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BGP - with AS Prepending/preferred path

Hello,

I'm currently having BGP running with 2 ISPs on 2 routers at 2 data centers as a failover setup with AS Prepending applied on a /24 advertisement on the router at the second data center. I have a 1Mb connection going to this data center through an ISP which has AT&T as their backbone network.

The thing is I try every single internet router out there to do a "show ip bgp *.*.*.*" and I get the right answer which SomeAS# 1stISP-AS# My-AS#, but if I use ATT's router to do a "show ip bgp" I get ATT-AS# 2ndIPS-AS# My-AS# My-AS# My-AS# My-AS# My-AS# <-- Prepending applied here.

I thought that AS-Prepending applied to the advertisement going through the 2nd ISP made the advertisement going through the 1st ISP the preferred path, shouldn't the 2nd ISP see this path also and ignore the path with AS Prepending applied until the connection to the 1st IPS fails?

Help!

Thanks,

Anthony

8 REPLIES
Bronze

Re: BGP - with AS Prepending/preferred path

Just because you see it does not mean it is the best path. For example, if you go to something like

route-views.oregon-ix.net

and do a

sh ip bgp regexp

you will see a list of the network blocks for the asn and all of the valid paths to the asn, but only one will be listed as the best. The far left as * listed and this means that this is a valid path to that network block. What you will need to look for will be a *> as this represents the valid and best path for the network block.

Hope this helps.

Re: BGP - with AS Prepending/preferred path

Hello Anthony,

So the second path does not show up at all in the ATT route server?

This means either

1) a filter in place blocking your update through 1stISP

2) aggregation either done by the 1stISP or ATT

3) LocPref/Weight used by ATT to prefer the path through 2ndISP (this would lead to the ATT route reflector only announcing the path you saw in the route server)

With 3) you should be able to see the LocPref from the value in the ATT route server

I would send an email to ATT asking for assistance. The address can be obtained from the whois database - search for the ATT-AS.

Hope this helps! Please rate all posts.

Regards, Martin

Hall of Fame Super Silver

Re: BGP - with AS Prepending/preferred path

Anthony

It depends somewhat on how many times you prepend your AS to the second ISP (and depends somewhat on how many ASes are between ISP-2 and ISP-1). If there are enough ASes between ISP-2 and ISP-1 (each AS gets added to the path as the advertisement is propagated) then the path may be equal or shorter for ISP-2 to come directly to you. It may also be that ISP-2 has set things up so that they route directly to their customers (and perhaps they pass that to ATT).

HTH

Rick

Re: BGP - with AS Prepending/preferred path

Hello Athony,

Rick is right, insufficient AS path length on the prepended path is 4) in the list.

Rick, I believe nothing ISP1 does internally with the BGP update would allow it to make it less preferable in the ATT AS, as long as ISP1 announces the update at all.

Ah this gives option

5) ISP1 sets f.e. LocPref if the update is received from ATT and does not announce the shorter path to ATT. Thinking twice, this comes down to option 4) because the shorter path is out there in the internet, hence in case 5) it would reach ATT through other provider AS.

Hope this helps! Please rate all posts.

Regards, Martin

Hall of Fame Super Silver

Re: BGP - with AS Prepending/preferred path

Martin

I am not sure that I follow your logic here.

My point has less to do with ISP-1 making the route less attractive to ATT and more with the possibility that ISP-2 might make it more attractive (pass some community to ATT that identifies this as from my customer, for example) and ATT makes it decision based on this. I am not sure that this is really the case - but it is one scenario which would explain the observed behavior.

HTH

Rick

Re: BGP - with AS Prepending/preferred path

Hi Rick,

this is what I think is included in option 3) ATT set LocPref (either by their own policy or triggered by communities). In any case it should bevisible in the route server of ATT.

Regards, Martin

New Member

Re: BGP - with AS Prepending/preferred path

Hello all,

It looks like AT&T sets up static routes to the /24 I'm advertising instead of using BGP.

Did a "show ip bgp regexp MyAS#" on AT&T's route server, it showed all routes through AT&T and nothing from the 1st ISP

Thank you for all your help.

Best regards,

Anthony

Re: BGP - with AS Prepending/preferred path

Hi Anthony,

it is highly unlikely that ATT uses static routes. My idea is, that your ISP1 sets Local Preference on all BGP updates learned from ATT. This would make it the prefered path and would explain that ISP1 will not announce the best path back to ATT.

So my suggestion now is to talk to ISP1 and ask for assistance. In case they announce your network with the shorter path to ATT all your issues should go away.

Hope this helps! Please rate all posts.

Regards, Martin

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