RouterA and RouterB are connected routers.
You issue the show clns neighbors command on RouterA and receive the following output:
System Id Interface SNPA State Holdtime ….. Type Protocol
RouterB Et0/0 0000.0000.000b Up 23 L1 IS-IS
Which of the following statements must be true? (Select the best answer.)
A. RouterA and RouterB are in the same area.
B. RouterA and RouterB are in different areas.
C. RouterB is configured for L1 routing only.
D. RouterB isconfigured for L1/L2 routing.
E. RouterA is a backbone router.
Explanation:
RouterA and RouterB are in the same area. Routers running the Intermediate SystemtoIntermediate System (IS-IS) routing protocol are placed into administrative domains called areas. Each ISIS router resides in only one area. The collection of all areas managed by a single organization is called a routing domain.
Each ISIS router is configured with a routing level. Level 1 (L1) routers are capable of intraarea routing, which delivers data within a single area. The output of the show clns neighbors command indicates that RouterB has established an L1 adjacency with RouterA; therefore, both routers must be in the same area.
Level 2 (L2) routers are capable of interarea routing, which delivers data between areas. If RouterA and RouterB were in separate areas and both routers were configured for L2 routing, you would have received the following output from the show clns neighbors command:
System Id Interface SNPA State Holdtime Type Protocol
RouterB Et0/0 0000.0000.000b Up 23 L2 IS-IS
Level 1/Level 2 (L1/L2) routers are capable of both intraarea and interarea routing and maintain a separate linkstate database for each. If RouterA and RouterB were in the same area and both routers were configured for L1/L2 routing, you would have received the following output from the show clns neighbors command:
System Id Interface SNPA State Holdtime Type Protocol
RouterB Et0/0 0000.0000.000b Up 23 L1L2 IS-IS
You can configure the routing level for an ISIS process by issuing the istype {level1 | level12 | level2only} command, and you can configure the routing level for an ISIS interface by issuing the isis circuittype {level1 | level12 | level2only} command. By default, all ISIS routing processes and interfaces are configured for L1/ L2 routing.
RouterB is configured for either L1 routing or L1/L2 routing. However, the output of the show clns neighbors command in this scenario does not indicate which routing level RouterB is configured to use. If either router was configured for L1 routing only, the Typefield of the show clns neighbors command would show L1, even if the other router were an L1/L2 router.
The output of the show clns neighbors command in this scenario does not indicate whether RouterA is a backbone router. ISIS requires that all Level 2 (L2) and L1/L2 routers be connected to form a backbone through the routing domain. If RouterA were configured for L1/L2 routing, RouterA would be a backbone router.
When an ISIS routing level mismatch, authentication mismatch, or maximum transmission unit (MTU) mismatch occurs, an ISIS adjacency will not form, but the output of the show clns neighbors command might instead show an End SystemtoIntermediate System (ESIS) adjacency. ESIS is used to discover end systems. If RouterA and RouterB were in different areas and if either router was configured for L1 routing only, you might see the following output after issuing the show clns neighbors command on RouterA:
System Id Interface SNPA State Holdtime Type Protocol
RouterB Et0/0 0000.0000.000bUp 23 IS ES-IS
Reference:
Cisco: Cisco IOS ISO CLNS Command Reference: show clns neighbors