IS-IS
Overview
IS-IS is a link-state interior gateway protocol that uses the shortest path first algorithm to determine a route. Routing decisions are made using the link-state information. IS-IS entities include:
End systems and intermediate system protocols allow devices and NEs to identify each other. The IS-IS protocol sends link state updates periodically through the network, so each device can maintain current network topology information.
Large networks, or autonomous systems, are supported by IS-IS using a two-level hierarchy. This divides a large area into more manageable, smaller areas. The first level (level 1) of routing is performed within an area. The second level (level 2) of routing is performed between areas, as shown in the following figure.
Figure 28-3: IS-IS routing areas example
Level 2 areas are also called backbones, similar to an OSPF backbone area. All traffic traversing different areas must traverse the backbone. A device can be configured as level 1, level 2, or both level 1 and 2. In this example, routers A, B, M, H, and X form the level 2 IS-IS backbone. The connection between routers A and B carries both level 1 and level 2 link-state PDUs (LSPs). However, level 1 devices are only aware of their own area’s topology, and must forward traffic to a level 1/2 device to forward the data to another area.
Two devices are in the same level 1 area when they have level 1 adjacency. Level 1 adjacency occurs when the area IDs are common and there is a level 1 connection between the devices. Level 2 adjacency occurs when it has at least one level 1 and 2, or one level 2 interface configured.
The NFM-P supports the configuration of IPv6 addresses for IS-IS adjacencies.
Note: If two neighboring devices in the same level 1 area run both level 1 and 2, they establish both a level 1 and a level 2 adjacency.
When LDP over RSVP is enabled for IS-IS, LSPs can be used by the IGP to calculate its SPF tree. The IGP then provides LDP with all of the ECMP IDP next-hops and tunnel endpoints that the IGP identifies as the lowest cost path to the destination. If an IGP calculation and an LDP over RSVP have the same cost, LDP chooses an LDP over RSVP tunnel over an IGP route and ECMP between the two types is not considered. The type and number of tunnels that are to be considered by LDP depend on the IGP costs, where the lowest cost between the tunnel endpoint and the target is selected.
After IS-IS is configured, routing occurs as follows:
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Hello PDUs are sent to IS-IS-enabled interfaces to discover neighbors and establish adjacencies.
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Link-state PDUs (LSPs) are created based on local interfaces and prefixes that are learned from adjacent devices.
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The devices flood LSPs to adjacent neighbors, and build a link-state database.
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A shortest path tree is calculated by the IS-IS and the routing table is built.
Flexible Algorithm support
Flexible IGP algorithms allow IGPs to compute constraint-based paths over the network. A flexible algorithm definition can be configured for each NE in the global NE properties. The flexible algorithm definition provides topology constraints on a per-NE basis. A flexible algorithm definition requires an administrative group; see To configure an administrative group policy.
A set of flexible algorithm definitions creates a flexible algorithm topology. If flexible participation has been configured on an ISIS instance, the IGP uses constraint-based shortest path first and the flexible algorithm definitions to find the best paths through the flexible algorithm topology.
IPv6 Traffic Engineering support
The NFM-P supports IPv6 Traffic Engineering (TE) extensions for IS-IS.
In addition, ISIS supports advertising which protocol is enabled on a given TE-link (SR-TE, RSVP-TE, or both) via the Application Specific Link Attributes sub-TLV. This allows the advertising router to send potentially different Link TE attributes for RSVP-TE and SR-TE applications.
To enable this feature, in summary, you must: