Cisco BPX 8650, Руководство по установке и настройке

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Cisco BPX 8600 Series Installation and Configuration
Release 9.3.30, Part Number 78-12907-01 Rev. E0, May 2005
Chapter 24 Configuring BXM Virtual Trunks
Connection Management
Handling VPC Failure Within the ATM Cloud
Any VPC within the ATM cloud generates a virtual trunk path failure in the Cisco network. This trunk
failure allows applications (such as connection routing) to avoid the problem trunk. Upon receiving
notification of the 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 failure clears
and the trunk communication failure test passes.
The ATM cloud notifies the Cisco network of the VPC failures using an ILMI traps or F4 end-to-end
AIS OAM flows. Prior to switch software Release 9.3.30, virtual trunk path failure could be detected
only by the receipt of an ILMI trap. This was a limitation because many public ATM clouds use AIS
OAM flows, instead of ILMI, to signal alarm conditions. As a result, virtual trunk path failure was often
detected indirectly through a communication failure test, causing a substantial delay in rerouting
connections or other corrective action. The current software provides the ability to detect F4 AIS flow
on a BXM virtual trunk, which reduces the failure detection time by a factor of ten compared to the
communication failure test.
The F4 AIS OAM cell detection enhancement is disabled by default. It can be enabled on a per trunk
basis using the
cnftrk command.For a detailed explanation of the cnftrk command and related
commands refer to the Cisco WAN Switching Command Reference , Release 9.3.30.
Note The F4 AIS OAM cell detection enhancement provides a new entry point into the virtual trunk path
failure alarm. As a result, more connection rerouting can occur. Users are advised to consider using
the Trunk Deroute Delay timer to avoid excessive rerouting during brief outages.
Connection Management
The cell addressing method for connections routed through a virtual trunk handles multiple types of
traffic flowing through an ATM cloud. The header format of cells can match the ATM-UNI or ATM-NNI
format because the port interface to the ATM cloud is a physical interface 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 to 4095. The VCI is a 16-bit value ranging from 1 to 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, virtual trunking is a modified
ATM UNI cell format (Strata-UNI) that 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.