Chapter 10: Troubleshooting IS-IS Routing Environments | Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)

Download CD Content

Intermediate System–to–Intermediate System (IS-IS) is designed for routing in Connectionless Network Service (CLNS) and IP-based networks. Large Internet Service Providers (ISPs) use the IS-IS routing protocol to leverage their interconnection support to large networks. ISPs need to connect both OSI and IP-based networks. As a result, IS-IS is the most appropriate routing protocol for business services similar to ISPs.

This chapter discusses various problems, their effective isolation, and the different methods to resolve problems in IS-IS routing environments.

Features of IS-IS

You should be familiar with the different protocol concepts of IS-IS to successfully isolate and troubleshoot problems. This section discusses the terminology used in the IS-IS routing protocol. The terms are:

Open Systems Interconnection (OSI): International standard protocol architecture.

Protocol Data Unit or Packet Data Unit (PDU): Single data unit of the corresponding protocol used across the network.

Link State Packet data unit (LSP): Data unit of the IS-IS protocol. LSP is analogous to the Link State Advertisement (LSA) in OSPF protocol. LSP refers to the packet containing routing information.

End System (ES): Host system in IS-IS network. ES is OSI terminology for host.

Intermediate System (IS): Router in IS-IS terminology.

Designated Intermediate System (DIS): Designated Router (DR) in IS-IS network.

Pseudo Node (PN): Broadcast link emulated as a virtual link by DIS.

SubNetwork Point of Attachment (SNPA): Data-link interface of the IS-IS network. This is the point at which a device is connected to the network.

Network Service Access Point (NSAP): OSI address. It consists of area address, system identifier, and NSEL selector byte. If NSEL for an NSAP is equal to 0, it is known as Network Entity Title (NET). NSEL for a router is always set to 0.

Network Layer Protocol ID (NLPID): Octet field that identifies a network layer protocol.

Adjacency: Nearby systems in IS-IS network.

End System–to–Intermediate System (ES-IS): Protocol similar to IS-IS that delivers the ISO defined CLNS.

IS-IS process tag: IS-IS process name given using the command router isis.

L1 router: Performs routing based on the area ID portion of the IS-IS packet.

Network Entity Title (NET): Unique address for an IS within an area. This address contains three fields: System ID, Area ID, and N-SEL.

System ID: ID used by L1 routers for locating a specific IS within an area.

Area ID: ID used by L2 routers for locating a specific area within an IS-IS network.

NSEL: Network level services offered to the IS. This value is 00 by default.

Number of manual areas: Areas configured using net command.

Number of active areas: Areas learned by adjacency relationships.

IS-IS level: Routing level of an IS-IS router.

Distance: Distance covered by the IS-IS route.

Maximum paths: Maximum number of parallel routes available to the same destination as in the routing table.

Number of SPF runs: Number of times of SPF execution for Level 1 (L1) and Level 2 (L2) routers.

The IS-IS routing domain is divided into two areas:

Level 1 (L1) area: Routing is performed between adjacent ISs within the same area.

Level 2 (L2) area: Connects two different L1 areas within the same routing domain.

There are three types of routers in IS-IS routing protocol: L1, L2, and L1-L2. A comparison of these three routers is given in Table 10.1.

Table 10.1: Comparing IS-IS Routers
L1 Router	L2 Router	L1-L2 Router
Nonbackbone router	Backbone router	Similar to OSPF Area Border Router
Knowledge of intra-area routes only	Knowledge of inter-area routes only	Knowledge of both intra-area and inter-area routes
Maintains L1 link state database	Maintains L2 link state database	Maintains two link state databases

The different types of data packets used in IS-IS routing protocol are shown in Table 10.2.

Table 10.2: Data Packets in the IS-IS Routing Protocol
IS-IS Packet	Function
IS-IS Hello Packet (IIH)	Establishes and maintains adjacency relationships between ISs.
Link State Packet (LSP)	Consists of two types of LSPs: non – psuedo node LSP and psuedo node LSP.
Complete Sequence Number Protocol data unit (CSNP)	Includes a list of LSPs known to the IS. CSNPs are generated and flooded by DIS periodically. Also explains every LSP in the link state database.
Partial Sequence Number Protocol data unit (PSNP)	Requests for missing route information after receipt of a CSNP. PSNP also acknowledges a routing update on point-to-point links.

Every IS generates and propagates a fresh non–psuedo node LSP when:

New neighbor IS comes alive into the IS-IS network and subsequently shuts down.
New IP prefixes are added or eliminated.
Link cost varies.
Update period for LSPs expires.

Every IS network generates and propagates the psuedo node LSPs when:

New neighbor IS comes alive into the IS-IS network and subsequently shuts down.
Update period for LSPs expires.
New LAN is connected to IS-IS routing domain.