IS-IS NSEL

[Fawzi Alabbas]

If you are studying IS-IS, you might note that we are setting the last 8 bit to (00). This last bit is called the NSEL, but why are we setting it to zero?

Well, when configuring NSAP addresses for routing purposes (especially with IS-IS), the NSEL is typically set to zero. This is because:

* Routing Purposes: The routers primarily deal with the network layer, not the transport or service layers. Setting the NSEL to zero indicates that the address is used for network routing purposes only.

* Default Behavior: In routing, we don't typically need to identify higher-layer services, so we use the NSEL value of zero as a convention to signal that the address is for routing and switching operations rather than identifying a specific service or application.

* Standard Practice: Most implementations of IS-IS and other OSI-based networking protocols assume that if the NSEL is zero, the NSAP refers to the network node itself, not any specific service.

So in summary, we are setting it to zero to indicate that the NSAP address refers to the node's general routing and switching function.

[paul driver]

Hello

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@Fawzi Alabbas wrote:
 This last bit is called the NSEL, but why are we setting it to zero?

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SPCOR -Chapter 5: IS-IS (page 111)  -- Implement IS-IS (IPv4 and IPv6)
the SEL field, also called an NSAP Selector or NSEL (the more commonly
used abbreviation), is a 1-octet-long field. It identifies the service in or above the net
work layer on the destination node that processes the datagram. For IP, you use 00. In
fact, if the device is a router, this field will always be 00.