One of the nice things about IS-IS is the ability to run IPv6 and IPv4 in the same protocol, over a single instance. So long as the two topologies are congruent, deploying v6 as dual stack is very simply. But what if your topologies are not congruent? The figure below illustrates the difference.
In this network, there are two topologies, and each topology has two different set of level 1/level 2 flooding domain boundaries. If topology 1 is running IPv4, and topology 2 is running IPv4, it is difficult to describe such a pair of topologies with “standard” IS-IS. The actual flooding process assumes the flooding domain boundaries are on the same intermediate systems, or that the two topologies are congruent.
One way to solve this problem today is to use IS-IS multi-topology, which allows the IPv6 and IPv4 routing information to be carried in separate TLVs so two different Link State Databases (LSDBs), so each IS can compute a different Shortest Path Tree (SPT), one for IPv4, and another for IPv6. Some engineers might find the concept of multi-topology confusing, and it seems like it might be overkill for other use cases. For instance, perhaps you do not care about incongruent topologies, but you do care about carrying IPv6 in a separate instance of IS-IS just to see how it all works, with the eventual goal of combining IPv4 and IPv6 into a single instance of IS-IS. Or perhaps there is some new feature you want to try on a production network in a different instance of IS-IS, without impacting the IS-IS instance that provides the production routing information. There are a number of use cases, of course—you can probably imagine a few.
How can these kind of use cases be solved in IS-IS? In EIGRP, for instance, the Autonomous System (AS) number is used as the EIGRP protocol port number on the wire, allowing multiple EIGRP processes to run in parallel on the same link (even though this capability has never been implemented, as far as I know). Some sort of parallel capability would need to be created for IS-IS; this is what RFC8202, IS-IS Multi-Instance, provides. Not only does RFC8202 provide this capability, it also illustrates an interesting use case for using TLVs in protocol design, rather than fixed length fields.
For readers not familiar with these concepts, fixed length field protocols marshal data into fields of a fixed length, each of which represents a single piece of information. The metadata required to interpret the data is carried entirely in the protocol specification; the protocol itself does not carry any information about the information, only the information itself. The positive attribute of working with fixed length fields is the amount of information carried on the wire is minimized. The negative is that any change in the protocol requires deploying a new version throughout the network. It is difficult to “ignore” bits that are carried without introducing failures. Further, in a fixed length field format, as new information is pushed into the protocol, either new packet formats must be created and handled, or the length of any given packet must be increased.
Type/Length/Value (TLV) formats carry the kind of information in the specification, but they carry information about the kind of information being carried, and the size of the information being carried, in the protocol itself. This means the packet format is larger, but the protocol is more flexible.
In the case of RFC8202, adding this kind of multi-instance capability in a fixed length field formatted protocol would require a shift in the packet format. In a TLV based protocol, like IS-IS, you can add new features can be added by adding a new TLV; this is precisely what RFC8202 does. To provide multi-instance capability, RFC8202 adds a new multi-instance TLV to the IS-IS PDU, which is the “outer packet format” used to carry every other kind of IS-IS information, including hellos, link state information, etc. This new TLV carries an instance ID, which differentiates each instance of IS-IS.
The instance IDs must be configured the same on each IS so they match in order to build adjacencies. Point to point and broadcast operation works the same as “standard” IS-IS, including Designated Intermediate System operation on each instance, etc. IS-IS would be implemented on each IS so each instance will have a separate LSDB, an a separate SPT would be computed across each of these LSDBs. The other key factor will be implementing multiple routing tables, and then finding some way to route traffic using the correct routing table. In the case of IPv4 and IPv6, this is fairly simple to sort out, but it would be more complex in other cases.
RFC8202 adds a new and interesting capability to IS-IS—it may take some time for vendors to implement and deploy this new capability, but this should make IS-IS more flexible in real world situations where multiple interior gateway protocol on a single network.