Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 234

path as the group IDs for the turquoise, black, orange, and red groups. When the receiving
switch in the turquoise group receives the connection request, it creates a detailed DTL for its
own group. This process repeats until the connection request is routed across all intermediate
groups.

Crankback and alternative routing is made more efficient in hierarchical networks. In a non-
hierarchical network, a rejected call must go all the way back to the DTL originator before an
alternate path can be calculated. With PNNI hierarchy, the call goes back to the entry point node
within the current level. Using Figure 5-23, if the call is rejected within the turquoise group
because a link within the turquoise group cannot meet the required QoS metrics, crankback is
started. The call returns to the entry node within the turquoise group, which then continues the
call setup process on a path that avoids the rejecting link.

ATM ES Addresses
To support SVCs and PNNI routing of connections, switches and interfaces must be assigned a
20-byte ATM address. Each ES must have at least one ATM ES address (AESA). The AESA
identifies the PNNI peer group, the switch ID, and the ES ID. When the originating end device
sends a SVC connection request, it identifies the ATM address of the destination device.

The following three types of addresses are used in private ATM networks:
•

Data Country Code (DCC) —DCC addresses always start with a prefix of 39. Different
authorities in each country administer DCC addresses. In the United States, the American
National Standards Institute (ANSI) administers DCC addresses.

•

International Code Designator (ICD) —ICD addresses always start with a prefix of 47.
The British Standards Institute administers ICD addresses.

•

Encapsulated E.164 —E.164 addresses always start with a prefix of 45. E.164 addresses
are essentially telephone numbers and are administered by the telephone carriers. E.164
addresses support ISDN.

An ATM address is divided into a prefix and an ES ID. Figure 5-24 shows the division between
the prefix and the ES ID.

The AESA and the level identify PNNI peer groups. The PNNI standard supports levels 0 through
104. The level number corresponds to the number of address bits that you use for routing. When
selecting a level number, you can use any number between 0 and 104, but it is easier to work with
numbers that represent 8-bit boundaries.

For example, level 56 indicates that the first 56 bits (7 bytes) of the ATM address identifies the
peer group level. Figure 5-25 shows examples of level 56 ICD addresses.