Vpls And Spanning Tree Protocol; Spanning Tree Operating Modes - Alcatel-Lucent 7750 SR Manual

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VPLS and Spanning Tree Protocol

Alcatel-Lucent's VPLS service provides a bridged or switched Ethernet Layer 2 network.

Equipment connected to SAPs forward Ethernet packets into the VPLS service. The 7750 SR

participating in the service learns where the customer MAC addresses reside, on ingress SAPs or

ingress SDPs.

Unknown destinations, broadcasts, and multicasts are flooded to all other SAPs in the service. If

SAPs are connected together, either through misconfiguration or for redundancy purposes, loops

can form and flooded packets can keep flowing through the network. Alcatel-Lucent's

implementation of the Spanning Tree Protocol (STP) is designed to remove these loops from the

VPLS topology. This is done by putting one or several SAPs and/or spoke SDPs in the discarding

Alcatel-Lucent's implementation of the Spanning Tree Protocol (STP) incorporates some

modifications to make the operational characteristics of VPLS more effective.

The STP instance parameters allow the balancing between resiliency and speed of convergence

extremes. Modifying particular parameters can affect the behavior. For information on command

usage, descriptions, and CLI syntax, refer to

Per VPLS instance, a preferred STP variant can be configured. The STP variants supported are:

While the 7750 SR initially uses the mode configured for the VPLS, it will dynamically fall back

(on a per-SAP basis) to STP (IEEE 802.1D-1998) based on the detection of a BPDU of a different

format. A trap or log entry is generated for every change in spanning tree variant.

Some older 802.1W compliant RSTP implementations may have problems with some of the

features added in the 802.1D-2004 standard. Interworking with these older systems is improved

with the comp-dot1w mode. The differences between the RSTP mode and the comp-dot1w mode

7750 SR OS Services Guide

— Rapid Spanning Tree Protocol (RSTP) compliant with IEEE 802.1D-2004 -

default mode

— Compliant with IEEE 802.1w

— Operation as in RSTP but backwards compatible with IEEE 802.1w

(this mode allows interoperability with some MTU types)

— Compliant with the Multiple Spanning Tree Protocol specified in IEEE 802.1Q-

REV/D5.0-09/2005. This mode of operation is only supported in an mVPLS.

Virtual Private LAN Services

Configuring a VPLS Service with CLI on page