ISIS multi-topology - interfaces with different IPv4/v6 AF enabled

As we know, the ISIS requires address family configuration must be matched between the ISIS router and interface configuration. So if you have both ipv4 and ipv6 AF configuration, you must do the same under interface.

Sometime you may have some of interfaces only enabled ipv4, but dual stack globally, in this case, you will need the ISIS multi-topology.

router isis VrfDefault
   net 49.0001.0000.0000.0002.00
   router-id ipv4 100.255.255.2
   is-type level-1
   !
   address-family ipv4 unicast
   !
   address-family ipv6 unicast
      multi-topology   <<< need multi-topology enabled here
!
interface Port-Channel211
   no switchport
   ip address 100.201.1.0/31
   ipv6 address 100:201:1::/127
   isis enable VrfDefault <<< this intf has both v4 and v6
!
interface Port-Channel251
   no switchport
   ip address 100.205.1.0/31
   isis enable VrfDefault <<< but this intf only v4
   isis multi-topology address-family ipv4 unicast <<<

And both neighbors are up!!

Router1(config-if-Po251)#show isis nei
Instance  VRF      System Id     Type Interface          SNPA              State Hold time   Circuit Id
VrfDef default  RouterDualStack  L1   Port-Channel211    P2P               UP    3           48
VrfDef default  RouterV4         L1   Port-Channel251    P2P               UP    2           63