Editing IS-IS (section)

== IPv6 support and multi-topology ==
Because IS-IS encapsulates its PDUs into Layer 2 frame, it does not depend on Layer 3 protocols, such as IPv4 or IPv6. This is different from OSPF, which uses IPv4. Therefore, when IPv6 came up, adding IPv6 support to OSPF would require re-writing the protocol. That is how OSPFv3 was created. 

In case of IS-IS, '''TLV 232''' for '''IPv6 interface address''' and '''TLV 236''' for '''IPv6 reachability''' were added to support IPv6. And of course, IPv6 needs to be enabled on the interface. 

In order to display supported Layer 3 protocols, also called NLPID (Network Layer Protocol ID), '''TLV 129''' is used. Here, '''IPv4''' has code of '''0xCC''', while '''IPv6''' - '''0x8E'''. 

There might be an issue, if IPv4 and IPv6 topologies do not overlap. This could happen due to misconfiguration or intentionally (if some routers between do not support IPv6). For this situations, multi-topology support is added to IS-IS. 

'''TLV 229''' was added to display '''supported multi-topologies''', such as IPv4 unicast and IPv6 unicast. 

If multi-topology is enabled, IS-IS will calculate separate SPF tree for IPv4 and IPv6. This means twice the resource usage, but from the other side, this prevents traffic blackholing. 

When multi-topology is enabled, then IS-IS will use '''TLV 222''' for '''Multi-topology IS''' reachability, '''TLV 235''' for '''Multi-topology IP reachability''' and '''TLV 236''' for '''Multi-topology IPv6 reachability'''.