Chapter 25 Configuring Bxm Virtual Trunks; Overview - Cisco BPX-BXM-155-8DX Installation And Configuration Manual

Configuring BXM Virtual Trunks
This chapter describes Broadband Switch Module (BXM) virtual trunks, a feature supported by the
BXM cards beginning with switch software Release 9.2:
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Note: Virtual trunking is a purchased feature; Cisco Customer Service must enable it on each node
where you intend to use virtual trunking.

Overview

Virtual trunking provides connectivity for Cisco switches through a public ATM cloud as shown in
Figure 25-1. Because a number of virtual trunks can be configured across a physical trunk, virtual
trunks provide a cost effective means of connecting across a public ATM network. Each virtual trunk
typically uses only part of a physical trunk's resources. Yet, like regular trunks, virtual trunks can carry
high-priority traffic.
The hybrid network configuration provided by virtual trunking allows private virtual trunks to use the
mesh capabilities of the public network in interconnecting the subnets of the private network.
A virtual trunk is simply "a trunk defined over a public ATM service." The trunk really does not exist as a
physical line in the network. You use an additional level of reference, called a virtual trunk number, to
differentiate the virtual trunks found within a physical port.
You establish connectivity through a public ATM cloud by allocating virtual trunks between the nodes on
the edge of the cloud. With only a single trunk port attached to a single ATM port in the cloud, a node uses
the virtual trunks to connect to multiple destination nodes on the other side of the cloud. From the
perspective of a Cisco node, a virtual trunk is equivalent to a VPC provided by the ATM cloud network,
which provides connectivity through the cloud.
The ATM equipment in the cloud must support Virtual Path switching and moving incoming cells based on
the VPI in the cell header. Within the cloud, one virtual trunk is equivalent to one VPC. Because the VPC
is switched with just the VPI value, the 16 VCI bits (from the ATM cell format) of the ATM cell header are
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001
Overview
How Virtual Trunking Works
Connection Management
General Procedure to Set Up a Trunk
Example: Virtual Trunk Across an ATM Network
Command Overview
Compatibility Between Cards in Virtual Trunks
C H A P T E R
Cisco BPX 8600 Series Installation and Configuration
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