Sunday, November 7, 2010


Now it's time to go through the feared part of IS-IS: That is the NSAP address applied to the routers. OSI uses CLNP addresses, and when it's applied on a router, it's called a NSAP (Network Service Access Point) address.
You only need one address per node (per IS), not per interface. This address tells what area the router is in. It tells the Cisco router what IS-IS area it's in and what's it's System ID (which is equivalent to the Router ID).

The NSAP address can be up to 20-bytes in length, compared to OSPF's 4-bytes.

The original implementation of OSI defined more than 5 sections the NSAP address would represent, compared to the 2 which IP addresses have.

Cisco's implementation only uses three of those fields:
- Area address
- System ID
- NSEL (NSAP Selector)

An example NSAP address is:

Every NSAP address assigned to a Cisco router will always have the last 2 bits set to 00. That is the NSEL. The NSEL of 00 shows that the address is assigned to an IS.

The System ID in this case is AA15.B322.1841. The System ID is always a fixed 6 bytes in length. As mentioned previously, the System ID is actually the equivalent of the Router ID.

Some Administrators like to put the MAC addresses as the System ID. Typically in lab environments, the System ID always ends up becoming the router's number.

Whatever is left represents the area. The best way to read the address is from right to left. In this case, the area is 49.1234. Areas starting from 49 represents a private address. These packets would theoretically be dropped by ISPs like how RFC 1918 packets would.

This example address is not the longest you can get. You can have up to 20 characters in an NSAP address.

IS-IS builds a topology of the OSI domain to find out how to reach various destinations based on the NSAP addresses. When routing, IS-IS checks the area address of the packet. If it's a different area, it would be routed based on Area ID. If it's in the same area, it would be routed based on the System ID.

If you are a L1 router, it would send it to the closest L1/L2 router. The L1/L2 router then sends the packet through the backbone to the other area.

For intra-area routing, L1 and L1/L2 routers simply route it based on the L1 database.

The term "packet" is a PDU, which is a Network Layer PDU. In OSI, a Network Layer Packet is simply referred to as a Network PDU. A Frame is known as a Data-Link PDU. Theses are the various packet types used by IS-IS:
-Link State Packet (LSP)
-Partial Sequence Number PDU (PSNP)
-Complete Sequence Number PDU (CSNP)

The Hello message is the foundation that forms neighbor relationships and serves as a keepalive.

The LSP is similar to OSPF's LSU. In each IS-IS LSP, there can be information for one or more routes.

CSNP acts like the OSPF's DBD.

PSNPs are typically used to request for information like a LSQ. It is also an acknowledgment for an LSP.

A typical Network PDU has the following structure:
LSP Header|TLV|TLV|TLV|...

The LSP header contains the PDU type, length, sequence number and lifetime. The TLVs contain neighbors, authentication information and attached subnet information. They simply added a new TLV for IP routing to work!

The original IS-IS and the Integrated IS-IS is really not that different at all! Integrated IS-IS still reads NSAP and routes NSAP! It simply has an extra TLV which allows the IS to match NSAP with IP. IS-IS is one of the first routing protocol to be released for IPv6 because they simply need to add a new TLV. On the other hand, OSPFv3 had to be completely re-engineered for IPv6!

The IS-IS network types are simple:

Broadcast mode assumes full connectivity. The Broadcast Mode elects a Designated IS (DIS) which is the same as DR/BDR. Broadcast Mode uses Multicast.

When dealing with NBMA networks without a full mesh, IS-IS simply uses point-to-point (sub-interfaces) design. Point-to-Point mode uses Unicast.

To keep databases tightly synchronized, CSNPs are sent once every 10 seconds in Broadcast mode, while only once initially (when link comes up) in Point-to-Point networks.

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