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RSTP link-type configuration based on number of RSTP sites over VPLS network

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TitleRSTP link-type configuration based on number of RSTP sites over VPLS network
Objective
Depending on how many RSTP sites (where RSTP protocol is enabled to prevent remote loop among multiple sites) are connected to VPLS cloud, proper RSTP link type needs to be configured over the port which connectes to service provider cloud in customer switch.
Environment
  • EXOS
Procedure
[Case 1] Two RSTP sites over VPLS
If two sites are connected via RSTP, then "point-to-point" link-type must be configured to allow fast convergence between two RSTP sites.


[Case 2] Three or more RSTP sites over VPLS
If three or more RSTP sites are connected over VPLS using RSTP protocol to prevent loop, then RSTP link-type for customer port which connects to CEP in the provider must be explicitly configured as broadcast link-type instead of point-to-point.

Though auto configuration for STP link-type exists, the auto configuration determines point-to-point or broadcast based on the full duplex of port instead of detecting multiple STP switches.

If point-to-point link-type is configured in customer switches, there is a possibility for temporary loop in a network because early agreement BPDU from either downstream bridges response to propose BPDU from upstream switch (close to root) can make upstream switch unblock port before downstream switch blocks all downstream ports.
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