Viewing Neighbor Information

Viewing Neighbor Information
For IS-IS, it is important to know who your neighbors are. Many times routing information not
being received by a router could be caused by neighbor adjacencies not being formed. To view
who a router’s neighbors are, use the following command:
Austin#show clns is-neighbors
System Id Interface State Type Priority Circuit Id
➥Format
0000.0C00.0C35 Ethernet1 Up L1 64 0000.0C00.62E6.03 Phase V
0800.2B16.24EA Ethernet0 Up L1L2 64/64 0800.2B16.24EA.01 Phase V
0000.0C00.3E51 Serial1 Up L2 0 04 Phase V
0000.0C00.62E6 Ethernet1 Up L1 64 0000.0C00.62E6.03 Phase V
In the preceding example, the router has three Level 1 neighbors and two Level 2 neighbors.
If you noticed, one of the neighbors is an L1/L2 router, which means this router will form a
Level 1 and a Level 2 adjacency with the router. The command also informs you if the state were
230 Chapter 7  Integrated IS-IS
something different. The detail parameter can be added to the end of the command line to
view the area address for each of the neighbors. You can also use the command show clns
neighbors to find out IS-IS and ES-IS neighbors.
If you would like to view the Hello PDUs a router is sending and receiving, the debug isis
adj-packets command can be used:
Austin#debug isis adj-packets
ISIS-Adj: Rec L1 IIH from 0000.0c00.40af (Ethernet0), cir type 3, cir id
BBBB.BBBB.BBBB.01
ISIS-Adj: Rec L2 IIH from 0000.0c00.40af (Ethernet0), cir type 3, cir id
BBBB.BBBB.BBBB.01
ISIS-Adj: Rec L1 IIH from 0000.0c00.0c36 (Ethernet1), cir type 3, cir id
CCCC.CCCC.CCCC.03
ISIS-Adj: Area mismatch, level 1 IIH on Ethernet1
ISIS-Adj: Sending L1 IIH on Ethernet1
ISIS-Adj: Sending L2 IIH on Ethernet1
ISIS-Adj: Rec L2 IIH from 0000.0c00.0c36 (Ethernet1), cir type 3, cir id
BBBB.BBBB.BBBB.03
Debugging IS-IS adjacency packets provides you with knowledge of what Hello PDUs the
router is sending and receiving. (IIH is an abbreviation for IS-IS Hello.) This can be a powerful
tool when you’re trying to find out why routers aren’t forming adjacencies. For instance, if you
had two routers that weren’t forming adjacencies with each other and one router had a physical
interface connected to the NBMA network and the other router had a subinterface connected
to the NBMA network, it might be hard to figure out why they weren’t forming adjacencies.
With the debug isis adj-packets command, you would be informed of a Hello PDU mismatch.
That was just one example of how the command can help you out with adjacency issues.