Configuring Integrated ISIS
Integrated IS-IS is unique among the IP routing protocols covered in this book for a couple of reasons. First, it is the only protocol that must be enabled both as a process and on individual interfaces. Second, it is the only IP routing protocol that was not originally designed for IP. Because Integrated IS-IS uses CLNS PDUs rather than IP packets, the configuration is not always as obvious as that of the other protocols.
An interesting side effect of the fact that Integrated IS-IS is a CLNS protocol is that the IP addresses of neighboring routers have no influence on the formation of adjacencies. As a result, IS-IS does not have the adjacency restrictions concerning secondary IP addresses that OSPF has. However, another result is that two interfaces with IP addresses from completely different subnets can become adjacent. IP will not work in such a situation, but the fact that the adjacency makes the link only "half broken" can cause some troubleshooting confusion.
Case Study: A Basic Integrated IS-IS Configuration
A basic Integrated IS-IS process is configured on a Cisco router in four steps:
1. Determine the area in which the router is to be located and the interfaces on which IS-IS is to be enabled.
2. Enable IS-IS with the command router isis.[19]
[19] The router isis command can also be given a name, such as router isis Warsaw. If IS-IS and ISO-IGRP are configured on the same router, one or both of the processes must be named. If ISO-IGRFP is not configured, naming is unnecessary.
3. Configure the NET with the net command.
4. Enable Integrated IS-IS on the proper interfaces with the command ip router isis. This command must be added not only to transit interfaces (interfaces connected to IS-IS neighbors) but also to interfaces connected to stub networks whose IP addresses should be advertised by IS-IS.
Figure 10.41 shows a small six-router internetwork divided into two areas. In the NETs, areas 1 and 2 will be encoded as 00.0001 and 00.0002, respectively, and the System IDs will be the MAC identifiers of the E0 or TO0 interfaces of each router. Table 10.4 shows the NETs encoded with this information.
Figure 10.41. Area 1 is encoded as 00.0001 in the NET, and area 2 is encoded as 00.0002. The System ID of each NET is the E0 or T00 MAC identifier.
Figure 10.41. Area 1 is encoded as 00.0001 in the NET, and area 2 is encoded as 00.0002. The System ID of each NET is the E0 or T00 MAC identifier.
- Table 10.4. The NETs used for the IS-IS configurations of the routers in Figure 10.41.
|
Router |
Area |
MAC |
Net |
|
Paris |
00.0001 |
0000.3090.6756 |
00.0001.0000.3090.6756.00 |
|
Berlin |
00.0001 |
0000.3090.c7df |
00.0001.0000.3090.c7df.00 |
|
London |
00.0001 |
0000.0c0a.2c51 |
00.0001.0000.0c0a.2c51.00 |
|
Rome |
00.0001 |
0000.0c0a.2aa9 |
00.0001.0000.0c0a.2aa9.00 |
|
Brussels |
00.0002 |
0000.0c76.5b7c |
00.0002.0000.0c76.5b7c.00 |
|
Amsterdam |
00.0002 |
0000.0c04.dcc0 |
00.0002.0000.0c04.dcc0.00 |
The configurations of Paris, London, Brussels, and Amsterdam are: Paris clns routing
interface Serial0
ip address 10.1.255.5 255.255.255.252 ip router isis
interface TokenRing0
ip address 10.1.2.1 255.255.255.0 ip router isis ring-speed 16
interface TokenRing1
ip address 10.1.7.1 255.255.255.0 ip router isis ring-speed 16
router isis net 00.0001.0000.3090.6756.00
London clns routing
interface Ethernet0
ip address 10.1.3.2 255.255.255.0 ip router isis
interface Serial0
ip address 10.1.255.6 255.255.255.252 ip router isis
router isis net 00.0001.0000.0c0a.2c51.00
Brussels clns routing
interface Ethernet0
ip address 10.1.3.1 255.255.255.0 ip router isis
interface Ethernet1
ip address 10.1.4.1 255.255.255.0 ip router isis
router isis net 00.0002.0000.0c76.5b7c.00
Amsterdam clns routing
interface Ethernet0
ip address 10.1.4.2 255.255.255.0 ip router isis
interface Ethernet1
ip address 10.1.5.1 255.255.255.0 ip router isis
interface Ethernet2
ip address 10.1.6.241 255.255.255.240 ip router isis
router isis net 00.0002.0000.0c04.dcc0.00
The configurations of Berlin and Rome are similar. A detail that you should notice is that CLNS routing is enabled in these configurations. The CLNS routing is necessary to handle the CLNS PDUs used by ISIS. However, the clns routing command was not entered as a configuration step. Rather, the router entered it automatically when IS-IS was enabled.
Figure 10.42 shows Paris' route table. Notice that the table contains both L1 and L2 routes. By default, Cisco routers are L1/L2 routers. This fact is also apparent by observing the routers' IS neighbor tables, as in Figure 10.43.
Continue reading here: Administrative Distances
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