IPv6 Single Topology Challenge

Load isis.ipv6.init.cfg

#IOS-XE
config replace flash:isis.st.challenge.init.cfg

#IOS-XR
configure
load bootflash:isis.st.challenge.init.cfg
commit replace
y

R1 and XR2 have enabled IPv6 support using ST mode. R2 and XR1 have not yet implemented IPv6. The operator has found that the adjacencies between R1-R2 and XR1-XR2 are now broken. Find a way to fix this without enabling IPv6 support on R2 or XR1, and without disabling IPv6 support on R1 or XR2.

Answer

#R1
router isis
 add ipv6
  no adjacency-check

#XR2
router isis 1
 address-family ipv6 unicast
  adjacency-check disable

Explanation

A router that enables IPv6 on an interface using ST mode requires that the neighbor supports both IPv4 and IPv6 in the protocols supported TLV. If the neighbor only supports IPv4, the router will not bring the adjacency up.

To get around this, we can configure the router to disable this adjacency check. This allows the router using IPv4/IPv6 ST mode to still form an adjacency with a IPv4-only neighbor.

#IOS-XE
router isis
 no adjacency-check

#IOS-XR
router isis 1
 address-family ipv6 unicast
  adjacency-check disable

Now R1 and XR2 will form the adjacency even though the NLPIDs (network layer protocol IDs) do not match:

Note that this feature can be used to seamlessly enable IPv6/ST mode in your network. When you bring up IPv6 on the first node, it won’t disrupt the existing adjacencies with all other nodes. Once you enable IPv6 ST mode everywhere, you can remove this feature.