Troubleshooting 2 - Adjacency

On R1 and R2 only, load flash:isis.tshoot2.cfg

#IOS-XE (R1, R2)
config replace flash:isis.tshoot2.cfg

Fix the broken adjacency between R1 and R2

Answer

#R1
router isis
 is-type level-2
int gi2
 no mtu 3000
int gi2.12
 isis net point-to-point

 
#R2
key chain ISIS_PASS
 key 1
  key-string ISIS_PASS
!
ipv6 unicast-routing
!
int gi2.12
 ip add 10.1.2.2 255.255.255.0
 ipv6 enable
 ipv6 router isis

There are several things wrong here. The following must match for an adjacency to come up:

• Common subnet for IPv4

• Protocols supported must match and multitopology mode must match

  • However, if one side uses ST and one uses MT transition, then both IPv4 and IPv6 will work

• Circuit types and router levels must match/be compatible

• Hello auth must match

• MTU must match

The following was broken in this situation:

• R2 has a misconfigured key-string

• R2 is in the wrong IPv4 subnet

• R1 does not use p2p as the circuit type

• R1 is level-1 only, while R2 is level-2 only

• R1 has a larger MTU on the interface. R2 won’t be able to accept fully padded Hellos from R1

• R1 is running IPv6 on the interface and R2 is not

  • IOS-XE uses single-topology by default, in which both IPv4 and IPv6 are required if one side has IPv6 enabled.

  • An alternative would be to enable wide metrics on both sides, and then use multi-topology on R1. This would allow the adjacency to come up without R2 supporing IPv6. This is because each protocol has its own topology now on R1.