Lab4 Shadow Session

Load unix:lab4.init.cfg

configure replace unix:lab4.init.cfg

R7 has been introduced as the RR. This causes the path to R6’s loopbacks via R5 to be hidden again. Create a shadow session between R3 and R7 so that R3 learns both paths (via R4 and via R5). Enable multipath on R3. New loopbacks (Lo1) have been preconfigured on R3 and R7 for this shadow session.

• R3 Lo1 = 10.0.0.33, fc00::33

• R7 Lo1 = 10.0.0.77, fc00::77

Answer

https://www.youtube.com/watch?v=eF1zqUhiPbs&list=PL3Y9eZjZCcsejbVWD3wJIePqe3NiImqxB&index=6

#R3
router bgp 65000
 template peer-session IBGP_SHADOW_SESS
  remote-as 65000
  update-source Loopback1
 exit-peer-session
 !
 neighbor 10.0.0.77 inherit peer-session IBGP_SHADOW_SESS
 neighbor fc00::77 inherit peer-session IBGP_SHADOW_SESS
 !
 add ipv4
  maximum-paths ibgp 2
  neighbor 10.0.0.77 activate
 !
 add ipv6
  maximum-paths ibgp 2
  neighbor fc00::77 activate

#R7
router bgp 65000
 template peer-session IBGP_SHADOW_SESS
  remote-as 65000
  update-source Loopback1
 exit-peer-session
 !
 neighbor 10.0.0.33 inherit peer-session IBGP_SHADOW_SESS
 neighbor fc00::33 inherit peer-session IBGP_SHADOW_SESS
 !
 add ipv4
  bgp additional-paths select backup
  neighbor 10.0.0.33 activate
  neighbor 10.0.0.33 inherit peer-policy IBGP_V4V6
  neighbor 10.0.0.33 advertise diverse-path backup
 !
 add ipv6
  bgp additional-paths select backup
  neighbor fc00::33 activate
  neighbor fc00::33 inherit peer-policy IBGP_V4V6
  neighbor fc00::33 advertise diverse-path backup

Explanation/Verification

Before we make the change, notice that R7 is hiding the backup path via R5. By default, the RR only sends its bestpath to all RR clients.

Only the bestpath is sent to R3:

We now setup the shadow session. This simply uses a different loopback for peering, Lo1. The RR advertises the “diverse-path backup” path on the shadow session, not the bestpath. This does not require Add Path to be configured and negotiated between peers. It is similar to advertise-best-external in a way. Also note that R7 does not need to locally install the backup path as a fast repair route, as it will not be in the data path.

#R7
router bgp 65000
 add ipv4
  bgp additional-paths select backup
  neighbor 10.0.0.33 advertise diverse-path backup
 !
 add ipv6
  bgp additional-paths select backup
  neighbor fc00::33 advertise diverse-path backup

On R3 we receive the backup path (with nexthop R5) over the shadow session in addition to the primary path (with nexthop R4) on the primary session.

R3 installs these locally as multipath in the RIB/FIB.

Note that in this lab, we can achieve multipathing by only running a shadow session with R3, because R3 is the router making the multipath decision. In another topology, you might need to run multiple shadow sessions.