IS-IS Overview

Intermediate System to Intermediate System (IS-IS) is a an inter-domain (also called interior gateway) routing protocol, similar to OSPF in the sense that it is also a link-state routing protocol and it uses the Dijkstra algorithm to create the network topology.

However, while OSPF is based on the TCP/IP stack and it works only over ip (and ipv6 since OSPFv3), IS-IS is a more versatile protocol which can also work with IP, but it is essentially designed for the OSI layer, to work as a network layer protocol, on the same layer as CLNS (connectionless-mode network service).

Other similarities with OSPF include - use of hello multicast packets to discover neighboring routers, support for VLSM and CIDR, authentication and multi-path. 

IS-IS levels

• ISIS does not have a backbone area as such (like OSPF). Instead the backbone is the contiguous collection of Level-2 capable routers with branches of Level 1-2 and Level 1 routers forming the individual areas.
• ISIS area borders are on links, not routers
• the routers can be classified as:
• Level-1 router : 
• Has neighbours only on the same area
• Has a level-1 LSDB with all routing information for the area
• Level-2 router:
• May have neighbours in the same or other areas
• Has a Level-2 LSDB with all routing information about inter-area
• Level-1-2 router:
• May have neighbours on any area.
• Has two separate LSDBs: level-1 LSDB & level-2 LSDB

Each router is identified with a unique Network Entity Title (NET). NET is a Network Service Access Point (NSAP) where the n-selector is 0, and then compare with each router having a unique Router-ID with IP protocols.

NSAP

• Length between 8 and 20 bytes 
• Area address can have up to 13 bytes
• System ID defines an IS or an ES in an area
• NSEL (n-selector) identifies a network service user; transport entity or the IS network entity itself

ISPs usually use only one area, because it is enough to cover a few hundred routers network.

NET address begins with 49, for "private" address range. So, the first 8 bits are usually 49, next 16 bits are the area number, next 48 represent the router loopback address and the rest of the 8 bits are set to 0.

So, an NSAP address commonly looks like: 

49.0001.1921.6800.1001.00, with 192.168.1.1 as loopback address in area 1.

Types of packets

IS-IS Hello PDU

IS-IS hello PDUs, also called Intermediate System-to-Intermediate System Hello (IIH) are used to detect neighbors and establish adjacencies with other routers.
There are three different formats: one for point-to-point hello packets, one for Level 1 broadcast links, and one for Level 2 broadcast links. Level 1 routers must share the same area address to form an adjacency, while Level 2 routers do not have this limitation. The request for adjacency is encoded in the Circuit type field of the PDU.

Link-State PDU

A link-state PDU (LSP) contains information about each router in the network and the connected interfaces. Also included is metric and IS-IS neighbor information. Each LSP must be refreshed periodically on the network and is acknowledged by information within a sequence number packet.

On point-to-point links, each LSP is acknowledged by a partial sequence number PDU (PSNP), but on broadcast links, a complete sequence number PDU (CSNP) is sent out over the network. Any router that finds newer LSP information in the CSNP must delete the out-of-date entry and update the link-state database.
A change in the topology means a change in one or more of the LSPs. The router that has experienced a link going up or down will resend its LSP to inform the other routers of the change.

Flooding is the process by which these new LSPs are sent throughout the network to ensure that the databases in all routers remain identical.

Complete Sequence Number PDU

The complete sequence number PDU (CSNP) lists all the link-state PDUs (LSPs) in the link-state database of the local router. Contained within the CSNP is an LSP identifier, a lifetime, a sequence number, and a checksum for each entry in the database. Periodically, a CSNP is sent on both broadcast and point-to-point links to maintain a correct database. Also, the advertisement of CSNPs occurs when an adjacency is formed with another router. Like IS-IS hello PDUs, CSNPs come in two types: Level 1 and Level 2.

When a device receives a CSNP, it checks the database entries again its own local link-state database. If it detects missing information, the device requests specific LSP details using a partial sequence number PDU (PSNP).

Partial Sequence Number PDU

A partial sequence number PDU (PSNP) is used by an IS-IS router to request LSP information from a neighboring router. A PSNP can also explicitly acknowledge the receipt of an LSP on a point-to-point link. On a broadcast link, a CSNP is used as implicit knowledge. Like hello PDUs and CSNPs, the PSNP also has two types: Level 1 and Level 2.

When a device compares a CSNP to its local database and determines that an LSP is missing, the router issues a PSNP for the missing LSP, which is returned in a link-state PDU from the router sending the CSNP. The received LSP is then stored in the local database, and an acknowledgement is sent back to the originating router.

Network Types 

The types of networks that IS-IS defines include:

•Point-to-point networks

•Broadcast networks

Point-to-point networks, such as serial lines, connect a single pair of routers. A router running IS-IS will form an adjacency with the neighbor on the other side of a point-to-point interface. A DIS is not elected on this type of link. The basic mechanism defined in the standard is that each side of the link declares the other side to be reachable if a hello packet is received from it. When this occurs, each side then sends a CSNP to trigger database synchronization.

Broadcast networks, such as Ethernet, are multiaccess in that they are able to connect more than two devices; all connected routers will receive a packet sent by one router. On broadcast networks, one IS will elect itself the DIS. Hello packets on broadcast networks are sent to the AllL1ISs or AllL2ISs MAC-layer broadcast addresses. The DIS is responsible for flooding;