Refer to the IS-IS configuration exhibit.
This is the typical IS-IS configuration of the routers in an AS using IS-IS as the IGP. This AS is in
the transition phase of integrating IPv6 into the network. During this transition phase, some of the
routers within the AS might be running IPv4 only, some might be running IPv6 only, and others
might be running both IPv4 and IPv6. To avoid any black holes for the IPv6 traffic, which
configuration change can be made?
A.
Disable IS-IS adjacency checks.
B.
Enable IPv6 adjacency over IPv4 IS-IS peering.
C.
Enable multi-topology IS-IS.
D.
Disable the IPv4 unicast address-family.
E.
Enable IS-IS wide metric to support the single-topology mode.
Explanation:
Disabling IPv6 Protocol-Support Consistency Checks
IS-IS performs consistency checks on hello packets and will reject hello packets that do not have the same set of configured address families. For example, a router running IS-IS for both IPv4 and IPv6 will not form an adjacency with a router running IS-IS for IPv4 or IPv6 only.
Note to self:
Disabling IPv4 Subnet Consistency Checks
Perform this task to disable IPv4 subnet consistency checking when forming adjacencies. Cisco IOS software historically makes checks on hello packets to ensure that the IPv4 address is present and has a consistent subnet with the neighbor from which the hello packets are received. If multitopology IS-IS is configured, this check is automatically suppressed, because multitopology IS-IS requires routers to form an adjacency regardless of whether or not all routers on a LAN support a common protocol.
Multitopology mode is the default IPv6 mode for IOS XR routers.
Multitopology mode is the default mode of operation when enabling the IPv6 address family within IS-IS.