How to configure rate-limited FRGP ATM PVCs
General
ATM PVC connections at the FRGP originate on the M20
router frgp-gw-1, but pass thru the ATM switch frgp-as-1 to be
distributed to SONET connections to other ATM switches (4-campus, for
example), CRS connections (DU and FLC, for example), or leased lines
(UW, for example). PVCs in the Juniper router must be configured with ATM
shaping to not overrun the switch or the downstream network, and PVCs
in the Cisco LS1010 switch must have "cttr" options configured to
match the Juniper. A complicating factor here is the different
ways the two vendors refer to ATM bit rates. Cisco counts a cell
as 53 bytes, whereas Juniper only counts the 48 byte
payload. Juniper ATM bit rates are configured to be 48/53 that of
Cisco or Qwest CRS rates.
Traffic class: UBR, VBR, or CBR?
UBR, or Unspecified Bit Rate, has no User Parameter Control (UPC)
limits, and is the best and easiest way to most effectively maximize
the use of leased circuits or switch-to-switch connections. It's also
the cheapest and most flexible Cell Relay Service option. UBR PVCs
accept packet bursts that dramatically reduce latency as compared to
CBR PVCs that accept cells only into isochronous transmission
slots. CBR cannot accept packet bursts. However, most managers want
bandwidth guarantees to assure they're getting what they paid for, so
most CRS circuits are purchased as CBR (Constant Bit Rate)
services. CBR is the most difficult service to fully utilize without
extensive and laborious tuning. Cells issued too rapidly into a CBR
CRS circuit will be dropped by the UPC policer at the ingress to the
CRS network. The only way to assure cells won't be dropped by a CBR is
to configure rates below the CBR CRS rate.
PVC VPI/VCI assignment
By ancient and venerable tradition,
each FRGP member is assigned a separate VPI. At one time, VCIs of 100
identified Abilene connections and were switched directly to the
Abilene router. VCIs of 200 and 300 identified the first and second
commodity connections. Its nice to keep this system, but Qwest names
PVCs incrementally from 1/33 and its nicer to keep a PVC's VPI/VCI
numbers unchanged as it transits the FRGP, so there are a few assigned
this way.
Juniper Configuration
The OC-12 ATM interface on frgp-gw-1 is at-0/2/0. PVCs are created on
a subinterface which is generally assigned by adding or,
alternatively, concatenating the VPI and VCI numbers. Usually we use
atm-snap encapsulation to allow easy IPv6 implementation, but the
atm-vc-mux encapsulation is more efficient and desirable for low-speed
links (TCP ACK in one cell rather than two!)(And SNAP shouldn't be
necessary for IPv6 anyway - its still IP - but that's what JUNOS
does). A limited number of VCs are allowed on the interface, so each
VPI must be set with the maximum number of VCs that will be supported.
Shaping with a limited queue-length must be configured to prevent
cell-buffer starvation; UBR can be faked by shaping with VBR PVC with
very liberal UPC (User Parameter Control). Calculate JUNOS bit rates
by multiplying Cisco or Qwest CRS rates by 48/53. For PVCs that stay
on leased lines, be sure to configure a maximum rate no larger than
the physical circuit.
Example: DU's connection
interfaces {
at-0/2/0 {
vpi 1 {
maximum-vcs 401;
}
unit 133 {
description "------------------------------------------------- DU PVC 1/33";
encapsulation atm-snap;
vci 1.33;
shaping {
cbr 48m;
queue-length 300;
}
family inet {
mtu 4470;
(extraneous stuff deleted)
}
family inet6 {
mtu 4470;
(more extraneous stuff deleted)
}
}
Cisco LS1010 Configuration
PVCs are technically one-way only, but Cisco and most other
manufacturers install PVCs in both directions with one command. LS1010
IOS always displays the PVC configuration on the highest number
interface (or, equivalently, the lowest line in the config), even when
the configuration was applied to a low interface number towards the
top of the config file. It can be disconcerting to have a newly
configured PVC vanish, so be sure to look for it on the highest numbered
interface.
No User Parameter Control (UPC) is required for UBR CRS PVCs or PVCs that travel on
leased circuits or dedicated connections. CRS CBR PVCs, however,
need to be rate-shaped to avoid cell drops by the CRS ingress
policer. A "Connection Traffic Table Row" must be configured with the
appropriate UPC values, and that "CTTR" must be applied to
the PVC. Here're a few example Connection Traffic Table Rows:
atm connection-traffic-table-row index 11 cbr pcr 12352
atm connection-traffic-table-row index 20 vbr-nrt pcr 35000 scr0 30000
atm connection-traffic-table-row index 21 ubr pcr 150000 mcr 25000
Note that the rates are in kbps even tho the "cr" stands for "Cell
Rate." Note also that UBR PVCs can be configured with a Peak Cell Rate and a
Minimum Cell Rate. The Minimum Cell Rate guarantees that the
UBR PVC will transmit at least the minimum rate. No burst
sizes are specified for UBR, which is a little bothersome
and could be a problem for CRS but never seems to be.
The "CTTR" is applied to the PVC in each direction and could
conceivably be different in each direction.
Make sure to include the "pd on" option to invoke packet
discard. Packet discard causes the switch to drop all subsequent cells
of a single packet after UPC forces the drop of any cell. Cell drops
relieve congestion much more effectively with packet discard turned
on. Here're a few example PVC configuration lines:
interface ATM4/0/0
atm pvc 1 32 pd on rx-cttr 15 tx-cttr 15 interface ATM3/1/0 1 32
atm pvc 1 33 pd on rx-cttr 12 tx-cttr 12 interface ATM3/1/0 1 33
atm pvc 1 34 pd on interface ATM3/1/0 1 34
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