HOW TO
• Relates to: CCNA | CCDA | CCNP | CCDP | CCIP | CCIE
This week's newsletter will be the last edition of a month-long series on
configuring Integrated IS-IS on Cisco devices. In past articles I have covered
IS-IS principles, configuring IS-IS in a single area scenario, and
troubleshooting IS-IS adjacency problems. This week, I will be writing about
configuring IS-IS in a multiple area scenario.
As I have stated in the
past, IS-IS is similar to OSPF. When there are multiple IS-IS areas, there is a
backbone area (similar to area 0 in OSPF networks) that all other areas connect
to. In the scenario that I will be walking through, there are two areas and
three routers connected in an IS-IS network. A diagram of the network layout is
available here.
RouterC
is a core router in the backbone area (area 51). RouterA is an inter-area router
in area 12, and RouterB is an edge router connecting area 12 and area 51. All
routers are loaded with a version of IOS that supports multi-area
IS-IS.
Similar to configuring IS-IS in a single area, IS-IS is enabled on
each router, a CLNP address is configured, and the IS-IS instance on each
interface that will participate in IS-IS routing. The following are the commands
used to configure RouterB: RouterB (config)# router isis area12
RouterB (config-router)# net 12.0000.0000.0000.000b.00
RouterB (config-router)# interface ethernet0/0
RouterB (config-if)# ip router isis area12
RouterB (config-if)# interface serial1/0
RouterB (config-if)# ip router isis area12
RouterB (config-if)# interface serial1/1
RouterB (config-if)# ip router isis area12 The area12 at the end of
lines 1, 4, 6, and 8 is the area tag used to identify the IS-IS instance. If the
IS-IS area tag applied to an interface does not match the tag for the IS-IS
router instance, IS-IS will not work properly. Since we are trying to configure
integrated IS-IS (IP routing over IS-IS), we have to make sure that IP routing
is enabled on the router. The actual area number based on the CLNP address for
RouterB is 12.0000.
Once IS-IS is configured on all of the routers, IP
packets should be routable from the Ethernet side of RouterA to the Ethernet
side of RouterC. However, further configuration is possible. Since RouterA will
only be participating in inter-area IS-IS, the router type can be changed from
the default of Level 1-2 to Level 1 (inter-area only). Also, since RouterC is a
core router used for intra-area routing, the router type could be changed to
Level 2. RouterC was configured as a Level 2 router with the following commands:
RouterC (config)# router isis area51
RouterC (config-router)# net 51.0000.0000.0000.000c.00
RouterC (config-router)# is-type level-2-only Notice the CLNP
address assigned to RouterC places it in area 51. The is-type command assigns
the router type for the specific IS-IS instance. An interface can be assigned a
specific IS-IS type with the isis circuit-type command. Options for both
commands are level-1, level-1-2 (default), or level-2-only. The IS-IS specific
components of the configurations for each router can be found here.
To
check the configuration of the routers the following commands can be used: show clns protocol-shows current configuration
show clns neighbors -shows clns adjacencies
show isis database-shows LSPs in the link-state database
show isis tobology-lists the system IDs of know IS-IS routers The
output of the show clns protocol on RouterA is as follows: RouterA#sh clns prot
IS-IS Router: area12
System Id: 0000.0000.000A.00 IS-Type: level-1
Manual area address(es):
12.0000
Routing for area address(es):
12.0000
Interfaces supported by IS-IS:
Serial0 - IP
Ethernet0 - IP
Redistributing:
static
Distance: 110 The output includes the area ID, the system ID,
IS-IS interfaces, and the metric (in this case the default 110).
Looking
at the IP routing table of RouterA, we see the following: RouterA#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is 192.168.1.6 to network 0.0.0.0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.4 is directly connected, Serial0
192.168.2.0/30 is subnetted, 1 subnets...
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