Content

| 7.4 |  | IP Routing with Integrated IS-IS |
| 7.4.1 |  | OSI, IP, and dual |

Integrated IS-IS supports the following three types of networks:

- OSI
- IP
- Dual, made up of both OSI and IP

The LSPs can contain many variable-length TLV fields describing OSI and IP state information.

Integrated IS-IS LSPs describe IP information in a similar manner to the way IS-IS describes ESs. There are specific TLV types for IP information. Like all modern routing protocols, Integrated IS-IS supports the following:

- Variable-Length Subnet Masks (VLSMs): the mask is sent with the prefix in the updates
- Redistribution of IP routes into and out of IS-IS
- Summarization of IP routes

Even if Integrated IS-IS is being used only for IP routing, a NET address is required for L2 forwarding and Dijkstra algorithm computation. OSI protocols are used to form the neighbor relationship between routers. SPF calculations rely on a configured NET address to identify the routers. The remainder of this page describes IS-IS routing in a pure OSI environment. If IP routing with IS-IS is required, the same process described below still takes place. TLVs are used to carry IP routing information, enabling IP routing by way of OSI routing.

To build the OSI forwarding database, the CLNS routing table, the synchronized link-state database is used to calculate the SPF tree to OSI destinations or NETs. The link metrics are totaled along each path to decide which is the shortest to any given destination. There are separate link-state databases for L1 and L2 routes. Therefore, SPF is run twice, once for each level, and separate SPF trees are created for each level. ES reachability is calculated with a partial route calculation (PRC) based on the L1 and L2 SPF trees. There are no OSI ESs if it is a pure IP Integrated IS-IS environment. The best paths are inserted in the CLNS routing table, OSI forwarding database.

Routing inside an L1 area is based on the system ID of the destination ISO (NSAP) address. OSI packets to other areas are routed to the nearest L1L2 router. L2 routing is based on the area ID and only considers the area cost. If an L1L2 router receives a packet from an L2 neighbor destined for its own area, it will route it based on the system ID, the L1 routing.

When routing a packet from one area to another area, the L1 routers route the packet to the nearest L1L2 router. L1 routers find the closest exit point from the area, based on receipt of default routes from the L1L2 routers in their area. The L1L2 router routes the packets into the L2 backbone based on the destination area ID. The packet travels across the L2 backbone to the destination area. Once it arrives in the destination area, L1 routing is again used to route the packet to its final destination inside that area. The interface between the L1 world and the L2 world takes place on an L1L2 router. The L1L2 router behaves as if it were both an L1 router, by routing to L1 destinations, and an L2 router, by routing between areas.

An IS-IS domain is the equivalent of an IP AS. IS-IS can support the interconnection of multiple domains. In a pure OSI environment, ISO IGRP interprets the IDI portion of CLNS routes and allows routing between domains. ISO-IGRP is a Cisco proprietary. There is also a standard OSI Interdomain Routing Protocol (IDRP), specified in ISO/IEC 10747, which provides the same function, but is not supported by Cisco. IDRP is used for L3 routing in an OSI environment. This protocol has never actually been deployed in a production environment. This is because by the time the ISO formalized IDRP, IP had already "won" as the routed protocol for the Internet. The current standard for interdomain routing in an IP environment is Border Gateway Protocol (BGP) Version 4. Module 9 will discuss this further.

Web Links

Configuring Integrated IS-IS

http://www.cisco.com/en/US/products/ sw/iosswrel/ps1828/products_configuration_ guide_chapter09186a00800ca56f.html

Open System Interconnection Protocols
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ ito_doc/osi_prot.htm#xtocid12

Overview of IDRP (ISO/IEC 10747)

http://www.zvon.org/tmRFC/RFC1629/Output/ chapter3.html#sub3