User guide

Multi-Topology IS-IS Overview Configuring IS-IS
page 3-34 OmniSwitch AOS Release 7 Advanced Routing Configuration Guide March 2015
Multi-Topology IS-IS Overview
Multi-topology (M-ISIS) support is necessary in IS-IS to support network domains in which non-dual
stack IS-IS routers exist. The default protocol behavior of IS-IS is to construct shortest paths through the
network using the routers' MAC addresses with no regard to the different IP address families supported.
This behavior may result in black-holed routing when there are some IPv4-only or IPv6-only routers in an
IS-IS routing domain, instead of all dual-stack routers.
M-ISIS mechanism runs multiple, independent IP topologies within a single IS-IS network domain, using
separate topology-specific SPF computation and multiple Routing Information Bases (RIBs).
M-ISIS routers advertise their MT capability by including a set of MT TLVs in their Hello PDUs. When
the router originates LSPs, it uses MT Reachable IS TLVs to list the topologies the router belongs to and
the neighbors in the same topologies. Any IS-IS router that does not advertise MT capability, or does not
use the MT TLVs is considered as belonging to the default topology.
On point-to-point interfaces, if two neighboring MT capable IS-IS routers have no common topologies in
common, no adjacency is formed. On broadcast interfaces, an adjacency is formed between two or more
neighboring IS-IS routers even if there is no topology in common.
M-ISIS Operation
Each Multi-topology runs its own SPF computation. The results of the SPF computation stored in a
separate Routing Information Base (RIB), identified by the MT ID.
By default, as per normal IS-IS protocol behavior, SPF computation for IPv4 and IPv6 prefixes results in a
common SPF tree. This approach holds true when all IS-IS routers in the network are dual-stack and share
a common topology (that is, IPv4 and IPv6 traffic traversing the same links, using corresponding IP
interfaces configured on the VLANs). This computation is the non MT-capable mode of IS-IS operation.
If M-ISIS capability is enabled, different SPF computations are performed for IPv4 and IPv6 based on the
different MT IDs. Specifically, one common IPv4 SPF computation is performed for the default behavior
as well as IPv4 prefixes learned in the context of MT ID 0. IPv6 SPF computation is performed for IPv6
prefixes learned in the context of MT ID 2. In this MT-specific computation, IPv6 prefixes learned
through the default behavior (that is, if advertised in IP6 Reachable Prefixes TLV type 236), is omitted
from this computation.
Enabling M-ISIS Capability
Use the following command to enable M-ISIS capability support for IS-IS. If enabled, IPv6 SPF
computation is performed separate from the IPv4 SPF computation.
-> ip isis multi-topology
Changing the multi-topology mode with this command will result in internal disabling and re-enabling of
IS-IS protocol, with the new mode of operation. This causes IS-IS adjacencies to be reset.
If M-ISIS mode of operation is not enabled, AOS IS-IS operates in a dual-stack mode, computing a single
SPF for IPv4 and IPv6. M-ISIS TLVs received in this mode are not processed for SPF calculations.
Note. M-ISIS is advised in networks containing ISIS enabled routers with a combination of IPv4 and IPv6
capabilities.