldn't even need a BGP session for regular routing, just static routes on both sides with a BGP session just for blackholing would work just as well.
    – Teun Vink
    Apr 6 '16 at 13:36
  • My ISP saying they won't do iBGP and you have to buy public AS number, can you tell me how I can do peering without buying AS. If I create private AS then is there any issue to ISP? Also you are saying we can keep existing default route and just create BGP peering and do blackholing, but how do I stop routing tables which will fill my small router memory?? Do you have any config example which fit in my design?
    – Satish
    Apr 7 '16 at 12:25
  • Yes, you do not want to run iBGP. You should be able to use a private AS number, but that is up to the ISP. The ISP controls what routes they send you -- that is how BGP is different than internal protocols like OSPF. They can send all, some or no routes to you.
    – Ron Trunk
    Apr 7 '16 at 13:14

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