Cisco Systems Network Router manual Part Number OL-3847-01 Rev. D0, April

Chapter 3 Address and Closed User Group Planning

Planning Address Configuration Settings

The network shown in Figure 3-4uses the same physical topology as that shown in Figure 3-3for an SPG WAN. The difference is that the physical network has been divided into five peer groups at level

56.The level will be explained later in this section. What is important to understand now is that the physical topology is still the same as when all nodes were in a single peer group. Dividing the physical WAN into multiple peer groups simply reduces the size of each peer group, which reduces the total number of PTSEs and the size of the PNNI database within each node. This improves PNNI network performance within each of the smaller peer groups, which leaves more bandwidth and node resources available for processing calls.

The level 40 peer group shown in Figure 3-4is a logical peer group that has been defined to enable communications between the peer groups at the lower levels. Each of the level 56 peer groups operate more efficiently because they do not have to keep up with changes in the other level 56 peer groups. However, because the level 56 peer groups do not know the details about the other level 56 peer groups, they cannot communicate with the other groups without help from a higher level process.

The level 40 peer group shown in Figure 3-4is created by adding a higher-level PNNI processes to one of the nodes in each level 56 peer group. Each higher level process operates as a logical group node (LGN) at this higher level, and together these nodes form a logical PNNI peer group at this level. The level 40 peer group nodes exchange PTSEs regarding the level 56 peer groups and maintain a database with routing information for communicating between the lower-level peer groups. Level 40 nodes do not store the routing details stored within the level 56 peer groups, because that information is already stored at the lower level. The level 40 nodes only store the information that the level 56 nodes need to locate and communicate with other peer groups.

If the network shown in Figure 3-4were to grow until there were more than 100 LGNs at level 40, the level 40 peer group could be divided into multiple peer groups and a higher level could be created to enable communication between the level 40 peer groups. This process can continue until the practical maximum of 10 levels is reached. When you consider that 100 level 40 peer groups equate to approximately 10,000 level 56 nodes (100 level 40 nodes times 100 level 56 nodes), it is easy to see how adding additional layers enables PNNI to scale.

Note These calculations are based on general guidelines. Peer groups on MGX and SES nodes can support up to 160 nodes. Also, remember that these calculations are for network nodes, not CPE. The actual number of CPE and calls supported is considerably higher.

In general, when you create an SPG or MPG network, you need to select a starting level for your PNNI network, which should be the lowest level you will ever need. You can always add higher levels to an SPG or MPG network, but creating lower levels requires a significant amount of reconfiguration.

The PNNI level is mathematically related to the ATM addresses used in a PNNI network. Valid levels are 1 through 104. These numbers specify the number of ATM address bits that are used for the peer group ID, which is described in the next section. Specifically, the level identifies the number of sequential most-significant ATM address bits that define the peer group ID.

Although the PNNI specifications provide for up to 104 PNNI levels, they also limit the practical application to 10 levels. Some PNNI experts suggest that four levels will be sufficient for most PNNI networks. For these reasons, and because it easier to translate bytes of an ATM address instead of bits, Table 3-3shows the recommended levels to use for PNNI networks.

	Cisco PNNI Network Planning Guide for MGX and SES Products, Release 5
3-8	Part Number OL-3847-01 Rev. D0, April, 2004