Cisco Network Router User Manual page 38

The Hierarchical PNNI Network Topology
Hierarchical networks can support thousands of nodes because each higher level summarizes
information for all lower levels. For example, suppose a level 64 peer group were added below Peer
Group 2 in
leader for Peer Group 2. The impact on the hierarchical network would be the following:
•
•
•
The following sections provide additional information the peer group leaders that operate at higher levels
in a PNNI hierarchy and introduce the border nodes that connect one peer group to another.
Peer Group Leaders
A peer group leader (PGL) is a higher level node (such as the level 40 nodes in
summarizes data for a child peer group (such as the level 56 nodes in
a peer group that operates one level below the PGL that supports it. Each PGL works with other PGLs
at the same level to build and maintain network data that it summarizes and distributes to its child peer
group. The PGL also receives summarized data from a parent PGL if another level exists above the PGL's
level. Network data from levels above the PGL is also summarized and distributed to child peer groups.
Network administrators can use configuration commands to control which node becomes the PGL. The
configuration process assigns a PGL election priority to each node in the peer group. When PNNI nodes
start up, an election is held to determine which node has the highest PGL priority, and that node becomes
the PGL. If the PGL node fails, a new election is held among the operating nodes to determine a new
PGL. There is just one peer group leader for each peer group.
Each higher level peer group is made up of LGNs that represent the peer groups at the next lower level.
These LGNs collect and manage information that is needed to communicate with the peer groups
represented. As with the lowest level, these LGNs elect a PGL, which is responsible for determining
communications paths to PNNI groups not represented within the peer group.
Network administrators add higher levels by adding LGNs with the addpnni-node command. The PGL
Note
election priority is configured with the cnfpnni-election command.
The PGL task adds to the work load of a PNNI node. The PGL must not only collect and manage data
for communications outside the peer group, it must also collect and manage data for communications
within the peer group. Because the PGL task adds to the work load of a PNNI node, it is good design
practice to choose peer group leaders (and backup peer group leaders) carefully. Consider reducing the
load on switches that serve as peer group leaders, and avoid using border nodes as peer group leaders.
Simple Node Representation
When a LGN presents its child peer group information to other peer groups, the default representation
is called simple node representation. To other peer groups, the local peer group is represented as a single
node with no nodal state parameters.
Cisco PNNI Network Planning Guide for MGX and SES Products, Release 5
1-6
Figure 1-3. All nodes in the new level 64 peer group would be summarized by the peer group
Peer Group 2 would support one less physical node because it would have to add one LGN to
represent the level 64 child peer group.
The level 64 child peer group could support as many as 155 physical nodes (160 nodes - 1 LGN for
each of the 5 peer groups at the higher levels.
There would still be plenty of room for adding more physical nodes to levels above and below those
shown in Figure 1-3.
Figure 1-4 illustrates simple node representation.
Chapter 1 Introduction to PNNI
Figure 1-3) that
Figure 1-3). A child peer group is
Part Number OL-3847-01 Rev. D0, April, 2004