Connection Management; Cell Header Formats - Cisco BPX-BXM-155-8DX Installation And Configuration Manual

Connection Management

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Connection Management
The cell addressing method for connections routed through a virtual trunk handles multiple type of
traffic flowing through an ATM cloud. The header format of cells may match the ATM-UNI or
ATM-NNI format because the port interface to the ATM cloud is a physical configured as either a UNI
or NNI interface, as specified by the administrator of the ATM cloud.

Cell Header Formats

Cells transmitted to a virtual trunk use the standard UNI or NNI cell format.
Before cells enter the cloud on a virtual trunk, the cell header is translated to a user configured VPI
value for the trunk, and a software configured VCI value which is unique for the cell. The trunk card at
the edge of the cloud ensures that cells destined for a cloud VPC have the correct VPI/VCI.
As cells are received from the cloud by the BPX or IGX in the Cisco networks at the other end of the
cloud, these VPI/VCIs are mapped back to the appropriate VPI/VCI addresses by the Cisco nodes for
forwarding to the next destination.
The VPI is an 12-bit value ranging from 1 through 4095. The VCI is a 16-bit value ranging from 1
through 65535.
The VPI value across the virtual trunk is identical for all cells on a single virtual trunk. The VCI value
in these cells determines the final destinations of the cells. On BNI cards, for virtual trunking a modified
ATM UNI cell format (Strata-UNI) stores the ForeSight information, as applicable, in the header of a
Strata-UNI cell format. A virtual trunk with a BNI at one end must terminate on a BNI at the other end.
Figure 25-5 shows three different cell header types, ATM-STI, ATM-UNI, and Strata-UNI through a
cloud. The ATM-NNI header which is not shown, differs in format from the ATM-UNI only in that there
is no GFCI field and those four bits are added to the VPI bits to give a 12-bit VPI.
The ATM-STI header is used with BNI trunks between BPX nodes within a Cisco switch subnetwork.
The ATM-UNI is the standard ATM Forum UNI supported by the BXM card along with standard NNI.
Virtual trunks terminating on BXMs or UXMs use the standard ATM-UNI or ATM-NNI header as
specified by the cloud administrator (such as, Service Provider). Virtual trunks terminating on BNIs use
the Strata-UNI header.
Cisco BPX 8600 Series Installation and Configuration
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Integrated Local Management Interface (ILMI)
ILMI provides data and control functions for the virtual trunking feature. It is not necessary to
configure the ATM ports to block signalling traffic to Cisoc equipment due to ILMI (Integrated Layer
Management Interface) signalling support.
Blind Addressing
Each virtual trunk is assigned a blind address. In general terms, the blind address is used by a node
to communicate to the node at the other end of a trunk. Specifically the blind address is used for
sending messages across a virtual trunk during trunk addition, and for sending messages for the
Trunk Communication Failure testing.
VPC Failure Within the ATM Cloud
Any VPC failure within the ATM cloud generates a virtual trunk failure in the Cisco network. This
trunk failure allows applications (such as connection routing) to avoid the problem trunk.
Upon receiving notification of a VPC failure, the trunk is placed into the "Communication Failure"
state and the appropriate trunk alarms are generated. The trunk returns to the "Clear" state after the
VPC clears and the trunk communication failure test passes.
Chapter 25 Configuring BXM Virtual Trunks
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001