BGP Multihoming with IP SLA and PBR scenario - different ISPs

GS,

Running into an issue with BGP Multihoming using this scenario:

R1 --> connects to ISP A (200.1.1.1) and ISP B (201.1.1.2)
R2 --> connects to ISP C (202.1.1.3)

1. The agreement is to receive partial routing from the ISPs
2. The PBR is used to specify which internal prefixes are leaving via ISP A, B
or C.
3. ISP C is used as backup for ISP A and B, but it does not have backup for
its own.
4. R1 and R2 are iBGP neighbors (10.1.1.x)

The simplified IP SLA and PBR configuration:

R1

ip sla monitor 1
 type echo protocol ipIcmpEcho 200.1.1.1
ip sla monitor 2
 type echo protocol ipIcmpEcho 10.1.1.2

route-map PBR permit 10
 match ip address PBR-ISP-A
 set ip next-hop verify-availability 200.1.1.1 1 track 1
 set ip next-hop verify-availability 10.1.1.2 2 track 2

ip sla monitor 3
 type echo protocol ipIcmpEcho 201.1.1.2
ip sla monitor 4
 type echo protocol ipIcmpEcho 10.1.1.2

route-map PBR permit 10
 match ip address PBR-ISP-B
 set ip next-hop verify-availability 201.1.1.2 1 track 3
 set ip next-hop verify-availability 10.1.1.2 2 track 4

R1 monitors ISP A and B, and long as they are reachable, traffic are forwarded
from R1, if ISP A or B are down, the next hop is R2 (10.1.1.2) which then uses
ISP C (202.1.1.3) as the backup. The R2 has the same configuration monitoring
ISP A and B IP addresses, so it does not use ISP C when not needed. It has a
list of prefixes already going out via ISP C primarily.

The issue is that, when the BGP session between R1 and ISP A is down, the IP
SLA is still able to ping IP address 200.1.1.1 via ISP B, so the PBR does not
fall back to ISP C, and R2 is also able to reach IP 200.1.1.1 and still thinks
R1 and ISP A BGP session is up. This is happening because the partial routing
received from the ISP B comes with a 200.1.0.0/18 supernet, and we cannot find
a way to block the specific prefix 200.1.1.1/32 individually.

Do you guys have seen any similar scenario or a solution to overcome this type
of issue when the BGP peering IP address segment is advertised by other ISPs?

Thank you
Marina Ferreira

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Received on Sun May 24 2009 - 00:08:24 ART