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Again, we expect the metric to change from the default of 10 to 500, but it’s showing<br />

a metric of 73. Why isn’t the metric value changing to 500? The reason is because<br />

IS-IS uses two kinds of metrics, narrow and wide. By default, the JUNOS software<br />

uses the IS-IS narrow metrics, which are defined in the original IS-IS standards documents<br />

as 8-bit values. With narrow metrics, the router can’t advertise a metric<br />

greater than 63. If it receives a metric value greater than 63, IS-IS clips it to 63. This is<br />

why the link-state database shows a metric of 63 and the neighbor’s routing table<br />

shows a metric of 73 (the default metric of 10 plus 63). To resolve this problem, configure<br />

IS-IS to use wide metrics:<br />

[edit protocols isis]<br />

aviva@RouterG# set level 1 wide-metrics-only<br />

Wide metrics, defined in RFC 3784, can be values greater than 63. Looking again at<br />

the link-state database shows that IS-IS is now advertising the metric value of 500:<br />

aviva@RouterG> show isis database extensive<br />

IS-IS level 1 link-state database:<br />

RouterG.00-00 Sequence: 0xe5, Checksum: 0x912c, Lifetime: 492 secs<br />

IP prefix: 10.0.1.0/24 Metric: 10 Internal Up<br />

IP prefix: 10.0.16.0/24 Metric: 10 Internal Up<br />

IP prefix: 192.168.12.1/32 Metric: 500 External Up<br />

IP prefix: 192.168.19.1/32 Metric: 0 Internal Up<br />

TLVs:<br />

...<br />

IP external prefix: 192.168.12.1/32, Internal, Metric: 500, Up<br />

The neighbor’s routing table also reflects the change:<br />

aviva@RouterA> show route<br />

inet.0: 6 destinations, 7 routes (6 active, 0 holddown, 0 hidden)<br />

+ = Active Route, - = Last Active, * = Both<br />

192.168.12.1/32 *[IS-IS/160] 00:14:56, metric 500<br />

> via se-0/0/3.0<br />

11.9 Leaking IS-IS Level 2 Routes into Level 1<br />

Problem<br />

Your IS-IS network has several areas, some which have multiple exit points to your<br />

core area, and you want to increase the routing efficiency of your Level 1 routers.<br />

Solution<br />

On the Level 1–Level 2 router, create a policy to leak a specific Level 2 route into the<br />

Level 1 link-state database:<br />

[edit policy-options policy-statement level2-leaking term 1]<br />

aviva@RouterJ# set from protocol isis<br />

aviva@RouterJ# set from level 2<br />

aviva@RouterJ# set from route-filter 10.0.21.0/24 prefix-length-range /32-/32<br />

Leaking IS-IS Level 2 Routes into Level 1 | 371<br />

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

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.

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