Section 13.2. IS-IS Extensions for Multi-Topology Routing


13.2. IS-IS Extensions for Multi-Topology Routing

At the time of writing of this chapter, support for Multi-Topology IS-IS (MT-ISIS)[2] is in the early stages of adoption. As a result, support differs from vendor to vendor. Cisco Systems, for example, supports MT-ISIS for IPv4 and IPv6 unicast topologies, whereas Juniper Networks supports MT-ISIS for IPv4 and IPv6 unicast and multicast topologies. By the time you read this, both vendors likely will support more diverse topology types.

[2] Tony Przygienda, Naiming Shen, and Nischal Sheth, "M-ISIS: Multi Topology (MT) Routing in IS-IS," draft-ietf-wg-multi-topology-07.txt, June 2004.

13.2.1. MT-ISIS Procedures

IS-IS is extended to support multiple topologies by defining a new MT Intermediate Systems TLV and new MT Reachable IPv4 and IPv6 Prefixes TLVs that serve the same functions as the Extended IS Reachability TLV (type 22) and Extended IPv4 and IPv6 Reachability TLVs (types 135 and 236), respectively, but which have additional fields for MTID information. Table 13.1 lists the MTIDs defined by the MT-ISIS draft.

Table 13.1. MTIDs Defined for MT-ISIS

MTID Value

Topology

0

Standard (default) topology (IPv4 unicast routing topology)

1

IPv4 in-band management

2

IPv6 unicast routing topology

3

IPv4 multicast routing topology

4

IPv6 multicast routing topology

53995

Reserved for IETF consensus

39964095

Reserved for development, experimental, and proprietary features


The table shows that like MT-OSPF, MTID 0 signifies the default or, in MT-ISIS terms, standard topology. Again, this topology consists of all routers and links in the IS-IS domain and helps with backward compatibility. Any router that does not advertise MT capability, or which uses types 22 and 135 TLVs rather than the MT TLVs is considered by MT-ISIS routers to belong to the default topology. At the same time, MT-ISIS routers can use type 22 and 135 TLVs to advertise their membership in the default topology, and use MT TLVs to advertise membership in topologies other that MTID 0. Routers that do not support the MT extensions accept the type 22 and 135 TLVs as usual and ignore the MT TLVs.

When an MT-ISIS router originates a Hello on an interface, it includes in the Hello a set of Multi Topology TLVs, one for each topology that the originating interface belongs to. When the router originates its LSPs, it uses MT Reachable IS TLVs to list the topologies the router belongs to and the neighbors belonging to the same topologies. If a neighbor's Hellos indicate that it belongs to a topology that the receiving router does not, or that the neighbor does not belong to a topology that the receiving router does, the router does not associate the neighbor with that topology when it originates its LSPs.

If a neighbor does not include any MT TLVs in its Hellos, the receiving router includes the neighbor only in the default topology. Adjacency behavior is a bit different between point-to-point and broadcast interfaces: If two neighbors have no topologies in common on a point-to-point interface, no adjacency is formed; but an adjacency is formed between two or more neighbors on a broadcast interface even if they do not have any topologies in common. This is because, as with DRs and MT-OSPF, the DIS election is independent of the MT-ISIS extensions. So because a router that does not support MT extensions can be elected DIS, all routers on a broadcast network must be adjacent at the same level.

There are two more similarities to MT-OSPF: If a router detects a change in a topology to which it belongs, it must reflood a new LSP to advertise the change. And, level boundaries are the same for all MT-ISIS topologies, just as area boundaries are the same for all MTOSPF topologies. The NET of each router also applies to all topologies to which the router belongs.

Although NETs and level boundaries must be consistent, a router can set the Overload bit separately for each topology to which it belongs. And a router can be an L1/L2 router for some topologies and not for others; as a result, an L1/L2 router can set the Attached bit separately per topology.

13.2.2. MT-ISIS TLVs

The Multi Topology TLV (type 229) is shown in Figure 13.5. The TLV lists the MTIDs, up to 127, to which the originating router belongs, and is carried in both Hellos and LSPs. The MTID for MT-ISIS is 12 bits, rather than the 7-bit MTID that MT-OSPF uses. An Overload (O) bit and an Attached (A) bit is associated with each MTID listed, so that overloading and L2 attachment can be advertised separately for each topology. The R bits are reserved.

Figure 13.5. The MT-ISIS Multi Topology TLV.


Figure 13.6 shows the MT Intermediate Systems TLV (type 222). This TLV serves the same purpose as the Extended IS Reachability TLV shown in Figure 5.33: It describes the originating router's links to neighboring routers and the cost of those links, and so serves as the fundamental information for the SPF calculations. The difference is that the MT IS TLV associates the listed neighbors with a specific MTID. (The 4 bits preceding the 12-bit MTID are reserved.) The format of the TLV, aside from the added MTID field, is identical to the Extended IS TLV. This TLV can appear multiple times, according to the number of MTIDs the originator supports.

Figure 13.6. The MT-ISIS MT Intermediate Systems TLV.


Similarly, the MT Reachable IPv4 Prefixes TLV (Figure 13.7) has the same function as the Extended IP Reachability TLV shown in Figure 5.34: It advertises internal and external prefixes and their metrics, but associates the prefixes with a specific MTID. The type of the TLV is 235, and it can appear multiple times according to the number of topologies supported by the originating router.

Figure 13.7. The MT-ISIS MT Reachable IPv4 Prefixes TLV.


Finally, the MT Reachable IPv6 Prefixes TLV (type 237) shown in Figure 13.8 is an MT version of the IPv6 Reachability TLV shown in Figure 13.21: It advertises IPv6 prefixes, but like the MT Reachable IPv4 Prefixes TLV, it lists the IPv6 prefixes relevant to a specified MTID. One difference with this TLV and the related IPv4 MT TLV is that "regular" type 236 IPv6 Prefix TLVs and these type 237 MT IPv6 Prefix TLVs cannot be mixed. An IS-IS domain should use one type or the other, but not both.

Figure 13.8. The MT-ISIS MT Reachable IPv6 Prefixes TLV.





OSPF and IS-IS(c) Choosing an IGP for Large-Scale Networks
OSPF and IS-IS: Choosing an IGP for Large-Scale Networks: Choosing an IGP for Large-Scale Networks
ISBN: 0321168798
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Jeff Doyle

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