Hello S Kumar,
the provider has given you a /30 IP subnet for the primary MPLS link.
Have they given you a second different /30 IP subnet for the secondary MPLS link ?
if so, all you need to do is to setup on the 1921 Standby a second eBGP session using the IP addresses of the second /30 IPv4 subnet.
And you add an iBGP session between your two routers so that if primary fails the secondary will be ready to provide an alternate path to the remote branch office.
However, you say that:
>> Now MPLS provider has given us a redundant link, only one link will be active at a time
So, I am afraid that the provider has given you a single IPv4 subnet to be used on both links.
If this is the case you can:
configure the same IP subnet and same IP address on C1921-Standby and you configure an eBGP session to the same remote IP address and remote-as as you are doing in C1921-Active.
However, this eBGP session will be idle until the primary link fails.
The iBGP session between the two C1921 on an internal LAN subnet is still needed.
However, there will be a transition time:
a) the primary link is up and C1921 Active sends traffic to remote branch office using primary MPLS path
b) the primary link fails
c) the secondary link is activated an C1921 Standby starts to setup the new eBGP session on the secondary link.
During this phase traffic to the remote branch office will go the ASA primary and will be routed inside the LAN to LAN VPN.
d) When the session eBGP on C1921 - Standby is established and C1921 learns the routes for the remote branch office it will propagate them after 60 seconds to the C1921 Primary on the iBGP session on the LAN.
e) Traffic to the remote branch will use the C1921-Standby if the the C1921-Primary prefers the iBGP learned route over the floating static route via ASA
Be aware that eBGP routes have AD 20 and iBGP routes have AD 200, so to make the iBGP route preferred over a floating static route you would need to use an admin distance of 210 on the backup static route or you would use the default static route to ASA primary.
In this case the most specific route is preferred and iBGP routes are preferred over the default route regardless of its AD settings.
Please note that when the primary MPLS link is restored the above listed events happen in opposite order and also in this case you will have a temporary usage of the LAN to LAN VPN before the eBGP session is restored on C1921-Active.
Hope to help
Giuseppe
