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 22.214.171.124
BGP routing table entry for 126.96.36.199/24, version 357980534
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
7018 40110 40110 40110 40110 40110 40110
188.8.131.52 (metric 10) from 184.108.40.206 (220.127.116.11)
Origin IGP, metric 0, localpref 80, valid, internal, best
You mentioned that the address belongs to lightweight. It seems that lightweight are aggregating the address. When both are up, 18.104.22.168/19 is advertised by lightweight and 22.214.171.124/24 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.
126.96.36.199/19 is a supernet that is owned by CableVision (Is Lightpath the same as CableVision?).
I believe the reason you don't see 188.8.131.52/24 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,
Harold Ritter Sr. Technical Leader CCIE 4168 (R&S, SP) firstname.lastname@example.org México móvil: +52 1 55 8312 4915 Cisco México Paseo de la Reforma 222 Piso 19 Cuauhtémoc, Juárez Ciudad de México, 06600 México
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 184.108.40.206/24 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.
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