I have following scenario where i already have 10G (A - BLUE Link) connectivity from ISP on SW1 and SW2 for redendency and i have configured eBGP and iBGP, Everything is good at this point, Now my company need more bandwidth and more public ips so we ask for another 10G link to our ISP which is B (RED Link) as per following diagram.

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I have following configuration at present with BLUE link peer ( because i didn't configure RED link yet)

router bgp 200
  address-family ipv4 unicast
    redistribute static route-map RTBH
    remote-as 100
    timers 10 30
    address-family ipv4 unicast
    remote-as 200
    update-source loopback0
    address-family ipv4 unicast


Once i configure link (B) RED on SW1 & SW2 then how do i announce my new public subnet B for neighbor B peer?

My link B public subnet is how do i tell BGP account it on link B ?

  • You should provide full configurations here, we can't even tell which neighbor is which from the snippet you provided. Based on what little config you're providing, any network configured with a 'network' statement would be advertised.
    – Teun Vink
    Jun 25, 2018 at 5:27
  • what other configuration you need? I believe we can use prefix-list to filter out route to specific neighbor for advertisement but not sure if any other method we can use
    – Satish
    Jun 25, 2018 at 14:28
  • 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 could post and accept your own answer.
    – Ron Maupin
    Jan 6, 2021 at 0:37

1 Answer 1


Here is a generic example:

ip prefix-list TO-A seq 10 permit le 32
ip prefix-list TO-B seq 10 permit le 32 ! the 'le 32' allows smaller subnets of that block
router bgp 200
  neighbor <A>
    ip prefix-list TO-A out
  neighbor <B>
    ip prefix-list TO-B out
  • That ip prefix-list TO-A seq 20 permit le 32 is a little dangerous. It could turn them into a transit (although most providers will reject those advertisements). Jun 25, 2018 at 16:22
  • True. It's a generic example, and in the OP's example there are no filters, so that problem already exists.
    – Ron Trunk
    Jun 25, 2018 at 16:31
  • I don't think that problem exists today because he only has one provider. (They'll reject transit routes because they see their own AS.) The OP does list his actual existing subnet as, so that could be used in place of both seq 10 and 20. Jun 25, 2018 at 16:33
  • Fair enough. I'll modify the prefix list
    – Ron Trunk
    Jun 25, 2018 at 16:34
  • so we don't need any deny statement in prefix-list, also i had question my A link is live and in production so doing this exercise does cause any issue also should i need to remove network in my existing config?
    – Satish
    Jun 25, 2018 at 16:40

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