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Cisco PNNI Network Planning Guide for MGX and SES Products, Release 5
Part Number OL-3847-01 Rev. D0, April, 2004
Chapter 1 Introduction to PNNI
The Single Peer Group Topology
Tip A node is a network device that communicates with other network devices. Cisco PNNI-compatible
devices serve as nodes in a PNNI network. In this document, the terms node and switch are often used
interchangeably. However, in most cases, the PNNI node is a component of a Cisco PNNI-compatible
device. For example, some Cisco MGX switches, Release 2.0 and later, can operate as both a PNNI node
and as an MPLS device.
The PNNI protocol communicates the state of a PNNI network in PNNI Topology State Elements
(PTSEs). PTSEs are discrete messages that contain information about one of the following types of
network components:
• PNNI nodes
• Reachable addresses
• PNNI links between nodes
To enable communications with other nodes, each switch needs to have all the PTSE information for
each switch in the network. Each node is responsible for flooding out its own PTSE information to all
the other switches in the network.
Since up-to-date PTSE information is required for optimal routing decisions to be made, there are several
different mechanisms in place to help ensure that all nodes have reasonably accurate PTSE information.
The five common reasons for updating PTSEs are as follows:
• Resources administratively added, removed or altered on a node.
• Resource failure such as an Loss of Signal (LOS) on a link.
• A significant change in link resources due to virtual circuits (VCs) routing or derouting.
• Periodic updates defined by the PTSE refresh and PTSE lifetime interval timers.
• A processor switch module (PXM) switchover.
PTSE information is passed between nodes using PNNI Topology State Packets (PTSPs). These packets
utilize the Routing Control Channel (RCC; VPI = 0 and VCI = 18) between adjacent nodes. The RCC is
also used for Hello packets and other PNNI messages. If the switch is unable to establish the RCC with
the adjacent node, then PTSE information is not exchanged. Once a node receives PTSE information,
the node stores the contents, or element information, in the database. This information is used to generate
precomputed routing tables that identify routes to other network devices. The PNNI database is also used
to perform on-demand routing when the appropriate routing table does not contain a viable path.
The Single Peer Group Topology
A single peer group topology is a PNNI network in which all nodes share PTSEs with all other nodes.
As each node is brought up in a single peer group network, that node learns about all the other nodes,
and the other nodes learn about the new node. All nodes are capable of determining routes to all other
nodes within the single peer group. Figure 1-1 shows an example single peer group topology.