Customer Service Center

• ISIS
• Multi-Topology
• RFC-5120

• Multi-Topology ID is an optional mechanism within Intermediate System to Intermediate Systems (IS-ISs) used today by many ISPs for IGP routing within their clouds. This MT extension can be used for a variety of purposes, such as an in-band management network "on top" of the original IGP topology, maintaining separate IGP routing domains for isolated multicast or IPv6 islands within the backbone, or forcing a subset of an address space to follow a different topology. (RFC5120)
• According to RFC 5120 MT ID TLV is optional. For STC the default behavior is to not include the MT TLV in IIH Hello messages. Most ISIS test cases don't require Multi-Topology and use default ISIS topology which is signified as MT ID#0 and not required to be explicitly advertised.
• ISIS Multi-topology RFC 5120 section 2 => "standard" IS-IS topology is defined to be MT ID #0 (zero)"
  • If MT #0 is the only MT on the interface, it is optional to advertise it in the new TLV. Thus, not including such a TLV in the IIH implies MT ID #0 capability only.

7.5.  Reserved MT ID Values

   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.
   -  MT ID #6-#3995:    Reserved for IETF consensus.
   -  MT ID #3996-#4095: Reserved for development, experimental and proprietary features [RFC3692].

On Spirent TestCenter, we can config this value to "0", "2", "0,2" or empty. regarding to RFC5120 different value means:

Identifier Description
0 IPv4 routing topology
2 IPv6 routing topology
If this field is empty then the multi topology field will be included in the ISIS PDU.