IS-IS has a two-layer hierarchy. We have Level-2, which is the backbone, and we have Level-1, which is the edge. A router can exist as a Level-1 router, or a Level-2 router, or a Level-1-2 router, where it sits in both Level-1 and Level-2. Most networks today, can quite happily use just Level-2. IS-IS is multi-protocol. Integrated IS-IS carries CLNS and IPv4 address families. RFC 5308 adds IPv6 address family support, and RFC 5120 adds multi topology support. IS-IS, of course is extended to carry IPv6 prefixes, through RFC 5308, and it can share topology with IPv4, where the IPv4 and IPv6 topologies are identical or we can use multi topology, which means that IPv6 topology is independent of IPv4. The latter is preferred, as it allows network operators to incrementally roll out IPv6 completely independent of IPv4. There are two types of links in IS-IS. There's a point-to-point link, where only one other router is on the link forming a point-to-point adjacency. And there's a multi-access network for example, Ethernet, where there's the potential for many other routers to be on the network with several other adjacencies. IS-IS and multi-access networks have optimizations to aid scaling. One router is elected to originate at the LSPs for the whole multi-access network, and this is called a Designated Intermediate System or DIS. Other routers on the multi-access network form adjacencies with the DIS. There is one designated router per multi-access network. It generates the the network link advertisements and assists in the database synchronization. And this lets us scale IS-IS for multi-access and Ethernet networks. To select the designated router it is configured by priority, and it's done per interface. Most network operators manually configure their chosen DIS with the highest possible priority. Typically there will be two high priority routers on a network, even though IS-IS only chooses one to be the DIS. Operators would select one router to be the highest priority, (127) and a backup router to the next highest priority, (126). In the absence of any priority set, the routers on the multi-access network will determine priority by the highest MAC address. And the best practice is really to try and avoid this. If an Ethernet card on the multi-access network changes, the device gets a new MAC address, and it's quite likely there'll be a different DIS. To avoid this lack of determinism, operators will manually configure the priority so the DIS is always the same device.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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