Reference Guide

The following illustration is an example of the ISO-style address to show the address format IS-IS uses. In this example, the first
five bytes (47.0005.0001) are the area address. The system portion is 000c.000a.4321 and the last byte is always 0.
Figure 53. 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restrictions of single-topology mode are no longer in effect.
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Intermediate System to Intermediate System