Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 233

When a switch receives a request to establish a connection across the network, it creates a des-
ignated transit list (DTL) that identifies the route the connection will try to take. The connection
request is sent to the next switch in the DTL. That switch has the choice to accept the connection
or to reject it based on the connection admission control (CAC) function and the requested QoS.
Rejecting a connection request is called crankback .

Because PNNI exchanges QoS metrics and topology information between switches, two or
more VCs can be established to the same prefix over different paths because the connection
requests had different QoS requirements.

Signaling between switches is across an NNI.

If a connection request is rejected, the switch making the request can try a different route. After
the connection is accepted, the process repeats until the final ES is reached. If the ES accepts the
connection, a signal is sent back to the origination ES, and the connection is established.

In large networks, the PNNI routing protocol tables can become too large for efficient operation.
So, PNNI supports a hierarchy of peer switch groups for large networks.

A peer group of ATM switches shares a common peer group ID as part of their ATM address.
The peer group level determines the portion of the address that comprises the peer group ID.
Figure 5-23 shows an ATM network comprising seven peer groups at three levels.

PNNI Hierarchy Figure 5-23

Level 40

PGL

1

6

2

4 5 3

Black

Level 48

3.1

3.3 3.4

3.2

Turquoise

Orange

Level 56

PGL

3.23 3.24 3.41

3.21 3.25

3.22 3.44

3.42 3.43

Blue Green Yellow Red

To reduce the amount of routing information being distributed to each switch, PNNI summarizes
the information for each group.

Within each group, an elected peer group leader (PGL) sends summarized address prefixes to
the PGL in each of the other groups. Prefixes are partial addresses that summarize a group of
addresses.

In the example shown in Figure 5-23, the PGL in the turquoise group advertises the summarized
ATM address for its group to the green, blue, black, orange, yellow, and red PGLs. (The colors
are labeled in Figure 5-23.) When a switch in the green group wants to make a connection to a
switch in the red group, it creates a detailed DTL for its own group, plus it identifies the remaining