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might be useful when migrating two previously independent IS-IS domains into a<br />

single domain.<br />

After this simple configuration, the IS-IS protocol takes over. You do not have to<br />

configure neighbors. IS-IS automatically discovers them and establishes adjacencies<br />

with its neighbors by first sending IS-IS Hello (IIH) packets to ensure that the two<br />

ends of the link can communicate.<br />

To check that IS-IS is running on the router interfaces, use the show isis interface<br />

command:<br />

aviva@RouterG> show isis interface<br />

IS-IS interface database:<br />

Interface L CirID Level 1 DR Level 2 DR L1/L2 Metric<br />

fe-0/0/1.0 3 0x2 RouterG.02 RouterG.02 10/10<br />

fe-1/0/0.0 1 0x3 RouterG.03 Disabled 10/10<br />

lo0.0 0 0x1 Passive Passive 0/0<br />

This output shows the two Fast Ethernet interfaces we configured for IS-IS, as well as<br />

the lo0 interface. The second column, L, shows that fe-0/0/1 is a Level 1–Level 2<br />

interface (represented by the number 3) and fe-1/0/0 is a Level 1 interface. The loopback<br />

address is also listed because a NET is configured on it but it does not participate<br />

in any IS-IS level (shown as 0 in the L column). The two DR columns show the<br />

name of the router that has been elected as the DIS for that level. Interface fe-1/0/0<br />

has no Level 2 DR (Disabled) because it is a Level 1 interface. You may wonder how<br />

IS-IS discovers the name of the neighbor because it is not an IP routing protocol and<br />

hence doesn’t support DNS. The answer is that the JUNOS software supports<br />

dynamic mapping of ISO system identifiers to the hostname. If you have configured<br />

a router name with the set system host-name command, this name, and not the<br />

router’s NET, is displayed in all IS-IS output. The JUNOS implementation of IS-IS<br />

includes the hostname in the LSP, using the dynamic hostname TLV, type 137, to<br />

cache the symbolic name of the router.<br />

In the DR columns for the lo0.0 interface, the interface is shown as Passive, which is<br />

the default when you configure IS-IS on the loopback interface.<br />

The last column shows the link’s Layer 1 and Layer 2 metrics, which are 10 by<br />

default.<br />

You can see a brief summary of the adjacencies the router has established with the<br />

show isis adjacencies command:<br />

aviva@RouterG> show isis adjacency<br />

Interface System L State Hold (secs) SNPA<br />

fe-0/0/1.0 RouterH 2 Up 21 0:5:85:c1:d1:d1<br />

fe-1/0/0.0 RouterA 1 Up 6 0:5:85:ca:ca:70<br />

The output shows the two interfaces we configured. The interface fe-0/0/1 participates<br />

in a Level 2 area, connecting to RouterH, and interface fe-1/0/0 connects to<br />

RouterA. Notice that the lo0 interface is not listed because it doesn’t form any adja-<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.<br />

Configuring IS-IS | 353

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