Change of ISP and private to public AS

[dfbnws]

Hi

I have a setup with two routers, running ebgp with the DC (DC is providing internet access) routers using private AS. DC itself has connection through ebgp with ISP_A, from looking glass i can see they are advertising my subnet x.x.x.x/24 with there own AS (Prenpending there own AS 5 times).

I now have a public AS available for use, and also have two new ISP connections, ISP_A, ISP_B, one for each router. The task is to move internet connectivity to new ISPs.

I tried using the local-as no-prepend replace-as command, and could influence outbound path through localpref, but inbound wouldn't work even after manipulating prepending... 

(Note: i have a new connection to ISP_A and in the current setup, DC is also using ISP_A to advertise my subnet)

Question is, Am i missing something in the above solution? how do i go about this without completely deleting bgp config and reconfig using public AS? i just want to have minumum downtime...

Although, once new ISPs are operational, old circuits will be removed after verification and only new cicruits using public AS will be used...

[Francesco Molino · Accepted solution]

Hi

It's always preferred to reconfigure it but it should work with the command you've tried.

Normally you should have the following config:

router bgp 65000

neigbour x.x.x.x remote-as 45222

neigbour x.x.x.x local-as 45999 no-prepend replace-as

Then you're neighbor will have:

neigbour x.x.x.x remote-as 45999

Is it the config you've tested? If so, gave you ran some debug?

Can you share outputs to see why it's not coming up.

[dfbnws]

Actually, the bgp is up (both old circuit with private AS and new circuit with Public AS), this config is still active and my outbound traffic is using new circuit.

But the issue is with inbound. During last activity i tried to lower prepending on the new circuit to be less than 5, as for the current provider, i see via looking glass, they are advertising my subnet after prepending 5 times. But it didnt work...

Is there any issue with inbound traffic control when you are peered to different AS? in this transitional situation which i am in, for the rest the world my subnet would look to be originating from two different AS, my old providers AS and my own new Public AS....

Unfortunately i dont have debug logs for the last time i tried this in production

[Francesco Molino]

Does your new ISP prepend the prefix? You said you tried to lower propending. Why are you perpending yourself? Validate with your ISP if they're doing anything on prefix received from you first. If don't, then by advertising it yourself and if prepend is lower than other ISP, your route should be preferred.

You can go here : http://www.routeviews.org/routeviews/index.php/collectors/

You'll be able to telnet a router with full bgp table and validate.

I'm not very familiar with looking glass

[dfbnws]

Thanks for helping with this, I tried looking for the my subnet with my own AS in bgp lookups from the routers list you provided but can't see them.

The only entry for my subnet in global bgp table is from the old ISP, where it's being advertised by them with their own AS, not my AS...

Anyway I can see whether the new ISP is allowing my subnet to go out to Internet via my own AS?

thanks again for your help

[Francesco Molino]

Ok as we don't know your subnet, we can't check it out.
You need to make sure your new ISP doesn't have any route-map/prefix-lists denying it to be redistributed.