Mstp And Rstp Considerations - Avaya 8800 Planning And Engineering, Network Design

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 802.1w or
802.1s, new configuration files cannot be loaded if the flag is changed back to regular STP.
• For best interoperability results, contact your Avaya representative.
June 2016 Planning and Engineering — Network Design
Comments on this document? infodev@avaya.com
Spanning tree
101