Lab4 “Shadow Session”

Load lab4.init.cfg

#R1, R6
configure replace unix:lab4.init.cfg

#R2-R5, R7
configure
load bootflash:lab4.init.cfg
commit replace
y

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).

For this task, you must enable multipathing on R3. Note that on IOS-XR, shadow sessions are not possible. IOS-XR appears to lack the functionality of “advertise diverse-path.” Instead, you will need to configure add path between R3 and R7. Interestingly, configuring add path on XR does not automatically reset BGP peerings.

Answer

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

#R3
router bgp 65000
 address-family ipv4 unicast
  additional-paths receive
  maximum-paths ibgp 2
 !
 address-family ipv6 unicast
  additional-paths receive
  maximum-paths ibgp 2
!
! Clear bgp with R7 to negotiate Add-Path
do clear bgp 10.0.0.7
do clear bgp fc00::7

#R7
route-policy SELECT_BACKUP
  set path-selection backup 1 advertise
end-policy
!
router bgp 65000
 address-family ipv4 unicast
  additional-paths send
  additional-paths selection route-policy SELECT_BACKUP
 !
 address-family ipv6 unicast
  additional-paths send
  additional-paths selection route-policy SELECT_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:

If R7 was an IOS-XE router, we could setup a shadow session using “advertise diverse-path backup” with a separate loopback on R3, like below:

#R7 (If it was IOS-XE)
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

However, on IOS-XR, I do not see a way to implement this feature. Instead we can run Add-Path between R7 and R3 only. The capability will only be negotiated between these two routers, so R7 won’t send additional paths to any other iBGP peers.

On IOS-XR, we enable Add-Path for the address-family. We also must use a route-policy to install/advertise routes to peers. The RPL determines which paths are selected for advertisement and installation locally. R7 does not need to install these paths locally, so it only uses the “advertise” keyword in the RPL.

#R3
router bgp 65000
 address-family ipv4 unicast
  additional-paths receive
 !
 address-family ipv6 unicast
  additional-paths receive

#R7
route-policy SELECT_BACKUP
  set path-selection backup 1 advertise
end-policy
!
router bgp 65000
 address-family ipv4 unicast
  additional-paths send
  additional-paths selection route-policy SELECT_BACKUP
 !
 address-family ipv6 unicast
  additional-paths send
  additional-paths selection route-policy SELECT_BACKUP

On R3 we also use iBGP multi-paths.

#R3
router bgp 65000
 address-family ipv4 unicast
  maximum-paths ibgp 2
 !
 address-family ipv6 unicast
  maximum-paths ibgp 2

We now must manually clear the session. IOS-XR will not automatically clear the session for us when we add Add-Path capability. This capability is negotiated in the BGP Open.

#R3
clear bgp 10.0.0.7
clear bgp fc00::7

We should now see that Add-Path is negotiated:

R3 is now receiving the additional paths via R5:

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