A link has been added between R3 and R4. Configure a multi-homed inter-AS option A setup, so that R7/R8 and R9/R10 have reachability. Everything has been preconfigured except for the inter-AS setup.
Use R1-XR1 as the primary link for VPN_A, and R3-R4 as the primary link for VPN_B. In the case of link failure, the other remaining link should provide backup.
This configuration looks quite complex but in reality it’s not too difficult. All ASBRs treat each other as CEs, as they always do in option A. Because we want VPN_A to use the R1-XR1 link primarily, and VPN_B to use the R3-R4 link primarily, we use LP on all ASBRs to enforce this.
We can verify this by doing a traceroute between the CEs. VPN_A uses the R1-XR1 link:
VPN_B uses the R3-R4 link:
If we shutdown the ASBR-ASBR link for R1-XR1, we can see that VPN_A traffic fails over to use the R3-R4 link.
#R1
int gi2.30
shut
int gi2.40
shut
#XR1
int gi0/0/0/0.30
shut
int gi0/0/0/0.40
shut
Likewise, if we bring this link back up and shutdown the R3-R4 links, VPN_B traffic fails over to use the R1-XR1 link.
#R3, R4
int gi2.30
shut
int gi2.40
shut
Summary
Dual-homing with inter-AS option A is really no different than dual-homing with IPv4 or VPNv4 CEs in general. We can use any BGP techinque such as LP, AS path prepending, MED, etc. to create primary and backup routes. This scenario is a popular choice among SPs because option A involves the least amount of coordination, and dual homing in this manner is an easy way to achieve redundancy.
Many situations may not have a primary/backup circuit, instead just using the default IGP lowest cost path to determine the best route. In fact, in our lab, if we remove all LP configuration, all traffic will use R3-R4 as primary due to the shortest IGP cost to the nexthop from the PEs R2 and XR2.
