M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)
Extracted elements (25)
MT extensions do not support MT-based partition repair to prevent unnecessary complexity. The overload, partition, and attached bits in the LSP header only reflect the status of the default topology, while per-MT overload/attach bits are carried in the MT TLV.
New TLVs were introduced instead of sub-TLVs inside existing TLV type-22 and type-135 because: (1) multi-topologies are often non-congruent so sub-TLVs don't save LSP space; (2) TLV type-22 is already near its size limit with existing sub-TLVs; (3) new TLVs can automatically inherit TE attributes defined for the standard topology without re-standardization.
Since it is not possible to prevent a non-MT-aware router from being DIS, distinct DIS elections per MT and special MT TLVs in pseudo-node LSPs are not supported; pseudo-node LSPs include all LAN nodes regardless of their MT capabilities.
The standard IS-IS topology is defined as MT ID #0, preserving backwards compatibility: absence of MT TLV in IIHs and LSP fragment zero implies MT ID #0 participation only, allowing MT-unaware routers to coexist without protocol changes.
If MT #0 is the only MT on an interface, it is optional to advertise it in the MT TLV in IIH packets. Not including the TLV implies MT ID #0 capability only, ensuring backward compatibility with non-MT IS-IS routers.
MT IS TLV encoding is aligned with extended IS reachability TLV type 22 (RFC 3784), and MT IPv4 Prefixes TLV is aligned with type 135, to simplify implementation effort by reusing existing TE encoding formats.
A generated pseudo-node LSP by DIS MUST contain in its IS Reachable TLV all nodes on the LAN as usual, regardless of their MT capabilities; there is no change to the pseudo-node LSP construction.
A router MUST include within its LSPs in the Reachable Intermediate Systems TLV only adjacent nodes participating in the corresponding topology, and MUST advertise such TLVs only if it participates itself in the corresponding topology.
Each adjacency formed MUST be classified as belonging to a set of MTs on the interface, achieved by adding a new TLV into IIH packets advertising which topologies the interface belongs to.
Each MT MUST run its own instance of the SPF decision process. A reverse-connectivity check within SPF MUST follow the according MT to assure bi-directional reachability within the same MT.
If an MT ID is not detected in the remote side's IIHs, the local router MUST NOT include that neighbor within its LSPs. The local router SHOULD NOT form an adjacency if they don't have at least one common MT over the interface.
In the MT IS TLV (222), the TLV MUST be ignored if the MT ID field is zero, to ensure consistent view of the standard unicast topology.
Lack of MT TLV in hellos and fragment zero LSPs MUST be interpreted as participation of the advertising interface or router in MT ID #0 only. If a router advertises MT TLV, it has to advertise all MTs it participates in, specifically including topology ID #0.
On a broadcast LAN, two routers SHALL always establish adjacency regardless of whether they have a common MT, to ensure all routers can correctly elect the same DIS.
Route leaking between IS-IS levels SHOULD only be performed within the same MT.
The nexthops derived from the MT SPF MUST belong to the adjacencies conforming to the same MT for correct forwarding. Results of each computation SHOULD be stored in a separate RIB to avoid forwarding loops with overlapping addresses.
MT Intermediate Systems TLV (TLV 222) extends IS reachability TLV type 22 with an additional 2-byte MT membership header containing 4 reserved bits and a 12-bit MT ID, followed by standard extended IS TLV format entries. It can contain up to 23 neighbors of the same MT if no sub-TLVs are used.
Multi-Topology Reachable IPv4 Prefixes TLV (TLV 235) extends extended IP reachability TLV type 135 with a 2-byte MT membership header (4 reserved bits + 12-bit MT ID) followed by standard extended IP reachability TLV format entries.
Multi-Topology Reachable IPv6 Prefixes TLV (TLV 237) extends IPv6 Reachability TLV type 236 with a 2-byte MT membership header (4 reserved bits + 12-bit MT ID) followed by standard IPv6 Reachability TLV format entries.
Multi-Topology TLV (TLV 229) is advertised in IIHs and LSP fragment 0 to indicate which topologies a router or interface participates in. It contains one or more 2-byte MT components, each with an OVERLOAD bit (O), ATTACH bit (A), reserved bits (R), and a 12-bit MT ID field.
IANA created the 'IS-IS Multi-Topology Parameters' registry with reserved MT ID assignments: #0 (standard topology), #1 (IPv4 in-band management), #2 (IPv6 routing), #3 (IPv4 multicast), #4 (IPv6 multicast), #5 (IPv6 in-band management), #6-#3995 (IETF consensus / Expert Review), #3996-#4095 (Private Use).
This document defines four new IS-IS TLV types registered in the IANA IS-IS TLV code-point registry: MT-ISN (222), M-Topologies (229), MT IP. Reach (235), MT IPv6 IP. Reach (237).
IS-IS authentication (e.g., RFC 3567 cryptographic authentication) SHOULD be applied if there is high risk from modification of multi-topology information. The authentication procedure for IS-IS PDUs is the same regardless of MT information.
When multiple topologies share an interface in the same address space, a misconfiguration or a packet with a spoofed source address can lead to packet loss or unauthorized use of premium network resources by causing incorrect forwarding table selection.
MT TLV (code 229) value field consists of 2-byte MT components: bit 15 = OVERLOAD (O), bit 14 = ATTACH (A), bits 13-12 = reserved (R), bits 11-0 = 12-bit MT ID. LENGTH SHOULD be 2 times the number of MT components.