bgp guru

[whanson]

I have two providers: AT&T and Lighpath. Lightpath is the owner of the subnet being advertised. I set up the BGP connections through two routers; one to AT&T and one to Lightpath. From the internet, I notice that a tracert and looking glass bgp shows me that AT&T is the preferred path. If I shut down the AT&T side, I can get back in but the curious thing is that I don't see the customers AS number in the path and the routes are the same as if AT&T is doing the routing. Can someone tell me what is happening. I am trying to make Lightpath the preferred path.

Looking glass view when AT&T is up and I've prepended customer AS many many times

sh ip bgp 216.255.104.1

BGP routing table entry for 216.255.104.0/24, version 357980534

Paths: (2 available, best #1, table Default-IP-Routing-Table)

Flag: 0x248

Not advertised to any peer

7018 40110 40110 40110 40110 40110 40110

205.171.0.96 (metric 10) from 205.171.0.149 (205.171.0.149)

Origin IGP, metric 0, localpref 80, valid, internal, best

Community: 209:888 209:889

Originator: 205.171.0.96, Cluster list: 205.171.0.149

7018 40110 40110 40110 40110 40110 40110

205.171.0.96 (metric 10) from 205.171.0.150 (205.171.0.150)

Origin IGP, metric 0, localpref 80, valid, internal

Community: 209:888 209:889

Originator: 205.171.0.96, Cluster list: 205.171.0.149

View after AT&T is brought down. I don't get it.

sh ip bgp 216.255.104.1

BGP routing table entry for 216.255.96.0/19, version 355929423

Paths: (2 available, best #1, table Default-IP-Routing-Table)

Not advertised to any peer

3561 6128

205.171.0.96 (metric 10) from 205.171.0.149 (205.171.0.149)

Origin IGP, metric 0, localpref 80, valid, internal, best

Community: 209:888 209:889

Originator: 205.171.0.96, Cluster list: 205.171.0.149

3561 6128

205.171.0.96 (metric 10) from 205.171.0.150 (205.171.0.150)

Origin IGP, metric 0, localpref 80, valid, internal

Community: 209:888 209:889

Originator: 205.171.0.96, Cluster list: 205.171.0.149

[olorunloba]

You mentioned that the address belongs to lightweight. It seems that lightweight are aggregating the address. When both are up, 216.255.96.0/19 is advertised by lightweight and 216.255.104.0/24 by AT&T. Since the AT&T is more specific, this will be used irrespective of the BGP attributes. Discus with your providers with respect to the summarisation.

[whanson]

thank you so much. Now I get it

[Harold Ritter]

216.255.96.0/19 is a supernet that is owned by CableVision (Is Lightpath the same as CableVision?).

I believe the reason you don't see 216.255.104.0/24 after you shutdown the link to AT&T is that the other SP only advertise its /19 supernet to their upstream provider (Savvis ASN 3561), which is also why the route via AT&T would always be preferred since more specific (/24 vs /19).

Hope this helps,

[sundar.palaniappan]

Hi,

The only way you can make lightpath/cablevision to be your preferred path for inbound traffic from the Internet is to get them to advertise your prefix of 216.255.104.0/24 to the Internet. Then you can use the prepend-path attribute to influence the route selection on the core BGP servers. When routes are aggregated the associated attributes are lost.

Hope that helps!

Regards,

Sundar

[whanson]

You are a peach. Thank you so much. Now I get it.