Tuning and Optimizing IS-IS

The Cisco IOS IS-IS configuration that configures an L1/L2 IS-IS router by default can result in the inefficient use of router and network resources and results in suboptimal routing. Although this configuration has the advantage of allowing all routers to communicate and converge without much administrative effort, it is not the most efficient way to build an IS-IS network, because routers with the default configurations send out both L1 and L2 Hellos and maintain both L1 and L2 LSDBs. Therefore, a network administrator must know how to tune IS-IS to conserve memory and bandwidth resources (only need to maintain the LSDB and send Hellos, LSPs, and SNPs for the necessary level) to achieve efficient and optimum performance.

Tuning IS-IS

The is-type level-1 and is-type level-2-only IS-IS router subcommands specify a router to act only as an internal area (L1) router or as a backbone (L2) router respectively.
The default configuration is is-type level-1-2; this is not shown in the router configuration.

Although a router can be an L1/L2 router, it might not required to establish both L1 and L2 adjacencies over all interfaces. If the router only needs to establish adjacency with an L1 router through a particular interface, it doesn’t need to send L2 Hellos out that interface, and vice versa. It wastes router and bandwidth resources to try to establish adjacencies that do not exist. Configure an interface to send only L1 or L2 Hellos using the isis circuit-type {level-1 | level-2-only} interface subcommand to tune IS-IS in such situations. Cisco IOS attempts to establish both L1 and L2 adjacencies over an interface (level-1-2) by default.
Warning: Changing the isis circuit-type will tear down existing adjacencies over the interface.

Unlike most other IP routing protocols, Integrated IS-IS does not take account of the line speed or bandwidth when setting the link metrics. All interfaces are assigned a metric value of 10, which results in suboptimal routing for networks with links of varying types and speeds. The isis metric {metric} {level-1 | level-2} interface subcommand changes the metric value (from 1 to 63). An interface can have different L1 and L2 metric values.

Below verifies the IS-IS configuration on RT1.
It has a default route i*L1 0.0.0.0/0 to the nearest L1/L2 router – RT2.

RT1#sh ip route

Gateway of last resort is 12.12.12.2 to network 0.0.0.0

     12.0.0.0/24 is subnetted, 1 subnets
C       12.12.12.0 is directly connected, FastEthernet1/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
i L1 192.168.2.0/24 [115/20] via 12.12.12.2, FastEthernet1/0
i*L1 0.0.0.0/0 [115/10] via 12.12.12.2, FastEthernet1/0
RT1#
RT1#sh clns protocol

IS-IS Router: 
  System Id: 1111.1111.1111.00  IS-Type: level-1
  Manual area address(es):
        49.0001
  Routing for area address(es):
        49.0001
  Interfaces supported by IS-IS:
        FastEthernet1/0 - IP
        FastEthernet0/0 - IP
  Redistribute:
    static (on by default)
  Distance for L2 CLNS routes: 110
  RRR level: none
  Generate narrow metrics: level-1-2
  Accept narrow metrics:   level-1-2
  Generate wide metrics:   none
  Accept wide metrics:     none
RT1#
RT1#sh isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
RT1.00-00           * 0x00000003   0x3A02        1123              0/0/0
RT2.00-00             0x00000004   0x1F62        1148              1/0/0
RT2.01-00             0x00000001   0x2630        1122              0/0/0
RT1#
RT1#sh isis neighbors

System Id      Type Interface IP Address      State Holdtime Circuit Id
RT2            L1   Fa1/0     12.12.12.2      UP    9        RT2.01
RT1#