IS-IS Multiarea Networks

Module Introduction

Before you start the hands-on part of this module, you should load the appropriate configuration and verify that the testbed is up and running by executing the corresponding robot file:

student@tour:~/trainings_resources/robot$ robot isis_multiarea/isis_multiarea_setup.robot

In order to get a better understanding, the lab setup is shown in the picture below.

Figure 1. IS-IS Level-2 Lab Setup

Configuring IS-IS Level-2 Routing

In the previous module we restricted the IS-IS Routing to Level-1 only by disabling the Level-2 adjacencies on the corresponding interfaces. If you want to configure a Level-2 only router, you have to disable Level-1 adjacencies in a similar way.

Exercise 1: Configuring Level-2 Routing

R1 is part of IS-IS Area 49.0001. Please configure Level-2 only adjacencies to R2 (hostif-0/0/1) and R3 (hostif-0/0/2). The authentication type is md5, the key-id 2 and the password rtbrick.

Click to reveal the answer

cfg> set instance default protocol isis system-id 1921.6800.0001
cfg> set instance default protocol isis area 49.0001/24
cfg> set instance default protocol isis hostname R1
cfg> set instance default protocol isis router-id 192.168.0.1
cfg> set instance default protocol isis authentication level-2 type md5
cfg> set instance default protocol isis authentication level-2 key1-encrypted-text $21e4946e31b406de98b3077aef03ed5a7
cfg> set instance default protocol isis authentication level-2 key-id1 2
cfg> set instance default protocol isis interface hostif-0/0/1/0 level-1 adjacency-disable true
cfg> set instance default protocol isis interface hostif-0/0/2/0 level-1 adjacency-disable true
cfg> set instance default protocol isis interface lo-0/0/0/0 level-1 adjacency-disable true
cfg> commit
cfg> show isis neighbor
Instance: default
  Interface          System         Level   State   Type    Up since                Expires
  hostif-0/0/1/0     R2             L2      Up      P2P     Tue Apr 04 08:55:12     in 22s 835945us
  hostif-0/0/2/0     R3             L2      Up      P2P     Tue Apr 04 08:55:16     in 26s 771878us

As R1 is now part of Level-2, it should have the full reachability information. Let’s take a look at the routing table for the loopback address of R4:

cfg> show route prefix 192.168.0.4/32
Instance: default, AFI: ipv4, SAFI: unicast
Prefix/Label                   Source            Pref    Next Hop                  Interface
192.168.0.4/32                 isis              18      172.16.0.6                hostif-0/0/2/0
                                                         172.16.0.2                hostif-0/0/1/0

From the output, we can see that R1 has two equally good paths to R4. The routing table does not provide any details about the metric. Remember, IS-IS is a link-state protocol preferring the path with the lowest metric. The routing table is populated by the IS-IS route table:

cfg> show isis route ipv4 unicast
Instance: default, AFI: ipv4, SAFI: unicast
  Prefix                Level   Metric     Type         Next Hop          Interface
  172.16.0.0/30             2   1000000    Internal     n/a               local
  172.16.0.4/30             2   1000000    Internal     n/a               local
  172.16.0.8/30             2   1000100    Internal     172.16.0.2        hostif-0/0/1/0
  172.16.0.12/30            2   1000100    Internal     172.16.0.6        hostif-0/0/2/0
                                                        172.16.0.2        hostif-0/0/1/0
  172.16.0.16/30            2   1000100    Internal     172.16.0.6        hostif-0/0/2/0
  192.168.0.1/32            2   1000000    Internal     n/a               local
  192.168.0.2/32            2   1000010    Internal     172.16.0.2        hostif-0/0/1/0
  192.168.0.3/32            2   1000010    Internal     172.16.0.6        hostif-0/0/2/0
  192.168.0.4/32            2   1000110    Internal     172.16.0.6        hostif-0/0/2/0
                                                        172.16.0.2        hostif-0/0/1/0

For the directly connected interfaces, IS-IS uses a default metric of 1000000. The total metric to reach 192.168.0.4/32 is 1000110. The metric can be modified on a per-interface and per-level basis using the syntax set instance <instance_name> protocol isis interface <ifl> <level> metric <metric>.

Exercise 2: IS-IS Metric

The IS-IS SPF calculation should result in two equally to node R4 (192.168.0.4/32). Configure the IS-IS interfaces in such a way, that the path to R4 via R2 is preferred. The metric should be set in such a way, that the path to 192.168.0.3/32 via hostif-0/0/2 is still best.

Click to reveal the answer

The overall metric for the path R1-R2-R4 is currently 1000000+100+10=1000110 and for the path R1-R3-R4 also 1000000+100+10=1000110. If we change the link metric for hostif-0/0/1 to 100, then metric(R1-R2-R4)=100+100+10=210. However, the metric(R1-R2-R3)=100+100+10 and metric(R1-R3)=1000000+10=1000010. In order to achieve our goal, we need to adjust the metric for hostif-0/0/2 to a value bigger than 100 but lower than 200:

cfg> set instance default protocol isis interface hostif-0/0/1/0 level-2 metric 100
cfg> set instance default protocol isis interface hostif-0/0/2/0 level-2 metric 150
cfg> commit
cfg> show isis route ipv4 unicast
Instance: default, AFI: ipv4, SAFI: unicast
  Prefix               Level   Metric     Type         Next Hop          Interface
  172.16.0.0/30            2       100    Internal     n/a               local
  172.16.0.4/30            2       150    Internal     n/a               local
  172.16.0.8/30            2       200    Internal     172.16.0.2        hostif-0/0/1/0
  172.16.0.12/30           2       200    Internal     172.16.0.2        hostif-0/0/1/0
  172.16.0.16/30           2       250    Internal     172.16.0.6        hostif-0/0/2/0
  192.168.0.1/32           2   1000000    Internal     n/a               local
  192.168.0.2/32           2       110    Internal     172.16.0.2        hostif-0/0/1/0
  192.168.0.3/32           2       160    Internal     172.16.0.6        hostif-0/0/2/0
  192.168.0.4/32           2       210    Internal     172.16.0.2        hostif-0/0/1/0

Summary

If you have completed the exercise, you can check the results by executing

student@tour:~/trainings_resources/robot$ robot isis_multiarea/isis_multiarea_verify.robot