IS-IS

Protocol Attributes

Protocol Type	Link-state
Algorithm	Dijsktra
Metric (IOS)	Default
Administrative Distance	115
Specification	ISO 10589
Supported Protocols	IPv4, IPv6, CLNS
Transport	Layer 2
Authentication	None, plain text, MD5

Background

IS-IS was developed by DEC as an international standard for the OSI to compete with TCP/IP in the 1980s. Integrated IS-IS refers to IS-IS which supports IP.

Terminology:

• IS - Intermediate System (router)
• ES - End System (host)

Connectionless Network Protocol (CLNP) is the network layer protocol defined in OSI. It is used by the Connectionless Network Service (CLNS). IS-IS routers use a CLNS address as a router ID.

OSI routing levels:

• Level 0 - Used to locate end systems
• Level 1 - Exchange of routes within an area (IS-IS)
• Level 2 - Backbone between areas (IS-IS)
• Level 3 - Between autonomous systems (Interdomain Routing Protocol (IDRP))

Like OSPF, IS-IS is also relies on the Dijkstra algorithm for path selection.

Packet Types

IS-IS packets use an 8-byte header with variable Type Length Value (TLV) fields for data. TLVs make IS-IS very flexible and easily extended to support other protocols.

Hellos

Hello types:

• End System Hello (ESH) - Used by ISO hosts to attach to routers; not used with IP networks
• Intermediate System Hello (ISH) - Used by routers to announce themselves to end systems
• Intermediate-to-Intermediate Hello (IIH) - Router to router; used separately at levels 1 and 2

Point-to-point hello packets are used over point-to-point links, and level 1 or 2 LAN hello packets are used on multiaccess links.

Link-State Packet (LSP)

Level 1 LSPs list a router's adjacencies.

Level 2 LSPs list a router's adjacencies and the areas it can reach.

Sequence Number Packet (SNP)

An advertisement containing one or more condensed LSPs.

SNPs are never flooded, only sent between neighbors.

Can be complete (CSNP) or partial (PSNP).

Addressing

ISO address types:

• Network Service Access Point (NSAP)
• Network Entity Title (NET)

Addresses are 8-20 bytes long, composed of three parts:

• Area - Similar to an IP subnet
• ID - Identifies a particular host
• SEL - Identifies a process on the host (similar to TCP/UDP ports)

Address fields:

• Inter Domain Part (IDP) - Used for external routing
  • Authority and Format Identifier (AFI) - Identifies the authority that dictates the format of the address (usually ''country code'', ''international code'', or ''private'')
  • Initial Domain Identifier (IDI) - An organization in the AFI (optional)
• Domain Specific Part - Used for routing within the AS
  • High Order DSP (HODSP) - The area within the AS
  • System ID - Identifies the system, 6-8 bytes (Cisco supports only 6-byte length), could be MAC or static length IP (192.168.0.1 ## 1921.6800.0001)
  • NSEL - 1-byte, identifies a network layer service

An NSAP with the NSEL set to 0x00 becomes a NET.

Addressing Rules

• ISO addresses are assigned to the system, not a particular interface
• One address per router is typical; limit is 3
• If a router has multiple NETs, they must all have the same system ID
• The area address must be identical for all routers in an area
• All routers must have a unique system ID of the same length (6 bytes for Cisco devices) within their area

Adjacencies

Requirements for an adjacency to form:

• MTUs must match
• Levels must match
• If level 1, routers must be in the same area
• System IDs must be unique
• Authentication type (if any) must match

IS-IS has only two network types: broadcast and point-to-point. It is recommended to configure NBMA networks as point-to-point with subinterfaces.

IS-IS Operation

Basic Principals of Area Routing

• Level 1 areas produce a summary into Level 2 & Level 1-2 routers and introduce a default route back into Level 1
• Level 1 routers match destination with closest match in the routing table and forward using the shortest path first like OSPF
• If destination is not found, Level 1 router forwards traffic to nearest Level 1-2 router using SPF
• Level 1-2 routers use routing table to send data across the Level 2 backbone using SPF
• For Level 2 routing updates to be exchanged, all Level 2 routers must be contiguous

Routing process

• Update
• Decision
• Forwarding
• Receiving

Update Process

Triggers:

• An adjacency comes up or goes down
• An interface changes state or receives a new metric
• A route changes (for example, due to redistribution)

An LSP generated by an update is flooded throughout an area on its respective level.

Three fields in an LSP are inspected to determine whether its information is more recent than what exists in the recipient's database:

• Remaining Lifetime - Time to live in the database; 20 minutes by default, refresh timer is 15 minutes
• Sequence Number - 32-bit sequential counter
• Checksum - Error checking

Decision Process

As implemented in Cisco IOS, IS-IS will consider six routes to a destination.

Internal paths are preferred over external paths, and level 1 paths are preferred over level 2 paths. If no path exists, the packet is sent to the nearest level 2 router.

Integrated (IP) IS-IS metrics:

• Default - Required; Cisco default for all interfaces is 10
• Delay - Not supported
• Expense - Not supported
• Error - Not supported

By default, interfaces are given a 6-bit metric, while a 10-bit (narrow) metric defines the total path cost. Cisco's implementation increases the metric size to 24-bits (wide).

Design Considerations

Area Design

Typical designs:

• Level 1 only - Sufficient for small networks but does not scale well
• Level 2 only - Provides for expansion through the addition of level 1 areas
• Level 1-2 only - Cisco default, allows for easy migration to a hierarchical design, hard on resources (all routers have two databases)
• Hierarchy - Intended implementation of IS-IS but can produce suboptimal routing

Route Summarization

Summarization reduces the need to flood LSPs.

Level 1-2 routers can summarize routes within their area. Summarization must be configured identically for all level 1-2 routers in an area.

Level 1 routes cannot be summarized within an area.