This question is little convoluted to explain but i will try my best, I have following scenario.

enter image description here

I have one ISP and i am getting 2x40G drops which you can see in diagram Link-A and Link-B (Both coming from same BGP AS) both Link-A & Link-B has associated subnet Subnet-A & Subnet-B both are /23, we run lots of public applicate so we need lots of public IP.


My ISP sending BGP Default route on both Link because we don't want to hold full BGP table. Now how do i tell my BGP Subnet-A take Link-A route and Subnet-B take Link-B route? In short link source base routing? is there any other method we can use to achive this schenatio

  • I am trying to avoid PBR because its kind of static route.
  • Should i use load-sharing ?
  • Can Local Preference solve this issue?
  • Or this solution is not feasible?
  • Can you put subnet a and b in different vrfs? If not can you put a and b on different switches? Jun 29 '18 at 18:00
  • VRF is little complicated because one of Link-A is live in production, we thought about VRF but seems too much work there.
    – Satish
    Jun 29 '18 at 18:06
  • Really you can think of both them in the default VRF right now so you could get away with just creating a new VRF just for Link/subnet B. Sometimes it takes more work to create a better network.
    – A-A-Ron
    Jun 29 '18 at 18:18
  • I need to buy switch to put them on separate switches, I have one more pair of Cisco 3064PX switches, does those switches can handle 40G traffic, if they can then i can terminate those link on them
    – Satish
    Jun 29 '18 at 18:18
  • Can't find a datasheet for a 3064PX, did find one for a 3064-X, Cisco Nexus 3064-X, 3064-T, and 3064-32T Switches Data Sheet, and that model does appear to have support for 40G.
    – A-A-Ron
    Jun 29 '18 at 18:29

You can make PBR more dynamic by tracking an IP SLA.

In this example, two IP SLAs are configured, one is to track the availability of over Link-A and the other tracks over Link-B

The PBR will send anything with source address in Subnet-A out to (Link-A next-hop) as long as the SLA monitoring reachability through that link is up. Anything sourced from Subnet-B will be sent out to (Link-B next-hop) as long as the SLA monitoring reachability though that next-hop is up.

If either of the SLAs are not up, routing will fall back to the global table and follow the default route in this case.

ip sla 1
 icmp-echo source-interface G0/1

ip sla 2
 icmp-echo source-interface G0/2

ip sla schedule 1 life forever start-time now
ip sla schedule 2 life forever start-time now

track 1 ip sla 1 reachability
track 2 ip sla 2 reachability

ip access-list standard Subnet-A

ip access-list standard Subnet-B

route-map Choose-ISP permit 10
 match ip address Subnet-A
 set ip next-hop verify-availability 1 track 1

route-map Choose-ISP permit 20
 match ip address Subnet-B
 set ip next-hop verify-availability 1 track 2
  • Thanks, in above scenario what will happened if my C1 switch dead or down, what will happen in that case?
    – Satish
    Jun 30 '18 at 20:01
  • You would need to configure the policy on both C1 and C2, if C1 was down it would follow the default OSPF route to C2 and then the same would be applied Jun 30 '18 at 21:23

So I understand you have a / 23 and want a particular block to exit link A and certain block exit the correct link b? You could advertise each / 24 in one of the links to ensure that link A / 24 always goes through that link, but this is not the best scenario because you would have a redundancy failure. Another option would be to announce the two / 24 on both sides, and, put a prepent in the prefix that you wish to make worse on that link. Thus, you will not stop announcing both sides, and the worsening LINK A will not gain from the one that is unpredicted and you reverse the other link. Now, if it is only the output of the machines in the subnet, you can perform a src nat so that subnet A clients leave the internet with the ip / 30 of your ISP of link A

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