BGP conditional advertisement with TWO ISPs assistence

[Mylancognizant1]

We have two Internet links R(50Mbps) and T(300 Mbps),
Our both edge routers are configured properly, as per configuration all traffic inbound and outbound should traverse via T link and R Link as backup link,Both ISP are getting default route through BGP routing table.

But we have observed that most of the international inbound traffic is coming via R link and our Rlink fully utilize and on toss even it is 10 times prepended from our side,

We did the troubleshooting and after analyzing the traffic pattern for inbound traffic  from multiple looking glasses  across the globe, R link is getting preferred due to international transit peering and their relationship with the upstream transit service providers.

I know there workaround, if we can use conditional advertisement in BGP. But not source , how to track route in case T links fails. Please help if you know any conditional routing setup 

[Martin Hruby]

Hello

You can implement conditional path advertisement in BGP by using an IP SLA probe tied with a track object and a dummy route. For example, let's say you want to track the reachability of a remote IP address 3.3.3.3:

ip sla 1
 icmp-echo 172.16.13.3
 frequency 5
ip sla schedule 1 life forever start-time now
!
track 1 ip sla 1 reachability

With a track object you can the add a dummy route into the routing table:

ip route 169.254.254.254 255.255.255.255 Null0 track 1

Once you have this, you can insert the dummy route into the BGP table and then configure an advertise-map (per-peer, in this example 172.16.12.2) which specifies which paths to advertise to a peer (in this example all paths) when and only when the dummy route is present in the BGP table. Make sure you don't advertise the dummy prefix outside by attaching a NO_ADVERTISE standard BGP community to it:

ip prefix-list ADVERTISE seq 5 permit 0.0.0.0/0 le 32
!
ip prefix-list CONDITION seq 5 permit 169.254.254.254/32
!
route-map R_DONT_ADVERTISE permit 10
 set community no-advertise
!
route-map R_ADVERTISE permit 10
 match ip address prefix-list ADVERTISE
!
route-map R_CONDITION permit 10
 match ip address prefix-list CONDITION
!
router bgp 65000
 network 169.254.254.254 mask 255.255.255.255 route-map R_DONT_ADVERTISE
 neighbor 172.16.12.2 advertise-map R_ADVERTISE exist-map R_CONDITION

With this configuration you can advertise all prefixes to a remote BGP peer, under a condition that the dummy prefix exists. And the dummy prefix only exists as long as the IP SLA probe can ping 3.3.3.3. Of course you need to adapt this to your environment, but I'm sure you get the idea.

Best regards,
Martin