Reference Guide

Intermediate System to Intermediate System | 491
Multi-Topology IS-IS (MT IS-IS) allows you to create multiple IS-IS topologies on a single router with
separate databases. This feature is used to place a virtual physical topology into logical routing domains,
which can each support different routing and security policies.
All routers on a LAN or point-to-point must have at least one common supported topology when operating
in Multi-Topology IS-IS mode. If IPv4 is the common supported topology between those two routers,
adjacency can be formed. All topologies must share the same set of L1-L2 boundaries.
You must implement a
wide metric-style globally on the Autonomous System to run Multi-Topology IS-IS
for IPv6 because the TLVs used to advertise IPv6 information in link-state packets (LSPs) are defined to
use only extended metrics.
The Multi-Topology ID is shown in the first octet of the IS-IS packet. Certain MT topologies are assigned
to serve predetermined purposes:
• MT ID #0: Equivalent to the “standard” topology.
• MT ID #1: Reserved for IPv4 in-band management purposes.
• MT ID #2: Reserved for IPv6 routing topology.
• MT ID #3: Reserved for IPv4 multicast routing topology.
• MT ID #4: Reserved for IPv6 multicast routing topology.
• MT ID #5: Reserved for IPv6 in-band management purposes.
Transition Mode
All routers in the area or domain must use the same type of IPv6 support, either single-topology or
multi-topology. A router operating in multi-topology mode will not recognize the ability of the
single-topology mode router to support IPv6 traffic, which will lead to holes in the IPv6 topology.
While in transition mode, both types of TLVs (single-topology and multi-topology) are sent in LSPs for all
configured IPv6 addresses, but the router continues to operate in single-topology mode (that is, the
topological restrictions of the single-topology mode remain in effect). Transition mode stops after all
routers in the area or domain have been upgraded to support multi-topology IPv6. Once all routers in the
area or domain are operating in multi-topology IPv6 mode, the topological restrictions of single-topology
mode are no longer in effect.
Interface support
MT IS-IS is supported on physical Ethernet interfaces, port-channel interfaces (static & dynamic using
LACP), and VLAN interfaces.
Adjacencies
Adjacencies on point-to-point interfaces are formed as usual, where IS-IS routers do not implement
Multi-Topology (MT) extensions. If a local router does not participate in certain MTs, it will not advertise
those MT IDs in its IIHs and so will not include that neighbor within its LSPs. If an MT ID is not detected
in the remote side's IIHs, the local router does not include that neighbor within its LSPs. The local router
will not form an adjacency if both routers don't have at least one common MT over the interface.