Cisco Systems Network Router manual Pnni Addressing Example

Chapter 3 Address and Closed User Group Planning

Address Planning Overview

Furthermore, PNNI switches exchange data with all other nodes in the peer group, so lots of address information would be transmitted constantly throughout the network as PNNI monitors the network topology.

Now let’s consider a more efficient example. Figure 3-1shows a PNNI network with some simplified addresses in place of the 20-byte ATM addresses.

Figure 3-1 PNNI Addressing Example

A3 A5

A.11

A1

A.12

Peer group A

A.21

A2

A.22

A4

A6

A.13	A.23
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For consistency, assume that the six switches shown in Figure 3-1connect to a total of 100 destinations. Notice that the destination addresses for the external lines connected to A.1 all use the prefix A.1, and the destination lines connected to A.2 use the prefix A.2. When you configure a common prefix for multiple addresses, you can reduce the size of the routing table and the topology database by storing routes to the address prefix, instead of routes to every destination. In this example, all nodes in Peer Group A store routes to the other switches, but there is no need to store additional routes for every destination address. The use of address prefixes is also called address summarization.

Address summarization also makes network management easier because you do not need to manually enter every AESA into the source nodes. Instead, you define a PNNI address prefix, which summarizes all destinations that share that prefix.

Address summarization does not preclude the use of non-conforming addresses. For example, if network management dictates the use of a specific non-conforming ATM address for a destination, that address can be manually entered at the switch, and PNNI will advertise a route to that device. The non-conforming address is called a foreign address. The support of foreign addresses makes PNNI more flexible, but keep in mind that excessive use of foreign addresses does impact switch performance.

Tip Chapter 4, “Planning Intermediate Route Selection,” describes how up to five routes can be stored in a total of 10 route tables for each destination. To understand the impact of foreign addresses, multiply the potential of 50 routes times the number of switches in a peer group, and then multiply that number times the number of foreign addresses. Address summarization is a key component in PNNI address planning.

When a call is placed to a destination address, PNNI refers to the destination addresses and prefixes in the routing tables or topology database. After the best route is chosen to the destination switch, the destination switch selects the appropriate destination interface by searching internal address tables for the longest prefix match. When a switch and its interfaces are configured with prefixes that enable PNNI to quickly locate the destination interface, PNNI routing is most efficient.

Although address summarization does make network management easier and routing more efficient, it can be misused and make PNNI routing less efficient. Consider the case where the same address prefix is assigned to multiple nodes. This is a valid configuration, but it can lead PNNI to unnecessarily reroute

	Cisco PNNI Network Planning Guide for MGX and SES Products, Release 5
3-2	Part Number OL-3847-01 Rev. D0, April, 2004