Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 474)

444 Multi-Service IronWare Switching Configuration Guide
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RSTP support for PB and PBB
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Core RSTP
RSTP will run in the PBB core for loop detection and avoidance. All ASs and CSs will participate in
the RSTP and one CS will be selected as the root for the core RSTP instance. The assumption is
there will be only one RSTP instance running in the PBB core and traffic flow will be through one CS
which is the root bridge.
A backbone service provider can either use RSTP or MSTP. Preferably MSTP shall be used, since
this will allow the service provider to use different active paths for different B-VLANs.
To avoid a potential loop in the core PBB network, RSTP will be enabled. The regular VLAN
corresponds to VPLS B-VLAN in AS/BEB bridges.
Edge RSTP
Dual homing ES to two ASs would require RSTP to avoid loops. An AS will have several RSTP
instances provisioned on the ES facing side. Here one AS will be acting as the root and the Edge
RSTP is completely separate and independent from the Core RSTP.
The Edge RSTP instance should be enabled on the VPLS instance on AS/ES.
For dual homing of the ES, the ASs can be connected using either of the following two methods.
1. Two AS/BEB switches connected via the S-tagged endpoint.
2. Two AS/BEB switches connected via the IB-tagged endpoint.
In each case, the RSTP behavior is different, which is explained below.
Figure 96 shows that BEB-1, BEB-2 and PB could be of same S-VLAN or different S-VLANs. Since
RSTP is enabled under the VPLS instance, all the VPLS VLANs which belong to that VPLS instance
will be considered as part of same RSTP instance. BPDUs will be transmitted on all the VPLS
endpoints with the VLAN tag associated with that end point.
FIGURE 96 RSTP convergence when two AS connected via S tagged endpoint
BEB BEB
PB
S tagged endpoints
DF DF
DF RF
RF AD
DF - Designated Forwarding
RF - Root Forwarding
AD - Alternate Discarding