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59-47
User Guide for Cisco Security Manager 4.4
OL-28826-01
Chapter 59 Configuring Router Interfaces
PVCs on Cisco IOS Routers
As shown in Figure 59-2, a virtual path is a bundle of virtual channels, all of which are switched
transparently across the ATM network on the basis of the common VPI. A VPC can be thought of as a
bundle of VCCs with the same VPI value.
Figure 59-2 ATM Virtual Path and Virtual Channel Connections
Every cell header contains a VPI field and a VCI field, which explicitly associate a cell with a given
virtual channel on a physical link. It is important to remember the following attributes of VPIs and VCIs:
• VPIs and VCIs are not addresses, such as MAC addresses used in LAN switching.
• VPIs and VCIs are explicitly assigned at each segment of a connection and, as such, have only local
significance across a particular link. They are remapped, as appropriate, at each switching point.
Using the VPI/VCI identifier, the ATM layer can multiplex (interleave), demultiplex, and switch cells
from multiple connections. Certain VPI/VCI identifiers are reserved for particular uses, such as the
Integrated Local Management Interface (ILMI).
Related Topics
• Understanding ATM Service Classes, page 59-47
• Understanding ATM Management Protocols, page 59-48
• Defining ATM PVCs, page 59-50
• PVCs on Cisco IOS Routers, page 59-46
Understanding ATM Service Classes
Version 4.0 of the Traffic Management Specification published by the ATM Forum defines five service
classes that describe the user traffic transmitted on a network and the quality of service that a network
needs to provide for that traffic. Security Manager supports the following ATM service classes:
• Available Bit Rate (ABR) This is a service class where ATM switches make no guarantee of cell
delivery, but do guarantee a minimum bit rate and that cell loss is kept as low as possible with the
use of a feedback mechanism. The ABR service category is designed for VCs that carry file transfers
and other bursty, non-real-time traffic that requires a minimum amount of bandwidth. This
bandwidth is specified via a minimum cell rate that must be available while the VC is configured
and active. For more details, see Understanding the Available Bit Rate (ABR) Service Category for
ATM VCs at:
http://www.cisco.com/en/US/tech/tk39/tk51/technologies_tech_note09186a00800fbc76.shtml.
• Constant Bit Rate (CBR) This is a service class where cells are transmitted in a continuous bitstream
to meet voice and video QoS needs. The CBR service class is designed for ATM virtual circuits
(VCs) that need a static amount of bandwidth that is continuously available for the duration of the
active connection. An ATM VC configured as CBR can send cells at peak cell rate (PCR) at any time
and for any duration. It also can send cells at a rate less than the PCR or even emit no cells. The
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