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IS-IS Areas
This article will demonstrate IS-IS area(s) and the adjacencies formed in different scenarios. All Cisco IOS routers are IS-IS Level-1-2 routers by default. A Level-1 router can form adjacency with another Level-1 or Level-1-2 router only, and a Level-2 router can form adjacency with a Level-2 or Level-1-2 router only.
By default, a Level-1-2 router does not forward Level-2 routes to Level-1 routers. However, Level-1 routes are forwarded to Level-2 routers. For a Level-1 router to reach a prefix in Level-2 area, it must forward the packets to Level-1-2 router.
All Level-1 routers within an area (including Level-1-2 routers) maintain an identical LSDB.
In IS-IS, a router resides completely within a single area, the Area ID is associated with the entire router rather than an interface.
Scenario 1: R1 is a Level-1 router, R2 is a Level-1-2 router and R3 is a Level-1-2 router
The IS-IS configuration on the routers is as below:
R1 routerinterface Loopback 0 ip address 1.1.1.1 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.1.1.1 255.255.255.0 ip router isis!router isis is-type level-1 net 49.0001.1111.1111.1111.00 passive-interface Loopback 0! R2 routerinterface Loopback 0 ip address 2.2.2.2 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.1.1.2 255.255.255.0 ip router isis!interface Serial 1/1 ip address 10.2.2.1 255.255.255.0 ip router isis!router isis net 49.0001.2222.2222.2222.00 passive-interface Loopback 0! R3 routerinterface Loopback 0 ip address 3.3.3.3 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.2.2.2 255.255.255.0 ip router isis!router isis net 49.0001.3333.3333.3333.00 passive-interface Loopback 0!The following output shows the adjacencies formed between R1, R2 and R3 routers. Notice that a Level-1 adjacency is formed between R1 and R2. But a Level-1-2 adjacency is formed between R2 and R3 as they are both L1/L2 routers.
Adjacencies formed
R1# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id2222.2222.2222 L1 Se1/0 10.1.1.2 UP 21 00R2# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id1111.1111.1111 L1 Se1/0 10.1.1.1 UP 29 003333.3333.3333 L1L2 Se1/1 10.2.2.2 UP 23 00R3# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id2222.2222.2222 L1L2 Se1/0 10.2.2.1 UP 25 01R1 router learns about prefixes beyond R2 router as Level-1 prefixes. R2 and R3 routers have both Level-1 and Level-2 LSDBs, and hence, R1 learns the prefixes.
R1 routing table
R1# show ip route isis | begin GatewayGateway of last resort is not set 2.0.0.0/32 is subnetted, 1 subnetsi L1 2.2.2.2 [115/10] via 10.1.1.2, 00:15:52, Serial1/0 3.0.0.0/32 is subnetted, 1 subnetsi L1 3.3.3.3 [115/20] via 10.1.1.2, 00:00:03, Serial1/0 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masksi L1 10.2.2.0/24 [115/20] via 10.1.1.2, 00:00:27, Serial1/0Scenario 2: R1 is a Level-1 router, R2 is a Level-1-2 router and R3 is a Level-2 router
The IS-IS configuration on the routers is as below:
R1 routerinterface Loopback 0 ip address 1.1.1.1 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.1.1.1 255.255.255.0 ip router isis!router isis is-type level-1 net 49.0001.1111.1111.1111.00 passive-interface Loopback 0! R2 routerinterface Loopback 0 ip address 2.2.2.2 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.1.1.2 255.255.255.0 ip router isis!interface Serial 1/1 ip address 10.2.2.1 255.255.255.0 ip router isis!router isis net 49.0001.2222.2222.2222.00 passive-interface Loopback 0! R3 routerinterface Loopback 0 ip address 3.3.3.3 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.2.2.2 255.255.255.0 ip router isis!router isis is-type level-2-only net 49.0001.3333.3333.3333.00 passive-interface Loopback 0!Since R3 is only a Level-2 router (essentially, link between R2 and R3 forms a Backbone Link), a L2 adjacency is formed between them.
Adjacencies formed
R1# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id2222.2222.2222 L1 Se1/0 10.1.1.2 UP 25 00R2# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id1111.1111.1111 L1 Se1/0 10.1.1.1 UP 26 003333.3333.3333 L2 Se1/1 10.2.2.2 UP 26 00R3# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id2222.2222.2222 L2 Se1/0 10.2.2.1 UP 28 01Since Level-2 routes are not leaked into Level-1 area, R1 does not learn of prefixes on R3. However, R3 will learn about prefixes in Level-1 area(s).
