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| 7.3 |  | IS-IS Operation |
| 7.3.6 |  | IS-IS adjacencies |

IS-IS uses Hello PDUs to establish adjacencies with other routers (ISs) and ESs. Hello PDUs carry information about the system, its parameters, and its capabilities.

ISs use IIHs to establish and maintain neighbor relationships. Once an adjacency is established, the ISs exchange link-state information with LSPs.

ISs also send out ISHs. ESs listen for these ISHs and randomly pick an IS, the first one heard, to forward all their packets to. OSI ESs require no configuration to forward packets to the rest of the network.

ISs listen to the ESHs and learn about all the ESs on a segment. ISs include this information in their LSPs. For particular destinations, ISs may send redirect messages to ESs to provide them with an optimal route off the segment.

Now consider adjacencies between ISs. Separate adjacencies are established for Level 1 and Level 2. If two neighboring routers in the same area run both Level 1 and Level 2, they will establish two adjacencies, one for each level. The L1 and L2 adjacencies are stored in separate L1 and L2 adjacency tables.

On LANs, two adjacencies are established with specific L1 and L2 IIH PDUs. Routers on a LAN establish adjacencies with all other routers on the LAN with the same area ID and level. This is unlike OSPF, where routers establish adjacencies only with the Designated Router. On LANs, IS-IS PDUs are multicast to the well-known MAC addresses. IIH PDUs announce the area ID. Separate IIH PDUs announce the Level 1 and Level 2 neighbors. For example, where a LAN has routers from two areas attached, the routers from one area accept Level 1 IIH PDUs only from their own area. Therefore, they only establish adjacencies with their own area routers. The routers from a second area similarly accept Level 1 IIH PDUs only from their own area. The L2 routers, or the L2 process within any L1L2 router, accept only L2 IIH PDUs and establish only L2 adjacencies.

On point-to-point WAN links there is a common IIH format, part of which specifies whether the Hello message relates to Level 1, Level 2, or both. The area ID is also announced in the Hello messages.

By default, Hello PDUs are sent every ten seconds. The timeout to declare a neighbor down is 30 seconds, which equals three missing Hello packets. These timers can be reconfigured using the isis Hello-interval and isis Hello-multiplier interface configuration commands.

Various combinations are possible for links between L1, L2, and L1L2 routers. L1 routers in the same area, which includes links between L1 only and L1L2 routers, exchange IIH PDUs specifying L1 and establish an L1 adjacency. L2 routers, when in the same area or between areas, and including links between L2 only and L1L2 routers, exchange IIH PDUs specifying L2 and establish an L2 adjacency. Two L1L2 routers in the same area establish both L1 and L2 adjacencies. The two routers maintain these with a common IIH PDU specifying both the L1 and L2 information. Two L1 routers that may be physically connected but are not in the same area will exchange L1 IIH PDUs. This includes an L1 only to an L1L2 router in a different L1 area. However, they ignore these as the area IDs do not match. Therefore, they do not establish an adjacency.

L1 only routers establish L1 adjacencies. L2 routers establish L2 adjacencies, between areas. L1L2 routers establish both L1 and L2 adjacencies with their L1L2 neighbors in the same area. L2 adjacencies exist independent of areas and must be contiguous; for example, Area 2 is not the backbone area. The backbone in IS-IS is exactly the contiguous set of routers and connections between L2 and L1L2 routers. The backbone may traverse multiple areas.

| Interactive Media Activity Matching: IS-IS Level 2 Adjacencies Upon completion of this activity, the student will be able to identify the Level 2 adjacencies between routers in an IS-IS network.