Specifications

5-12
Cisco ATM Services (AXSM) Configuration Guide and Command Reference for MGX Switches
Release 5.2, Part Number OL-6484-01 Rev. C0, September 2005
Chapter 5 AXSM Command Reference
addcon
Restrictions
Frame discard applies to connections that use ATM AAL5 adaptation (ITU-T I.363.5). Although
enabling frame discard on an AAL5 cell stream is not mandatory, it helps improve the useful throughput
on a VC by discarding complete frames during times of congestion on the switch. Without frame discard
enabled on an AA5 cell stream, corrupted AAL5 frames (containing dropped cells) can reach upper
layers and trigger numerous re-sends. Conversely, enabling frame discard on other (non-AAL5) types of
cell streams can bring uncertain results. In a worst case, total discard of end-to-end traffic of a non-AAL5
stream can occur in either direction.
The service module hardware does not support frame-based discard on VPCs. Only VCCs support
frame-based discard.
Note An important caveat exists for VPCs that were added with frame discard enabled prior to version 3.0.23
or 4.0.10 (the releases where the two types of frame discard became available). The switch lets you
enable frame discard on a VPC even though hardware does not support it. If such a VPC (with frame
discard enabled) already exists on the node when you upgrade to 3.0.23, 4.0.10, or later, you cannot
subsequently modify the VPC unless you delete it then re-add it with frame discard disabled. To avoid
the need to delete a VPC, you must disable frame discard on any such VPCs before upgrading to 3.0.23,
4.0.10, or later releases.
Policer Settings and Consequences
This section describes two types of conformance tests that occur when you enable frame discard through
this frame discard parameter. The tests are PCR and SCR conformance tests. The text is taken from ATM
Forum standards.
The PCR conformance test is performed using GCRA1 in exactly the same manner as normal cell
policing. For this test, the Action should be set to discard. If the PCR conformance test is deemed to be
non-compliant, the action will be to discard of the cells in the current frame.
In other words, a “partial packet action” can be taken when cells in the current frame fail this
conformance test. The PCR conformance test implements a partial packet discard (PPD). The policer
does a complete frame discard if the first cell of the packet was discarded as a result of PCR failure
The SCR conformance test is performed using GCRA2, although it differs slightly from the normal cell
policing.The SCR conformance test is performed only at the start of a frame. If the first cell of a frame
is a conforming CLP=0 cell, then all remaining cells will be as if they are conforming to the SCR
conformance test.
The SCR conformance test can be programmed to tag non-conforming CLP=0 cells. If the first cell of a
frame is a non-conforming CLP=0, then that cell and all other cells in that frame (including the EOM)
will be tagged. In other words, the tagged action taken by this conformance test is determined at frame
boundaries only. If the SCR conformance test is programmed to discard, the policer can discard at any
point in the frame and is not restricted by frame boundaries.
Frame Discard Setting Policer Behavior (frame discard in
addcon)
Congestion Thresholds (SCT setting)
A = 0, B = 0 Cell-based policing CLP lo/hi thresholds
A = 0, B = 1 Cell-based policing EPD thresholds
A = 1, B = 0 Frame-based policing CLP lo/hi thresholds
A = 1, B = 1 Frame-based policing EPD thresholds