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Juniper Configuring a Level 1 Only Router
 
You want to set up the Level 1only routers.
For routers that participate only in a Level 1 area, disable Level 2 on the router:
 
[edit protocols isis]
source@RouterA# set level 2 disable
source@RouterA# set interface ge-0/0/0.0
source@RouterA# set interface lo0.0
 
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]
source@RouterA# show
isis {
     level 2 disable;
     interface ge-0/0/0.0;
     interface lo0.0;
     }
 
These routers have only a Level 1 link-state database:
 
source@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:
 
source@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  ge-0/0/0.0 RouterG
10.0.0.0/24        1     328       20 int  ge-0/0/0.0 RouterG
10.0.1.0/24        1     328       20 int  ge-0/0/0.0 RouterG
192.168.12.1/32    1     328       10 int  ge-0/0/0.0 RouterG
192.168.19.1/32    1     328       10 int  ge-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:
 
source@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 ge-0/0/0.0
10.0.0.0/24        *[IS-IS/15] 00:30:06, metric 20
                    > to 10.0.16.2 via ge-0/0/0.0
10.0.1.0/24        *[IS-IS/15] 00:30:06, metric 20
                    > to 10.0.16.2 via ge-0/0/0.0
10.0.16.0/24       *[Direct/0] 2d 20:28:21
                    > via ge-0/0/0.0
10.0.16.1/32       *[Local/0] 4d 18:42:46
                      Local via ge-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 ge-0/0/0.0
192.168.19.1/32    *[IS-IS/15] 00:30:06, metric 10
                    > to 10.0.16.2 via ge-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 ge-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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