Discussion

cencies. The State column shows that both adjacencies are operational (Up). The
Hold column shows the amount of time remaining before the router closes the adjacency.
By default, IS-IS sends Hello packets, which act as adjacency keepalives, every
three seconds for DIS routers and every nine seconds for Level 1 routers. Non-DIS
routers send Hello packets less frequently in case IS-IS needs to re-elect a DIS. While
a DIS is being elected, there is likely to be traffic loss. Having a longer hello timer
interval on the non-DIS systems remedies this problem. The default hold time is
three times the hello interval, or 9 seconds, and 27 seconds for DIS and Level 1 routers,
respectively. The SNPA column shows the subnetwork point of attachment, which
is the MAC address of the next hop.
The detail version of this command gives a bit more insight into the adjacencies:
aviva@RouterG> show isis adjacency detail
RouterH
Interface: fe-0/0/1.0, Level: 2, State: Up, Expires in 21 secs
Priority: 64, Up/Down transitions: 1, Last transition: 17:16:43 ago
Circuit type: 3, Speaks: IP, IPv6, CLNS, MAC address: 0:5:85:c1:d1:d1
Topologies: Unicast
Restart capable: Yes
LAN id: RouterG.02, IP addresses: 10.0.1.1
RouterA
Interface: fe-1/0/0.0, Level: 1, State: Up, Expires in 7 secs
Priority: 64, Up/Down transitions: 1, Last transition: 16:57:54 ago
Circuit type: 1, Speaks: IP, IPv6, MAC address: 0:5:85:ca:ca:70
Topologies: Unicast
Restart capable: Yes
LAN id: RouterA.02, IP addresses: 10.0.16.1
The output shows the other two IS-IS routers. RouterG reaches RouterH over interface
fe-0/0/1, and it connects to RouterA over interface fe-1/0/0. The State field
shows that the adjacencies are operational (Up), and the Expires field shows the
amount of time remaining before the router closes the adjacency. The second line
shows the router’s DR priority, how many times the adjacency has gone down and
come back up, and when the last up-down transition occurred. The third line
shows the Circuit type, which is the IS-IS level. A value of 3 indicates the router is a
Level 1–Level 2 router, a value of 2 is a Level 2–only router, and a value of 1 is a
Level 1–only router. The Speaks field shows the protocols that the router is running,
and the MAC address field shows the subnetwork point of attachment, which is
the MAC address of the next hop. The last line shows the IS-IS identifier of the
router on the LAN and the router’s IP address.
If the IS-IS adjacency doesn’t come up, there are a few things to check when troubleshooting.
First, make sure the physical interface is operational. Here, the adjacency
with RouterH is down:
aviva@RouterG> show isis adjacency
354 | Chapter 11: IS-IS
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