IS-IS

What is IS-IS

• Intermediate System To Intermediate System
• An ISO standard open protocol.
• Link State and Shortest Path
• Good for large flat networks

Terms

IS

• Intermediate System
• A router

NSAP

• Network Service Access Point

NET

• Network Entity Title
• A router
• Also refers to the address NSAP

ES

• End Station
• A PC, or a server.

Station Routing

• AKA, intra-area
• Routing within a L1 area.

Area Routing

• AKA, inter-area
• Routing within a L2 area.
• The L2 area.

Types of Routers

L2 Routers

IS-IS doesn’t refer to a backbone, but L2 routers perform the same function. They should be center-of-topology.

L1L2 Routers

These routes have topology information for the L1 area and the L2 area.

These are kind of like ABRs in OSPF.

L1 Router

These are the Area routers. They do not flood their link state databases into L2.

• Intra-area
• Default route out (sets the attached bit)
• Redistribution is allowed

Example

                                             ┌──────┐
                                             │ L1   │
                                             └───┬──┘
                                                 │
┌──────┐     ┌──────┐        ┌──────┐        ┌───┴──┐     ┌─────┐
│  L1  ├─────┤ L1L2 ├────────┤  L2  ├────────┤ L1L2 ├─────┤ L1  │
└───┬──┘     └───┬──┘        └──────┘        └───┬──┘     └─────┘
    │            └──────┐                  ┌─────┘
┌───┴──┐             ┌──┴───┐          ┌───┴──┐
│  L1  │             │  L2  │──────────┤  L2  │
└──────┘             └──────┘          └──────┘

Topologies

Single Topology

• All Routed Protocols must be configured on all enabled interfaces.
• e.g. v4 and v6 on all interfaces.

Multi-Topology

• Some interfaces can be v4, others can be v6, others can be both.

Addressing scheme

packet-beta
0-7: "AFI"
8-23: "Area ID"
24-71: "System ID"
72-79: "SEL"

AFI

• Authority and Format Identifier - 1 byte
• 49 means local authority, and hexadecimal (binary is encoded).

Area ID

• Variable, and … includes the AFI.

System ID

• 6 bytes, can fit a MAC address or a v4 address.
• Must be unique in an area for L1.
• Must be unique in a domain for L2.

SEL

• Selector - 1 byte.
• This is always 00 to mean router.

Example

net 49.0001.0000.0A00.0001.00

So long as the NSAP is unique, its OK because we aren’t routing CLNS.

Priority is used for the CLNS election. Circuit ID, who won the election

Etc

ISIS does not ride IP, it rides CLNS. To do Multipoint NBMA you need to include CLNS resolution.

L1 areas must match

IS-IS Narrow

The Cisco default link cost is 10.

These are the limits:

• 63 per link
• 1 023 per path

IS-IS Wide

• 16 777 215 per link
• 4 294 967 295 per path

Cisco’s implementation of the wide metric uses the bits ISO set aside for delay, expense and error.

Config

Enable wide metrics:

metric-style wide

Metric Transition Commands

Used when migrating from narrow to wide without a hard cutover:

IS-IS Authentication

• Plaintext
  • Link, Area, or Domain
    • Link is between routers
    • Area is every router must have a matching password
    • L2 and L1/L2 router use domain authentication.

Notes

Default route injected via route-map.

References

RFC 1195: Use of OSI IS-IS for routing in TCP/IP and dual environments | RFC Editor

RFC 5308: Routing IPv6 with IS-IS | RFC Editor

ISO/IEC 10589:2002 - Information technology — Telecommunications and information exchange between systems

ISO/IEC 8348:2002 - Information technology — Open Systems Interconnection — Network service definition