Fore Systems forerunner series, Configuration Manual

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ForeRunner ATM Switch Network Configuration Manual
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Configuring PVCs
Co
nfigu
ri
ng PVCs
1.7.2 Non-conforming Cells: Tagging vs. Dropping
Second, it is important to understand the concept of tagging and dropping. Each ATM cell has
a Cell Loss Priority (CLP) bit which indicates if the network can drop it under congested con-
ditions. When the CLP bit is set to 0 (or CLP=0), the cell is assessed for compliance with traffic
parameters associated with the CLP=0 stream. If the traffic parameters dictate that non-com-
pliant cells should be “tagged,” the CLP bit is set to 1 (or CLP=1) by the UPC contract, which
means that upon experiencing congestion further in the network, these CLP=1 cells are
dropped in preference to CLP=0 cells.
Table 1.1 shows the various traffic contracts and user-configurable actions to be executed upon
the detection of non-conforming cells.
1.7.3 UPC Traffic Contract Parameters
The ATM Forum has defined different types of traffic contracts to be used in conjunction with
these leaky buckets. The parameters that make up these types of contracts are defined as follows:
• pcr0 - PCR for cells with CLP=0
• pcr01 - PCR for the aggregate of the CLP=0 cells and the CLP=1 cells (all cells)
• scr0 - SCR for cells with CLP=0
• scr01 - SCR for the aggregate of the CLP=0 cells and the CLP=1 cells (all cells)
• mbs0 - MBS for cells with CLP=0
• mbs01 - MBS for the aggregate of the CLP=0 cells and the CLP=1 cells (all cells)
• tag - sets CLP bit=1 for CLP=0 cells that fail the PCR0 test for CBR0 contracts or
the SCR0/MBS0 test for VBR0 contracts
Table 1.1 - Summary of Traffic Contract Variables and Policing Actions
Traffic
Type
PCR SCR MBS Bucket 1 Bucket 2
Policing
on Switch
Fabric
Action on
non-conforming cells
Bucket 1 Bucket 2
CBR PCR01 PCR01,
CDVT Yes Drop
CLP=0+1
CBR0 PCR0,
PCR01 PCR01,
CDVT PCR0, CDVT Yes Drop
CLP=0+1 If enabled, Tag CLP=0;
else, Drop CLP=0
VBR PCR01 SCR01 MBS01 PCR01,
CDVT SCR01,
BT01+CDVT Yes Drop
CLP=0+1 Drop CLP=0+1
VBR0 PCR01 SCR0 MBS0 PCR01,
CDVT SCR0,
BT0+CDVT Yes Drop
CLP=0+1 If enabled, Tag CLP=0;
else, Drop CLP=0