Per-Vlan Spanning Tree Plus; Mstp And Rstp Considerations - Avaya 8800 Planning And Engineering

Per-VLAN Spanning Tree Plus

PVST+ is the Cisco-proprietary spanning tree mechanism that uses a spanning tree instance
per VLAN. PVST+ is an extension of the Cisco PVST with support for the IEEE 802.1Q
standard. PVST+ is the default spanning tree protocol for Cisco switches and uses a separate
spanning tree instance for each configured VLAN. In addition, PVST+ supports IEEE 802.1Q
STP for support across IEEE 802.1Q regions.
For more information about PVST+, see Avaya Ethernet Routing Switch 8800/8600
Configuration — VLANs and Spanning Tree, NN46205-517.

MSTP and RSTP considerations

The Spanning Tree Protocol provides loop protection and recovery, but it is slow to respond
to a topology change in the network (for example, a dysfunctional link in a network). The Rapid
Spanning Tree protocol (RSTP or IEEE 802.1w) reduces the recovery time after a network
failure. It also maintains a backward compatibility with IEEE 802.1D. Typically, the recovery
time of RSTP is less than 1 second. RSTP also reduces the amount of flooding in the network
by enhancing the way that Topology Change Notification (TCN) packets are generated.
Use to configure multiple instances of RSTP on the same switch. Each RSTP instance can
include one or more VLANs. The operation of the MSTP is similar to the current Avaya
proprietary MSTP, except that the Avaya version has faster recovery time.
In MSTP mode, eight instances of RSTP can be supported simultaneously for the Ethernet
Switch 460/470 or Ethernet Routing Switch 1600. Instance 0 or Common and Internal
Spanning Tree (CIST) is the default group, which includes default VLAN 1. Instances 1 to 7
are called Multiple Spanning Tree Instances (MSTI) 1 to 7. You can configure up to 64
instances, of which only 25 can be active at one time.
RSTP provides a new parameter called ForceVersion for backward compatibility with legacy
STP. You can configure a port in either STP-compatible mode or RSTP mode:
• An STP-compatible port transmits and receives only STP BPDUs. Any RSTP BPDU that
the port receives in this mode is discarded.
• An RSTP port transmits and receives only RSTP BPDU. If an RSTP port receives an STP
BPDU, it becomes an STP port. User intervention is required to bring this port back to
RSTP mode. This process is called Port Protocol Migration.
You must be aware of the following recommendations before implementing 802.1w or 802.1s:
• 25 STP groups are supported.
• Configuration files are not compatible between regular STP and 802.1w/s modes. A
special bootconfig flag identifies the mode. The default mode is 802.1D. If you choose
Planning and Engineering — Network Design
Spanning tree
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