R1 and R3 routing tables
R1# show ip route isis | begin GatewayGateway of last resort is not set 2.0.0.0/32 is subnetted, 1 subnetsi L1 2.2.2.2 [115/10] via 10.1.1.2, 00:57:58, Serial1/0 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masksi L1 10.2.2.0/24 [115/20] via 10.1.1.2, 00:42:33, Serial1/0R3# show ip route isis | begin GatewayGateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnetsi L2 1.1.1.1 [115/20] via 10.2.2.1, 00:10:18, Serial1/0 2.0.0.0/32 is subnetted, 1 subnetsi L2 2.2.2.2 [115/10] via 10.2.2.1, 00:17:37, Serial1/0 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masksi L2 10.1.1.0/24 [115/20] via 10.2.2.1, 00:17:37, Serial1/0Notice that R1 (Level-1-only) router has no visibility of routes in other areas within the same domain. It relies on a default-route to R2 router to reach prefixes beyond R2 router.
Scenario 3: R1 is a Level-1 router in Area 49.0001, R2 is a Level-1-2 router in Area 49.0001 and R3 is a Level-1-2 router in Area 50.0001
The configuration on routers is as below:
R1 routerinterface Loopback 0 ip address 1.1.1.1 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.1.1.1 255.255.255.0 ip router isis!router isis is-type level-1 net 49.0001.1111.1111.1111.00 passive-interface Loopback 0! R2 routerinterface Loopback 0 ip address 2.2.2.2 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.1.1.2 255.255.255.0 ip router isis!interface Serial 1/1 ip address 10.2.2.1 255.255.255.0 ip router isis!router isis net 49.0001.2222.2222.2222.00 passive-interface Loopback 0! R3 routerinterface Loopback 0 ip address 3.3.3.3 255.255.255.255 ip router isis!interface Serial 1/0 ip address 10.2.2.2 255.255.255.0 ip router isis!router isis net 50.0001.3333.3333.3333.00 passive-interface Loopback 0!Although the Area IDs dont match on R2 and R3 routers, they still form a Level-2 adjacency between each other.
Adjacencies formed
R1# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id2222.2222.2222 L1 Se1/0 10.1.1.2 UP 29 00R2# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id1111.1111.1111 L1 Se1/0 10.1.1.1 UP 25 003333.3333.3333 L2 Se1/1 10.2.2.2 UP 28 00R3# show isis neighborsSystem Id Type Interface IP Address State Holdtime Circuit Id2222.2222.2222 L2 Se1/0 10.2.2.1 UP 27 01Notice in below output that both routers learn about their neighbor's Area IDs in IS-IS Hello messages.
CLNS neighbor output
R2# show clns neighbors detailSystem Id Interface SNPA State Holdtime Type Protocol1111.1111.1111 Se1/0 *HDLC* Up 24 L1 IS-IS Area Address(es): 49.0001 IP Address(es): 10.1.1.1* Uptime: 00:02:21 NSF capable Interface name: Serial1/03333.3333.3333 Se1/1 *HDLC* Up 21 L2 IS-IS Area Address(es): 50.0001 IP Address(es): 10.2.2.2* Uptime: 00:02:09 NSF capable Interface name: Serial1/1R3# show clns neighbors detailSystem Id Interface SNPA State Holdtime Type Protocol2222.2222.2222 Se1/0 *HDLC* Up 22 L2 IS-IS Area Address(es): 49.0001 IP Address(es): 10.2.2.1* Uptime: 00:02:25 NSF capable Interface name: Serial1/0R2 (L1/L2) router signals R1 router that it can reach other area(s) by setting the Attached (ATT) Bit in the Level-1 LSPs. This causes R1 router to install a default-route to R2 router.
ATT bit
R1# show isis databaseIS-IS Level-1 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL1111.1111.1111.00-00* 0x00000065 0x37B1 971 0/0/02222.2222.2222.00-00 0x0000006A 0xF635 978 1/0/0R1# show ip route isis | begin GatewayGateway of last resort is 10.1.1.2 to network 0.0.0.0i*L1 0.0.0.0/0 [115/10] via 10.1.1.2, 00:19:37, Serial1/0 2.0.0.0/32 is subnetted, 1 subnetsi L1 2.2.2.2 [115/10] via 10.1.1.2, 00:33:50, Serial1/0 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masksi L1 10.2.2.0/24 [115/20] via 10.1.1.2, 00:33:50, Serial1/0Page updated
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