Cisco Systems 8600 Series manual Virtual Trunking Example

BPX Switch Operation

Virtual Trunking

Virtual trunking provides the ability to define multiple trunks within a single physical trunk port interface. Virtual trunking benefits include the following:

•Reduced cost by configuring the virtual trunks supplied by the public carrier for as much bandwidth as needed instead of at full T3, E3, or OC3 bandwidths.

•Utilization of the full mesh capability of the public carrier to reduce the number of leased lines needed between nodes in the Cisco WAN switching networks.

•Choice of keeping existing leased lines between nodes, but using virtual trunks for backup.

•Ability to connect BNI trunk interfaces to a public network using standard ATM UNI cell format.

•Virtual trunking can be provisioned via either a Public ATM Cloud or a Cisco WAN switching ATM cloud.

A virtual trunk may be defined as a “trunk over a public ATM service”. The trunk really doesn’t exist as a physical line in the network. Rather, an additional level of reference, called a virtual trunk number, is used to differentiate the virtual trunks found within a physical trunk port. Figure 1-5shows four Cisco WAN switching networks, each connected to a Public ATM Network via a physical line. The Public ATM Network is shown linking all four of these subnetworks to every other one with a full meshed network of virtual trunks. In this example, each physical line is configured with three virtual trunks.

Figure 1-5 Virtual Trunking Example

Cisco	Cisco
sub-network	sub-network
ATM-UNI	ATM-UNI

Leased line

(backup)

Public ATM

Network

ATM-UNI	ATM-UNI
Cisco	Cisco
sub-network	sub-network

Virtual trunk

Leased line

H8227

Introduction 1-13