Which of the following descriptions about IS-IS (IPv6) are correct? (Multiple choice)

Question

Options

• ABy default, IS-IS enables the multi-topology feature
• BIn order to support the processing and calculation of IPv6 routes, IS-IS adds NLPID to 129 TLV.
• CIn order to support the processing and calculation of IPv6 routes, IS-IS has added TLVs 232 and
• DIS-IS works at the data link layer and only needs to add new ILVs to support IPv6.

• A
  24% (8)
• B
  76% (26)

Explanation

B, C, and D are correct because IS-IS operates at the data link layer (Layer 2) rather than the network layer, meaning it doesn't rely on IP for transport - so supporting IPv6 simply requires extending the TLV framework, not redesigning the protocol. Specifically, the IPv6 NLPID (0x8E) is added to TLV 129 (Protocols Supported) to signal IPv6 capability (B), and new TLV 232 (IPv6 Interface Address) and TLV 236 (IPv6 Reachability) were introduced to carry IPv6-specific routing information (C and D).

A is wrong because multi-topology (MT) IS-IS is not enabled by default - it must be explicitly configured to maintain separate topologies for IPv4 and IPv6; by default, IS-IS uses a single topology for all address families.

Memory tip: Contrast IS-IS with OSPF - because OSPF runs on top of IP, Cisco/IETF had to create an entirely new version (OSPFv3) to support IPv6. IS-IS runs below IP, so it only needed new TLVs bolted on - remember "IS-IS: same protocol, new TLVs; OSPF: whole new version."

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