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A DIS is elected for each pseudonode. Every router elects itself as the DIS, and the router with the highest priority becomes the DIS. By default, all Cisco routers have a priority of 64. In the case of a tie, the router with the highest MAC address becomes the DIS.

The DIS has two functions: It creates and updates the pseudonode. The DIS also conducts flooding over the broadcast network.

The DIS multicasts Complete Sequence Number Protocol (CSNP) data units every 10 seconds. As the name indicates, the CSNP contains headers of all the LSPs. Unlike OSPF, no backup DIS exists and the election process is dynamic. A dynamic process indicates that, if a higher-priority router appears after a DIS is elected, this router is automatically elected as the DIS.

Because IS-IS does not have as many types of link states in its database as OSPF does, synchronization of its database is not expensive. Unlike OSPF, the CSNP is sent every 10 seconds; as in the case of OSPF, the database synchronization happens only at the time of initial adjacency.

A pseudonode LSP is flooded in only two cases:

• When a new neighbor is added to the broadcast network. If the new IS has higher priority, this new one becomes the DIS for the network. The neighbor information changes, so the LSP is flooded.

• When the refresh timer for the LSP has expired; the refresh time is 20 minutes.

Using IS-IS Non-Pseudonode

A non-pseudonode is created by IS for propagating information about all other types of links connected to the router that are not broadcast networks, such as point-to-point networks and stub networks. A non-pseudonode LSP carries information about all neighbors, attached prefixes, and metrics of the attached links.

NOTE

A non-pseudonode could be equated to a router LSA in OSPF. In this case, the IS informs the router about different types of links that are attached to it, and the cost of reaching those links. It also carries a complete list of neighbors attached to it.

Non-pseudonode LSP is generated in four cases: when any neighbor is added or deleted; when an IP prefix has changed; when there is a metric change on the connected network; and at every refresh interval.

Understanding Level 1 and Level 2 Routing

In level 1, IS nodes are based on the ID portion of the address. All level 1 routers route within their own area. They recognize the destination within their area by reading the destination address. If the destination is within the same area, the packet is routed to the destination. If the destination is not within the same area, it is sent to the closest level 2 router.

In IS-IS, all level 1 areas are stub areas, so no information is sent to level 1 routers that are outside their areas. All routers within the level 1 area maintain identical databases. A level 1 router will have the area portion of its address manually configured. It will not create neighbors with a node whose area addresses do not match its area ID.

NOTE

For migration reasons, if the level 1 router has an area address of 1, 2, and 3; and the neighbor has an area address of 2 and 4, the two routers will form a neighbor adjacency because they share one area number in common.

The level 1 routers that belong to the same area should be connected. In an unlikely case that a level 1 area becomes partitioned, an optional partitioned repair function allows the partition to be repaired via use of level 2 routes.

For an IP subnet, each level 1 router exchanges link-state packets that identify the IP address reachable by every router. Information about each IP address is sent in the link-state packet and includes the IP address, the subnet mask, and the metric. Each level 1 router is manually configured with the IP address, the subnet mask, and the IS-IS metric.

IS-IS carries subnet information in the LSP, which enables the network administrator to configure VLSM. When a packet must be sent to a destination that matches more than two IP prefixes, the packet is routed based on the longest prefix. A default route could be announced into IS-IS with the network mask of all zeros.

In level 2, the IS nodes are routed based on the area address. All level 2 routers route toward areas without considering the internal structure of the area. A level 2 router could also be a level 1 router for some areas.

A level 2 router accepts another level 2 router as a neighbor, regardless of the area address. If the area address does not overlap on a link, the link is considered a level 2 link only, so the router will send only level 2 LSPs.

NOTE

Level 2 routers form the backbone of IS-IS. All level 2 routers must be contiguous. If level 2 routers become partitioned, no provision exists for using level 1 routers to repair level 2 partitions.

If a single level 2 router loses connectivity to the level 2 backbone, the level 2 router will indicate in its level 1 LSPs that it is not attached. By doing this, the level 2 router indicates to all other level 1 routers that it is not attached. This signals all level 1 routers to use some other level 2 router to connect to the networks outside that area.

Cisco routers default to L1 and L2, which means that the router must maintain two databases: one for level 1 and another for level 2. This enlarges the backbone more than is required. Always be sure to configure level 1 only when the router is not connected to the backbone. Running both L1 and L2 is unnecessary and is not scalable.

IS-IS Packets

As discussed earlier, link-state protocols maintain information received from neighbors. Any information received from a neighbor is maintained in a database. Because link-state protocols require the information to be constant among neighbors, IS-IS has different protocol packets. Essentially, IS-IS has four packet types:

• Hello packets

Each of these is discussed in more detail in the following sections.

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