Reference Guide

Figure 44. ISO Address Format
Multi-Topology IS-IS
Multi-topology IS-IS (MT IS-IS) allows you to create multiple IS-IS topologies on a single router with separate
databases. Use this feature 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 (AS) to run multi-topology IS-IS for IPv6
because the Type, Length, Value (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 does not recognize the ability of the single-topology mode router to support IPv6
traffic, which leads 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. After 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topological restrictions of single-topology mode are no longer in effect.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, physical synchronous optical network technologies (SONET)
interfaces, port-channel interfaces (static and dynamic using LACP), and virtual local area network (VLAN) interfaces.
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