Intermediate System to Intermediate
System
The intermediate system to intermediate system (IS-IS) protocol that uses a shortest-path-first 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
• Configuration Information
• IS-IS Metric Styles
• Configure Metric Values
• Sample Configurations
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-first algorithm.
NOTE: This protocol supports routers passing both IP and OSI traffic, though the Dell Networking
implementation supports only IP traffic.
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 traffic within
an area, while Level 2 routers route traffic between areas. At its most basic, Level 1 systems route traffic within
the area and any traffic 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 traffic 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 different protocol data units (PDU). For IP traffic, 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.
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