The PReparation for configuring any routing protocol requires a sound understanding of the network topology and a coherent addressing scheme. When you have these, the basic configuration of Integrated IS-IS is as follows: Step 1 Enable the router process Integrated IS-IS with the router isis command. Step 2 Configure the NET address, thus assigning the area with the net networkaddress router subcommand. Step 3 Enable Integrated IS-IS for ip on the relevant interfaces with the ip router isis interface subcommand. Figure 7-9 illustrates a simple network to support the working configuration example. The figure shows the topology of the network and the addressing scheme that was used. Figure 1 Basic Integrated IS-IS Configuration
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Example 7-1 shows the basic configuration required to run Integrated IS-IS. The relevant commands are highlighted for easy identification, and a brief eXPlanation of the code is inserted after an exclamation mark to make the code easier to read. Note the router process is started and the NET address defined immediately beneath this router process because the IS-IS address is assigned to the router and as sUCh is a part of the routing process. IS-IS is started on the interface after the IS-IS router process is defined with a NET address. Example 7-1 Basic Integrated IS-IS Configuration
Working Example for Router A interface Ethernet0 ip address 140.100.96.1 255.255.255.0 ! assign the IP address and mask ip router isis ! start IS-IS on the interface ! interface Serial0 no ip address encapsulation frame-relay no fair-queue clockrate 56000 ! interface Serial0.1 point-to-point ip address 140.100.64.1 255.255.255.0 ! assign the IP address and mask ip router isis ! start IS-IS on the interface frame-relay interface-dlci 629 ! interface Serial0.2 point-to-point ip address 140.100.32.1 255.255.255.0 ! assign the IP address and mask ip router isis !start IS-IS on the interface frame-relay interface-dlci 931 ! ! router isis net 49.0001.0000.0000.000a.00 ! Start the IS-IS routing process and define the ISO address for the entire router. ! ip classless
Remember that by default Cisco routers are configured as Level 1-2 routers to avoid suboptimal routing. Also the clns routing command is not required for IP-only IS-IS. In some versions of the Cisco IOS software, clns routing is written into the configuration file automatically by Cisco, when the routing process for IS-IS is configured.
Changing the Router from Level 1-2 Changing the routing level in Integrated IS-IS is done to overcome Integrated IS-IS consuming resources on both the routers and the media. Using the topology in Figure 7-10, it is possible to configure routers D, E, and F as Level 1 routers because they are internal to their areas. The command is just is-type level-1. This is a subrouter command and as such is placed under the router isis command. Figure 2 Changing the Level of Routing Both for the Entire Router and at the Interface Level
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It is also possible to change the level of routing to Level 1 at the interface level by issuing the isis circuit-type level-1 command under the interface. In Figure 7-10, this is done on the Routers A, B, and C, the Ethernet interface pointing is configured to be running Level 1 routing, while the serial interface runs only Level 2 routing. Level 1-2 routers send LSPs with an ATT bit in their Level 1 LSPs, indicating that they are attached to another area. This creates a default route in the Level 1 router, pointing to the nearest Level 2 router. This allows the nearest Level 2 router to be the transit router for all data destined for another area. As illustrated in Figure 7-10, Example 7-2 shows Router A configured as Level 1 on the Ethernet interface pointing to Router D; whereas, the other interfaces are configured as Level 2-only. The relevant commands are highlighted for easy identification. Example 7-2 Changing Router A Routing Level on an Interface Basis
Working Example for Router A interface Ethernet0 ip address 140.100.96.1 255.255.255.0 ip router isis isis circuit-type level-1 !Configure Level 1 routing on the interface ! interface Serial0 no ip address encapsulation frame-relay no fair-queue clockrate 56000 ! interface Serial0.1 point-to-point ip address 140.100.64.1 255.255.255.0 ip router isis frame-relay interface-dlci 629 isis circuit-type level-2-only ! Configure Level 2 routing on the interface ! interface Serial0.2 point-to-point ip address 140.100.32.1 255.255.255.0 ip router isis frame-relay interface-dlci 931 isis circuit-type level-2-only !Configure Level 2 routing on the interface ! ! router isis net 49.0001.0000.0000.000a.00 ! ip classless
Example 7-3 shows the configuration command to change the level of routing on Router D to be Level 1. Note that this command appears under the router isis command, changing the behavior of the entire process rather than at the interface level. This example is illustrated in Figure 7-10. The relevant commands are highlighted for easy identification.
Example 7-3 Changing the Level of Routing for the Entire Router
Working Example for Router D interface Ethernet0 ip address 140.100.96.2 255.255.255.0 no ip directed-broadcast ip router isis ! interface Serial0 no ip address no ip directed-broadcast no ip mroute-cache shutdown no fair-queue ! interface Serial0.1 no ip directed-broadcast ! interface Serial1 no ip address no ip directed-broadcast shutdown ! interface BRI0 no ip address no ip directed-broadcast shutdown ! router isis net 49.0001.0000.0000.000d.00 is-type level-1 !Define the router as a Level 1 router ! ip classless
Configuring Summarization Configuring summarization is straightforward. The complexity is in the addressing scheme design. To configure summarization, enter the summary-address command followed by the summary address and mask under the router process. Figure 7-11 shows the topology used to support the working example shown in Example 7-4. In Figure 7-11, the IP routes from Router B are summarized into areas 0001 and 0003. Router B is capable of summarizing routes because the router is both a Level 1 and Level 2 router, acting as an ABR in OSPF. Example 7-4 shows Router B summarizing routes 140.100.104.0 through to 140.100.107.0 with a mask summarized from /24 to /22. The relevant commands are highlighted for easy identification.
Figure 3 Summarizing IP Addresses Between IS-IS Areas Example 7-4 Summarization of IP Routes from Router B
Working Example for Router B interface Ethernet0 ip address 140.100.104.1 255.255.255.0 no ip directed-broadcast ip router isis isis circuit-type level-1 ! interface Serial0
