IPv6 and Multi-Topology IS-IS
What the exam tests
The exam tests single-topology versus multi-topology operation, the IPv6 TLVs, and when multi-topology is required.
Core rules
- Single-topology IS-IS runs one SPF for both IPv4 and IPv6, so the IPv4 and IPv6 topologies must be congruent.
- IPv6 interface addresses use TLV 232; IPv6 reachability uses TLV 236.
- Multi-topology uses TLV 229 with topology IDs; MT ID 0 is IPv4 and MT ID 2 is IPv6.
- Multi-topology runs a separate SPF per topology and allows non-congruent IPv4 and IPv6 designs.
- Adjacency formation still uses the same IIH process; protocol support is advertised in the IIH.
- Nokia SR OS requires IPv6 to be enabled in the IS-IS instance and multi-topology to be enabled where non-congruent topologies are used.
Worked example
A router has IPv4 on every link but IPv6 only on a subset. In single-topology mode, the IPv6 SPF follows the IPv4 topology and may forward IPv6 traffic over links that have no IPv6 addresses, causing black holes. Enabling multi-topology lets the IPv6 SPF use only links that advertise IPv6 reachability, so paths differ correctly.
Traps
Assuming IPv6 uses a different hello; enabling IPv6 without enabling it on the interface; forgetting that single-topology requires congruent topologies; mismatching MT IDs or not enabling MT at all.