Configuring a Level 1Only Router

2017-11-03 09:05:07

Problem

You want to set up the Level 1only routers.

Solution

For routers that participate only in a Level 1 area, disable Level 2 on the router:

[edit protocols isis] aviva@RouterA# set level 2 disable aviva@RouterA# set interface fe-0/0/0.0 aviva@RouterA# set interface lo0.0

Discussion

To configure routers that are only in a Level 1 area, just disable Level 2 routing for all of IS-IS, not for a specific interface. The configuration looks like this:

[edit protocols] aviva@RouterA# show isis { level 2 disable; interface fe-0/0/0.0; interface lo0.0; }

These routers have only a Level 1 link-state database:

aviva@RouterA> show isis database IS-IS level 1 link-state database: LSP ID Sequence Checksum Lifetime Attributes RouterG.00-00 0x198 0x3ea3 2389 L1 L2 Attached RouterA.00-00 0x1ad 0xee5f 1190 L1 RouterA.02-00 0x1a6 0x4efc 1190 L1 3 LSPs IS-IS level 2 link-state database: 0 LSPs

The Level 1 link-state database has three LSPs, while the Level 2 database is empty.

Level 1 IS-IS routers know only about paths to routers within their level. For routes outside the level, they install a default route that points to the closest Level 1Level 2 router. To confirm that the router knows about other levels, first check the link-state database for an LSP that has the Attached (ATT) bit set. The show isis database output above shows that RouterG is connected to another level:

LSP ID Sequence Checksum Lifetime Attributes RouterG.00-00 0x198 0x3ea3 2389 L1 L2 Attached

Then check that the default route is installed:

aviva@RouterA> show isis route IS-IS routing table Current version: L1: 328 L2: 322 IPv4/IPv6 Routes ---------------- Prefix L Version Metric Type Interface Via 0.0.0.0/0 1 328 10 int fe-0/0/0.0 RouterG 10.0.0.0/24 1 328 20 int fe-0/0/0.0 RouterG 10.0.1.0/24 1 328 20 int fe-0/0/0.0 RouterG 192.168.12.1/32 1 328 10 int fe-0/0/0.0 RouterG 192.168.19.1/32 1 328 10 int fe-0/0/0.0 RouterG

You see from this output that all the routes are Level 1 routes and the default route 0.0.0.0/0 is directed to RouterG, which is the nearest Level 1Level 2 router. (In this example, it also happens to be the only Level 1Level 2 router in area 20.)

Also check that the default route is installed in the main routing table, inet.0:

aviva@RouterA> show route table inet.0 inet.0: 10 destinations, 10 routes (10 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 0.0.0.0/0 *[ IS-IS/15] 00:30:06, metric 10 > to 10.0.16.2 via fe-0/0/0.0 10.0.0.0/24 *[IS-IS/15] 00:30:06, metric 20 > to 10.0.16.2 via fe-0/0/0.0 10.0.1.0/24 *[IS-IS/15] 00:30:06, metric 20 > to 10.0.16.2 via fe-0/0/0.0 10.0.16.0/24 *[Direct/0] 2d 20:28:21 > via fe-0/0/0.0 10.0.16.1/32 *[Local/0] 4d 18:42:46 Local via fe-0/0/0.0 10.0.21.0/24 *[Direct/0] 4d 20:24:04 > via se-0/0/3.0 10.0.21.1/32 *[Local/0] 4d 20:24:09 Local via se-0/0/3.0 192.168.12.1/32 *[IS-IS/160] 00:30:06, metric 10 > to 10.0.16.2 via fe-0/0/0.0 192.168.19.1/32 *[IS-IS/15] 00:30:06, metric 10 > to 10.0.16.2 via fe-0/0/0.0 192.168.42.1/32 *[Direct/0] 2d 20:53:53 > via lo0.0

The output above shows that the static route via interface fe-0/0/0 goes to RouterG (10.0.16.2) and was learned from IS-IS. The default route is advertised as an internal Layer 1 route and has a default routing preference value of 15 rather than 18, which is the default for external Layer 2 routes. Because the JUNOS software chooses routes with the lowest preference, it will select an IS-IS internal route over an external one.

In both the show isis route and show route command outputs, the only routes included are those with the area. There is no information about links outside the area.

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