Virtual Private LAN Services
7210 SAS M Services Guide Page 269
Enhancements to the Spanning Tree Protocol
To interconnect 7210 SAS devices (PE devices) across the backbone, service tunnels (SDPs) are
used. These service tunnels are shared among multiple VPLS instances. Alcatel-Lucent’s
implementation of the Spanning Tree Protocol (STP) incorporates some enhancements to make
the operational characteristics of VPLS more effective. The implementation of STP on the router
is modified in order to guarantee that service tunnels will not be blocked in any circumstance
without imposing artificial restrictions on the placement of the root bridge within the network. The
modifications introduced are fully compliant with the 802.1D-2004 STP specification.
When running MSTP, spoke SDPs cannot be configured. Also, ensure that all bridges connected
by mesh SDPs are in the same region. If not, the mesh will be prevented from becoming active
(trap is generated).
In order to achieve this, all mesh SDPs are dynamically configured as either root ports or
designated ports. The PE devices participating in each VPLS mesh determine (using the root path
cost learned as part of the normal protocol exchange) which of the 7210 SAS devices is closest to
the root of the network. This PE device is internally designated as the primary bridge for the VPLS
mesh. As a result of this, all network ports on the primary bridges are assigned the designated port
role and therefore remain in the forwarding state.
The second part of the solution ensures that the remaining PE devices participating in the STP
instance see the SDP ports as a lower cost path to the root rather than a path that is external to the
mesh. Internal to the PE nodes participating in the mesh, the SDPs are treated as zero cost paths
towards the primary bridge. As a consequence, the path through the mesh are seen as lower cost
than any alternative and the PE node will designate the network port as the root port. This ensures
that network ports always remain in forwarding state.
A combination of the above mentioned features ensure that network ports are never blocked and
maintain interoperability with bridges external to the mesh that are running STP instances.
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