How to Configure and Monitoring IS-IS on Junos Devices

By Walter J. Goralski, Cathy Gadecki, Michael Bushong

IS-IS is a link-state interior gateway routing protocol. Like OSPF, IS-IS runs the Dijkstra shortest-path first (SPF) algorithm to create a database of the network’s topology and, from that database, to determine the best (that is, shortest) path to a destination.

The packets that IS-IS routers send to each other describing the network topology are called link-state protocol data units (link-state PDUs, or LSPs). In addition to describing the network topology that the router knows about, the link-state PDUs include IP routes, checksums, and other information.

To configure IS-IS on a router running the Junos operating system, you enable family iso processing on the interfaces on which you want IS-IS to run, and then you tell the router to form IS-IS adjacencies over that interface. Here’s how you do so for the ge-1/2/0.0 interface:

user@junos-router# set interfaces ge-1/2/0 unit 0 family iso
user@junos-router# set protocols isis interface ge-1/2/0.0

Once you have completed the configuration of IS-IS, you can use these commands to monitor IS-IS routing on your network:

Command Purpose
show isis adjacency Displays the router’s IS-IS neighbors.
show isis database Displays entries in the IS-IS link-state database.
show isis interface Displays the interfaces on which IS-IS is configured.
show isis route Displays the contents of the IS-IS routing table.
show route protocol isis Displays the routes learned by IS-IS.