BGP Redundant Config - Two Carriers - MPLS and Fiber (Layer 2)?

[bhill_tachc]

Hi!

I am trying to come up with the correct BGP configuration to provide redundant links with two different ISP's off of the same router; however, the transport methods are somewhat different. The scenario examples I have come across don't quite describe our situation.

One of these carriers is MPLS-based and hangs off of of a serial interface on our router (DS3 card) and communicates with the provider-edge router. The second carrier is fiber with an Ethernet handoff going into the FE 0/0 port on the router; the fiber network is configured as layer 2 so that all sites are seen as being on the same network segment (we can confirm this with a "sh cdp neighbor"). We do need to be able to segment these sites and will need an appropriate routing protocol. Because the routers at both the core and branch sites are already running BGP for the MPLS link we thought to continue to use the protocol for both links.

An example of the setup is:

Our idea (in theory) was to advertise two neighbors in BGP and let the protocol determine the faster link to take (i.e., fiber); however, when we fail the fiber link all traffic stops flowing rather than diverting to the MPLS link. We have tried distribution lists, multiple default route statements, modify the hold down timer, etc., but can't quite seem to get the configuration to fail-over correctly when one of the primary links fails.

Does anyone have any ideas?

Thanks!

[Giuseppe Larosa]

Hello Brian,

over the L2 network you can have an iBGP session, over the L3 VPN service you have an eBGP service.

the risk here is to have the intended backup link preferred over the iBGP link.

I would have expected: " the issue is the backup path is preferred"

BGP works differently then IGPs like OSPF or RIP.

BGP neighbors are configured and not dynamically discovered

You can use  different private AS numbers in the different sites to have comparable eBGP sessions over L2 with eBGP sessions over L3 VPN.

then you can use weight or local preference to prefer the routes learned over the session running over the L2 path.

router bgp 65001

neighbor x.x.x.x remote-as 65002

neighbor x.x.x.x local-preference 300

neighbor y.y.y.y remote-as 7018

network 10.10.0.0 mask 255.255.0.0

you need network commands to advertise the IP networks within the site

you will do something similar on second site

router bgp 65002

neighbor x.x.x.z remote-as 65002

neighbor x.x.x.z local-preference 300

neighbor w.w.w.w remote-as 7018

network 10.20.0.0 mask 255.255.0.0

Hope to help

Giuseppe