Cell Transmit Address Translation; Cell Receive Address Lookup; Selection Of Connection Identifier; Routing Vpcs Over Virtual Trunks - Cisco BPX-BXM-155-8DX Installation And Configuration Manual

Connection Management

Cell Transmit Address Translation

All cells transmitted to a virtual trunk have a translated cell address. This address consists of a VPI
chosen by the user and a VCI (ConId) chosen internally by the software. The trunk firmware is
configured by the software to perform this translation.

Cell Receive Address Lookup

The user-chosen VPI is the same for all cells on a virtual trunk.
At the receiving end, multiple virtual trunks can send cells to one port. The port must be able to
determine the correct channel for each of these cells.
The VPI is unique on each trunk for all the cells, but the VCI may be the same across the trunks. Each
port type has a different way of handling the incoming cell addresses. Only the BXM and UXM are
discussed here.

Selection of Connection Identifier

For connections, the associated LCNs are selected from a pool of LCNs for the entire card. Each virtual
trunk can use the full range of acceptable conid values. The range consists of all the 16-bit values (1
through 65535) excluding the node numbers and blind addresses. A port uses the VPI to differentiate
connections which have the same conid.
You can change the number of channels per virtual trunk once the trunk has been added to the network.
Decreasing the number of channels on an added virtual trunk will cause connection reroutes whereas
increasing the number of channels on an added virtual trunk will NOT cause connection reroutes.

Routing VPCs over Virtual Trunks

A VPC is not allowed to be routed over a virtual trunk. The routing algorithm excludes all virtual trunks
from the routing topology. The reason for this restriction is due to how the virtual trunk is defined within
the ATM cloud.
The cloud uses a VPC to represent the virtual trunk. Routing an external VPC across a virtual trunk
would consist of routing one VPC over another VPC. This use of VPCs is contrary to its standard
definition. A VPC should contain multiple VCCs, not another VPC. In order to avoid any non-standard
configuration or use of the ATM cloud, VPCs cannot be routed over a virtual trunk through the cloud.

VPC Configuration with the ATM Cloud

In order for the virtual trunk to successfully move data through an ATM cloud, the cloud must provide
some form of connectivity between the trunk endpoints. The ATM equipment in the cloud must support
virtual path switching and move incoming cells based on the VPI in the cell header.
A virtual path connection (VPC) is configured in the cloud to join two endpoints. The VPC can support
either CBR, VBR, or ABR traffic. A unique VP ID per VPC is used to moved data from one endpoint
to the other. The BPX nodes at the edge of the cloud send in cells which match the VPC's VPI value.
As a result the cells are switched from one end to the other of the ATM public cloud.
Within the ATM cloud one virtual trunk is equivalent to one VPC. Since the VPC is switched with just
the VPI value, the 16 VCI bits (from the ATM cell format) of the ATM cell header are passed
transparently through to the other end.
Cisco BPX 8600 Series Installation and Configuration
25-10
Chapter 25 Configuring BXM Virtual Trunks
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001