Administrator Guide

Intermediate System to Intermediate System
The intermediate system to intermediate system (IS-IS) protocol that uses a shortest-path-rst algorithm. Dell Networking supports both
IPv4 and IPv6 versions of IS-IS.
Topics:
• IS-IS Protocol Overview
• IS-IS Addressing
• Multi-Topology IS-IS
• Graceful Restart
• Implementation Information
• Conguration Information
• IS-IS Metric Styles
• Congure Metric Values
• Sample Congurations
IS-IS Protocol Overview
The IS-IS protocol, developed by the International Organization for Standardization (ISO), is an interior gateway protocol (IGP) that uses a
shortest-path-rst algorithm.
NOTE
: This protocol supports routers passing both IP and OSI trac, though the Dell Networking implementation supports only
IP trac.
IS-IS is organized hierarchically into routing domains and each router or system resides in at least one area. In IS-IS, routers are designated
as Level 1, Level 2 or Level 1-2 systems. Level 1 routers only route trac within an area, while Level 2 routers route trac between areas. At
its most basic, Level 1 systems route trac within the area and any trac destined for outside the area is sent to a Level 1-2 system. Level
2 systems manage destination paths for external routers. Only Level 2 routers can exchange data packets or routing information directly
with external routers located outside of the routing domains. Level 1-2 systems manage both inter-area and intra-area trac by maintaining
two separate link databases; one for Level 1 routes and one for Level 2 routes. A Level 1-2 router does not advertise Level 2 routes to a
Level 1 router.
To establish adjacencies, each IS-IS router sends dierent protocol data units (PDU). For IP trac, the IP addressing information is included
in the IS-IS hello PDUs and the link state PDUs (LSPs).
This brief overview is not intended to provide a complete understanding of IS-IS; for that, consult the documents listed in Multi-Topology
IS-IS.
IS-IS Addressing
IS-IS PDUs require ISO-style addressing called network entity title (NET).
For those familiar with name-to-network service mapping point (NSAP) addresses, the composition of the NET is identical to an NSAP
address, except the last byte is always 0. The NET is composed of the IS-IS area address, system ID, and N-selector. The last byte is the N-
selector. All routers within an area have the same area portion. Level 1 routers route based on the system address portion of the address,
while the Level 2 routers route based on the area address.
The NET length is variable, with a maximum of 20 bytes and a minimum of 8 bytes. It is composed of the following:
23
Intermediate System to Intermediate System 399