BGP advertisement in 2 ISP

[mudvayne15]

Hi All,

Apologies for the long post as I want to ask some experts on the ISP side if this would be possible to implement. 

We have advertised our IP block to AS 7468 from AS 38792, but AS 4637 is the one who is advertising it on the Internet. 

Now, we have installed a backup link and wanted to do the same as the primary internet link. AS 9304 will be the one advertising our IP block to the Internet.

route-views.optus.net.au>show ip bgp 116.214.110.0
BGP routing table entry for 116.214.110.0/23, version 1429404782
Paths: (2 available, best #2, table default)
  Not advertised to any peer
  7474 7473 4637 7468
    203.202.143.34 from 203.202.143.34 (203.202.143.34)
      Origin IGP, localpref 100, valid, external
      Community: 7473:2 7473:21045 7473:33935 7474:1202 7474:1212 7474:1402
  7474 7473 4637 7468
    203.202.143.33 from 203.202.143.33 (203.202.143.33)
      Origin IGP, localpref 100, valid, external, best
      Community: 7473:2 7473:21045 7473:33935 7474:1202 7474:1222 7474:1402 7474:1525
route-views.optus.net.au>

I have advised them to prepend AS-PATH so that their link would always stay as a backup. 

Now my concern is if in case the physical link goes down on GW1. Will, it re-route to the second ISP? 

[mudvayne15]

Update: I tested the failover but the preferred route is still AS 4637, I unplug the cable of GW1. 

What can we advise to the ISP to fix this?

[Shahab Khan]

Hi,

You can do one more thing here. I see you have a /23 subnet. From GW1 router advertise 2 /24 subnet instead of /23 & from GW2 advertise /23 subnet. This way also you can achieve the same goal.

[paul driver · Accepted solution]

Hello

---

@mudvayne15 wrote:

route-views.optus.net.au>

I have advised them to prepend

AS-PATH

so that their link would always stay as a backup. 

Now my concern is if in case the physical link goes down on GW1. Will, it re-route to the second ISP? 

---

As/when GW 1 is lost and it is not dual connected - AS7468 will lose any prefixes that was sent from GW1 so those same prefixes will be preferred over the backup AS9809 however the path which is taken to GW2 depends on where the return path originates as it could take any mpls router within that cloud but eventually it will come in via AS9809 to GW2

[MHM Cisco World]

friend, I see your network it have two GW and two ISP, 
but 

show ip bgp x.x.x.x

 
I see two path exactly same and two Public IP?

[MHM Cisco World]

anyway, after you check your network I have one point, 
I see that you get path with community set, you can use community here to prefer one path, 
One side using

route-map OUT 

and set community (I prefer use

AS-PATH 

number)
other Side using

route-map IN 

and match community and set LP for path.

this give you one primary and one backup path.