Defining the System ID

By now, it is clear that the SysID is one of three key components of an NSAP address, which is required to enable IS-IS routing on a router. The other components are the Area ID and the NSEL. According to ISO 10589, the SysID can be of variable length between 1 to 8 bytes. However, Cisco's implementation of IS-IS uses a fixed 6-byte length in conformance with the GOSIP 2.0 standard. It is probably not by coincidence that the 6-byte length specified by GOSIP 2.0 matches the length of a LAN MAC address. Therefore, you can use one of the LAN MAC addresses on a router as its SysID, essentially embedding a MAC address (a Layer 2 address) in the NSAP, which is a Layer 3 address. Of course, on an IP router with many LAN interfaces, you need to decide which MAC address to use as the SysID. However, the SysID doesn't have to be a MAC address.

Although an IP address has no visual relationship with the data-link address, except indirectly through mapping by the Address Resolution Protocol (ARP), a link between the NSAP and the data-link address (SNPA) can be easily established for single-point-attached CLNS hosts by using their MAC addresses at the point of attachment as the SysID. Because a router might have many active LAN interfaces, direct one-to-one LAN address-to-NSAP mapping might not always be possible. Network operators can follow any convenient method to define SysIDs for their routers while conforming to the requirements and caveats listed in the following section.

Requirements and Caveats

The following is a list of requirements and caveats that must be followed to define NSAP for IS-IS routing in general and in particular on Cisco routers:

  • Each node in an IS-IS routing area must have a unique SysID.

  • The SysID of all nodes in an IS-IS routing domain must be of the same length. The length of the SysID is 6 bytes (fixed) on Cisco routers. All routers connecting to the Level 2 backbone must have unique SysIDs relative to each other.

NOTE

SysIDs are required to be unique only within a specific area; therefore, a router in one area can potentially share the same SysID with a router in another area without any conflict unless they are both connected to the backbone for Level 2 routing. In practice, however, most service providers keep SysIDs unique for each router in the entire domain regardless of whether the network is a single-area domain or has multiple areas.


In Chapter 5, "The IS-IS Link-State Database," the IS-IS Link-State database is discussed and it is noted that the identifiers for link-state packets are tied to the SysIDs of the originating routers. This explains the need for unique SysIDs for nodes in the same area (Level 1) or the backbone (Level 2).

One popular way to define unique SysIDs is by padding a dotted -decimal loopback IP address with zeros to transform it into a 12-digit address, which can then be easily rearranged to represent a 6-byte SysID in hexadecimal, by regrouping the digits in fours and separating them with dots. Example 4-5 shows an excerpt from the configuration on a Cisco router. Typically, routers are configured with loopback addresses for other purposes, such as BGP routing or network management. Example 4-5 elaborates the procedure for defining an NSAP based on the loopback address.

Example 4-5 Example of System ID
 Interface Loopback 0 IP address 192.168.1.24 Router isis Net 49.0001.1921.6800.1024.00 

In Example 4-5, the loopback address is transformed as follows :

Step 1. Each octet in the dotted-decimal notation of the loopback IP address that is not three digits is prefixed with zeros, padding it to three digits, as follows:

 192.168.1.24 ---> 192.168.001.024 
Step 2. After Step 1, you have 12 digits, which you can easily rearrange into three groups of 4 digits, as follows:

 192.168.001.024 ---> 1921.6800.1024 
Step 3. 1921.6800.1024 can then be used as the unique SysID of the router. The area prefix and NSEL suffix are added to obtain the complete NSAP address, as shown in Example 4-6.

Example 4-6 Obtaining the Complete NSAP Address
 1921.6800.1024  ---> 49.0001.1921.6800.1024.00  ------- -------------- --           ^          ^         ^                                       AreaID     SysID      NSEL 

This method of defining the NSAP or NET is frequently used by Internet service providers that run the IS-IS protocol as IGP. Even though this might not be the only intuitive way, the method offers operational convenience by associating multiple applications to a single loopback address on the router. A single loopback address can be used as the BGP router ID, the basis for the IS-IS SysID, the MPLS/TE router ID, and for network management applications. OSPF also uses a loopback address on a router, when available, as the router ID; in typical situations, however, OSPF and IS-IS are not used together as IGPs in the same network, even though that is possible for merging two originally separate networks or during a routing protocol migration from one to the other. Because the Area ID and the NSEL are the same for all nodes in the same area, it's the unique SysID that provides uniqueness to the NET of a router. An IS-IS router must have at least one NSAP configured, and the SysID must be unique for the area in which it belongs. However, ISO 10589 allows up to three NSAP addresses per node ”all of which must have the same unique SysID of the node, differentiated only by the area prefix. The concept of having multiple NSAP addresses on the same router is called multihoming in IS-IS. Multihoming is addressed further in the next section. It is important to understand that multihoming does not allow a router to be connected to multiple separate areas. Instead, configuring multiple NSAPs with different area prefixes on a router merges the different areas, and the router continues to belong to a single, but unified, area.



IS-IS Network Design Solutions
IS-IS Network Design Solutions (Networking Technology)
ISBN: 1578702208
EAN: 2147483647
Year: 2005
Pages: 144
Authors: Abe Martey

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net