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Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide
OL-16506-17
Chapter 16 Configuring MPLS Layer 2 VPNs
Virtual Private LAN Services
Reverse Layer 2 Gateway Protocol
Layer 2 Gateway Protocol (L2GP) is a recommended IEEE standard (802.1ah) to address the issues that
arise when two independent, bridged domains are connected redundantly through an arbitrary number
of links. L2GP defines how the forwarding gateways are selected, so that only redundant ports are
blocked and there are no temporary loops. The transition should be at least the same speed in which STP
L2GP resolves the the transient loop problem during the reconvergence because it does not require
cooperation from the outside domain.
Reverse Layer 2 Gateway Protocol (R-L2GP) is a variation of an L2GP. In case of an R-L2GP, the pseudo
information of the R-L2GP is transmitted by Network-facing Provider Edges (nPEs) instead of User
Provider-Edges (uPEs). R-L2GP provides a mechanism to send out static preconfigured Bridge Protocol
Data Units (BPDUs) on each ring access port of nPEs to stimulate a per-access ring instantiation of the
protocol. R-L2GP enables the Provider Edges (PEs) to avoid the burden of running Multiple Instance
Spanning Tree Protocol (MST) when multiple independent access networks that run MST connect to a
pair of redundant PEs.