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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.
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 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.
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