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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.

enter image description here

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

router bgp 200
  router-id 192.168.255.20
  log-neighbor-changes
  address-family ipv4 unicast
    network 74.100.100.0/23
    redistribute static route-map RTBH
  neighbor 74.1.1.10
    remote-as 100
    timers 10 30
    address-family ipv4 unicast
      send-community
  neighbor 192.168.255.19
    remote-as 200
    update-source loopback0
    address-family ipv4 unicast
      next-hop-self

Question

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 74.20.20.0/23 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 '18 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 '18 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 can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 8:52
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Here is a generic example:

ip prefix-list TO-A seq 10 permit 74.100.100.0/23 le 32
! 
ip prefix-list TO-B seq 10 permit 74.20.20.0/23 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
| improve this answer | |
  • That ip prefix-list TO-A seq 20 permit 0.0.0.0/0 le 32 is a little dangerous. It could turn them into a transit (although most providers will reject those advertisements). – user3629081 Jun 25 '18 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 '18 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 74.100.100.0/23, so that could be used in place of both seq 10 and 20. – user3629081 Jun 25 '18 at 16:33
  • Fair enough. I'll modify the prefix list – Ron Trunk Jun 25 '18 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 74.100.100.0/23 in my existing config? – Satish Jun 25 '18 at 16:40

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