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This chapter provides an overview of the PNNI routing protocol for ATM networks, gives definitions for key terms used in the PNNI protocol, and describes the AtmDirector features for managing PNNI routing. This chapter also describes the PNNI topology map and provides instructions for obtaining node and link information.
PNNI is a dynamic source routing protocol for ATM internetworks that provides routing between ATM switches and groups of switches. The PNNI routing protocol was designed with two goals mind:
The following subsections briefly describe the main features of the PNNI protocol.
PNNI routing is based on switched virtual circuits (SVCs), used elsewhere in ATM networking. SVC routing requires that signaling establish a path to a destination before the originating system sends any data on that path.
PNNI is a dynamic, rather than static, routing protocol. While static routing protocols require operator intervention to accommodate network changes, dynamic routing protocols can adapt to changing network conditions by advertising their reachability and other topology state information. Cisco's implementation of PNNI on the LightStream 1010 switch can interoperate with the Interim Inter-switch Signaling Protocol (IISP), a static routing protocol, to provide routing among multiple peer groups.
Hop-by-hop routing computes a path using a table of "next" hops and the destination node address provided by the source node. Source routing, used by PNNI, takes a different approach: The whole path is specified by the source node, the path information is included in the call setup message, and signaling follows the path accordingly.
PNNI supports QoS routing, which allows selection of network routes or paths based on parameters requested for the connection. Parameters such as maximum cell delay, maximum cell delay variation, maximum cell loss ratio, and so on, are combined with a user-assigned administrative weight to calculate route selection.
A PNNI topology consists of the following basic elements:
To implement dynamic source routing on ATM networks with support for QoS route selection, PNNI uses a number of unique mechanisms. These mechanisms, described in the following subsections, are primarily responsible for learning, disseminating, and maintaining information about topology state, reachability, and routing metrics.
The PNNI Hello Protocol, modeled on the OSPF protocol, is used to support the hierarchical organization of network nodes. To discovery the identity of an adjacent switch, hello packets are exchanged containing the appropriate information. If the switches discover they are members of the same peer group, they form an inside link. If they discover they are members of different peer groups, they exchange additional information and create an outside link.
Once the Hello Protocol establishes that a link is functional, the adjacent switches exchange summary packets containing header information for all PNNI Topology State Elements (PTSEs) in their respective databases. Synchronizing the databases enables the switches to maintain the same view of the topology.
PNNI Topology State packets (PTSPs) are used to disseminate information by means of the flooding mechanism. PTSPs contain reachability, link status, and node status information needed to calculate QoS paths.
Reachability information is the first step in routing a PNNI request for a connection. The call setup message is directed to a node that advertises a prefix that matches the leading portion of the destination address. The longest matching reachable address prefix is always used. Reachable addresses can be internal or external. Internal addresses are known to PNNI to be local, for example, summary end systems attached to the switch. External addresses are addresses for which the reachability information came from elsewhere. A link to an IISP network is one example of an external address.
In addition to topological reachability information, PNNI also advertises detailed information about metrics and attributes for links. These metrics and attributes are described in Table 5-1.
PNNI supports scalability through a hierarchical organization of nodes into peer groups. A peer group is represented at the next level of the hierarchy as a logical group node by aggregated topology information. This information is summarized and advertised to higher and lower levels by a specially elected node called the peer group leader.
AtmDirector displays PNNI topologies based on the discovery of PNNI nodes and links. PNNI discovery proceeds in two phases:
The AtmDirector main window displays the names of the PNNI routing domains in the network and the peer groups that belong to each domain. You can launch a topology map for any PNNI domain or peer group from the AtmDirector main window. The topology map includes the devices and links that participate in the PNNI routing.
To display a PNNI topology map:
The topology service is launched for the selected PNNI domain or peer group, and the topology map is displayed (Figure 2-5).
The PNNI topology map displays the following elements:
From the PNNI topology map you can obtain configuration information for a node and connectivity information for a link.
For configuration and related information about a node, see, "Viewing the Node Configuration and Information" in Chapter 12, "Configuring and Monitoring PNNI Nodes."
The Link Information window shows the end nodes of a link and identifies the type of link between the nodes.
To display link information, follow these steps:
Step 1 Display the PNNI topology by double-clicking a PNNI domain or peer group name in the AtmDirector main window.
Step 2 Select a link from the topology.
Step 3 Select PNNI Options>Link Information.
The Link Information window opens (Figure 5-1). The fields in this window are described in Table 5-2.
Step 4 Click OK when you have read the link information.
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