Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual page 561

Chapter 6: Layer 2 Services over MPLS Overview
manual configuration and MPLS signaling protocol (LDP) state that had to be
maintained.
Now, you can emulate physical connectivity between two ATM ports that are not
directly connected. This emulation is made possible by transporting ATM cells
belonging to a subset of matching ATM VCs on both the ATM ports over a single
pseudowire. Multiple VCs over a single psuedowire is useful in scenarios when ATM
switches are connected using a high-speed packet switched network, instead of
expensive physical cables.
Support for configuration of multiple ATM VCs over a single pseudowire is based on
RFC 4816, Pseudowire Emulation Edge-to-Edge (PWE3) Asynchronous Transfer Mode
(ATM) Transparent Cell Transport Service. Although this RFC requires all the ATM
cells, corresponding to all possible ATM virtual circuits, received on an ATM port to
be transported on the associated single pseudowire, the current implementation
does not enable all possible ATM virtual circuits on an ATM port to be transported.
This condition occurs because of hardware limitations on the ATM line modules
supported on ERX14xx models, ERX7xx models, and the ERX310 router. As a result,
it is necessary to explicitly open virtual circuits on the segmentation and reassembly
(SAR) scheduler to enable ATM cells corresponding to those virtual circuits to be
received and transported over a pseudowire. Because of the scaling limitations on
the number of virtual circuits that can be opened on the SAR scheduler, all possible
ATM virtual circuits for a single ATM port cannot be opened on the SAR device.
Therefore, to enable multiple VCs over a single pseudowire to be configured on the
ERX routers, you must specify the subset of ATM virtual circuits on a port that must
be carried on the single pseudowire.
To configure the subset of ATM virtual circuits, you must configure a VPI/VCI range
using the new mpls-relay atm vpi-range vpiStart vpiEnd vci-range vciStart vciEnd
command in global configuration mode. You can configure this VPI/VCI range only
for ATM ports for which you have associated a pseudowire using the MPLS Martini
circuit configuration. Before the support for multiple ATM VCs over a single
pseudowire was available, MPLS Martini circuit configuration was allowed only on
ATM subinterfaces and not on ATM ports. To enable a subset of ATM virtual circuits
to be transported over a single pseudowire, you must add the MPLS Martini circuit
configuration on an ATM port (associating the ATM port with the single pseudowire)
and then specify the subset of ATM virtual circuits whose cells need to be transported
on the single pseudowire using the VPI/VCI range configuration.
You can specify a maximum of four non-overlapping VPI/VCI ranges for each ATM
port. The cumulative number of ATM virtual circuits in the specified VPI/VCI ranges
must not exceed the scaling limitation of the SAR scheduler. The SAR scheduler
limitation is not for each port, but for the entire line module. The VPI/VCI range
configurations specified on the ATM ports on both ends of the pseudowire must
match. Even if the VPI/VCI range configurations do not match on both ends of the
pseudowire, Label Discovery Protocol (LDP) brings up the pseudowire. However, on
the remote provider edge (PE) router, ATM cells received from the pseudowire that
are not within the configured ranges are discarded.
For more information on the guidelines to be followed when you configure VCI/VPI
ranges for transportation of a subset of ATM VCs on a single pseudowire, see
"Guidelines for Configuring VPI/VCI Ranges of ATM Virtual Circuits" on page 527.
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Multiple ATM Virtual Circuits over a Single Pseudowire Overview