O'Reilly logo

Junos® OS For Dummies®, 2nd Edition by Michael Bushong, Cathy Gadecki, Walter Goralski

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

Running IS-IS

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.

Unlike OSPF, which was developed and standardized by the Internet Engineering Task Force (IETF), IS-IS is an ANSI ISO protocol and was originally based on the Digital Equipment Corporation DECNET Phase V Network Technology.

IS-IS uses a slightly different terminology than OSPF for naming its protocol packets. 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.

Similar to OSPF, all IS-IS routers place the information in the received link-stated PDUs into their link-state database, and all routers have the same view of the network's topology. IS-IS runs the SPF algorithm on the information in the link-state database to determine the shortest path to each destination on the network, placing the destination/next-hop pairs that result from the SPF calculation into the IS-IS routing database.

Using IS-IS addresses only when needed

Unlike other IP routing protocols, which typically run on TCP, UDP, or IP, which are OSI Layer 3 or Layer 4 protocols, IS-IS runs directly ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required