I have two router core1 and core2 connected with ISP router border1 and border2 using eBGP peer and i have 5 public subnect (/24). My ISP sending me default route instead of full BGP table.

Question: I have 5 public IP subnets (prefix) and now i want 3 prefix traffic go out from core1 and remaining 2 subnet traffic go out from core2 and bore core1/2 connected over iBGP so how do i influance outgoing traffic using local-pref? (we can use AS prepend to inflaunce inbound traffic)

How do i create prefix-list to make my first 3 subnet use core1 ?

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  • 1
    "we can use AS prepend to inflaunce inbound traffic" In my experience AS prepend doesn't work this well. Many providers filter out routes that have more than 3 times the same AS. If your two /24 belongs to the same /23 the best way is to announce the /24 on the link you want the traffic to come through and the /23 on the other link.
    – JFL
    Jun 6, 2020 at 8:46
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    Please note that your use of the terminology "border" as in border router in the above diagram is incorrect and may lead to confusion. Your core-1 and core-2 routers are also your ASBRs -- your border routers. Jun 6, 2020 at 11:42
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    – Ron Maupin
    Dec 16, 2020 at 23:21

2 Answers 2


Your post states your ISPs send you default routes only. Therefore, you'll need to configure static routes toward the desired transit provider to direct traffic for the 5 routes of interest to ISP A or B.

Assume is a prefix you want to route to the Internet via ISP A which has next-hop address

route-map redist-to-bgp permit 100
  match tag 1
route-map redist-to-bgp deny 9999
router bgp 64496
  redistribute static route-map redist-to-bgp
ip route tag 1
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    I have only single ISP and they have two border1 and border2 router for residency and reason i need to split my subnet out for both router to utilize bandwidth, so local-pref won't work in my scenario right?
    – Satish
    Jun 6, 2020 at 18:40
  • You don't even need to use local-preference. You'll only have one route -- a static route via the link you have selected. The redistribute command will import that route into BGP. Presumably, you have an iBGP session between your core1 and core2 routers and the -> route will be propagated by iBGP to the other core router. You're not learning this /24 route from both ISP circuits via eBGP -- you are configuring it statically. There's only the one route so you don't have to local-pref it up. Jun 6, 2020 at 21:39

So you are getting default from both circuits and you want traffic from certain subnets to use a specific circuit to send traffic out? Assuming the traffic hits the right border it will use the circuit connected to it because eBGP trumps ibgp for the same prefix. This means your routing decision is at the cores not the borders so make sure the link from core 1 to border 1 is lower cost than core 1 to border 2 and vice versa - this will mean anything connected to core 1 will go via border 1 and circuit 1 and anything connected to core 2 will go via border 2 and circuit 2. You can probably do this with your IGP but would need to see more of your config to advise.

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