Beta Draft Confidential
About ATM Logical Ports
VPs and VCs
ATM Services Configuration Guide for CBX 3500, CBX 500, GX 550, and B-STDX 9000
1/19/05
2-13
Setting the Number of Valid Bits in the VPI/VCI
The Number of Valid Bits setting applies to the VPI and VCI range that you can use
for VCCs (both PVCs and SVCs). The default values of VPI = 4 and VCI = 10 mean
that you can use VCCs over the range of VPI = 0 – 15 (4 bits of VPI) and a VCI range
of VCI = 32 – 1023 (10 bits of VCI). The values have no effect on VPCs, which you
can provision anywhere over the VPI = 0 – 255 range; you can provision VPCs over
the VPI = 0 – 4095 range if you use the NNI cell header format.
For the CBX 3500 and CBX 500, the valid range for the VPI field is 0 – 8 and the
valid range for the VCI field is 6 – 14; for the GX 550, the valid range of the VPI field
is 0 – 12 and the VCI field is 6 – 13. You may have to adjust these values in the
following situations:
•
In cases where the required VPI/VCI(s) of the attached devices are outside the
range that the default values provide (VPI = 0 – 15 and VCI 32 – 1023).
•
If you use this logical port as a feeder for OPTimum trunks or virtual UNIs, the
VPI value limits the number of OPTimum trunks you can create on this physical
port. The VCI value limits the number of circuits you can route over each
OPTimum trunk.
This OPTimum trunk/circuit trade-off is shown by the following formulas:
For example, if you set the VPI value to 3 and the VCI value to 11, you can have
up to 7 virtual paths on the port, and up to 2,016 virtual channels on each path.
VPI/VCI Bit Allocation
When configuring a Direct Trunk or UNI logical port on a CBX 3500, CBX 500, or
GX 550, you select the number of bits in the Number of Valid bits in VPI/VCI field.
The highest order bit set (1) is used for reserving address space for
Multipoint-to-Point Tunnel (MPTs). It does not matter what the bit allocation for
VPI/VCI is, a set portion of the bit is used for establishing 16 MPTs across a trunk.
The number of bits configured for VPI will directly affect the total number of transit
VCs.
Maximum virtual paths = 2
P
– 1
(
P
represents the value in the Valid Bits in VPI field)
Maximum virtual channels = 2
C
– 32
(
C
represents the value in the Valid Bits in VCI field)
Note – On a CBX 500 only, P+C
≤
14.
Summary of Contents for CBX 3500
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