Enabling Pvst/Pvst+ S - Foundry Networks Switch and Router Installation And Configuration Manual

Foundry Switch and Router Installation and Configuration Guide
PVST
Each spanning tree (that is, each instance of STP) has one device called the root bridge. The root bridge is the
control point for the spanning tree, and sends STP status and topology change information to the other devices in
the spanning tree by sending BPDUs to the other devices. The other devices forward the BPDUs as needed.
The format of an STP BPDU differs depending on whether it is a Cisco PVST BPDU or a Foundry BPDU. Foundry
and Cisco devices also can support single STP BPDUs, which use another format.
• A Foundry device configured with a separate spanning tree in each VLAN sends BPDUs in standard IEEE
802.1D format, but includes a proprietary four-byte tag. The tag identifies the VLAN the BPDU is for.
• A Cisco device configured for PVST sends the BPDUs to multicast MAC address 01-00-0C-CC-CC-CD. If
the device is configured for PVST+, then the device sends BPDUs for all VLANs except VLAN 1 to 01-00-0C-
CC-CC-CD. The device sends BPDUs in VLAN 1 to 01-80-C2-00-00-00, the single STP address (see below
and "PVST+").
• A Foundry device configured for single STP (IEEE 802.1Q) sends untagged BPDUs to the well-known STP
MAC address 01-80-C2-00-00-00.
NOTE: Cisco devices can be configured to interoperate with devices that support IEEE 802.1Q single STP, but
the devices cannot be configured to run single STP.
Foundry's PVST support enables Foundry and Cisco devices that have separate spanning trees in each VLAN to
interoperate. The Foundry PVST support is automatically enabled when a port receives a PVST BPDU and does
not require configuration on the Foundry or Cisco device.
When PVST is enabled on a Foundry port, that port sends BPDUs in PVST format instead of Foundry's spanning
tree format.
PVST+
Foundry devices and Cisco devices support separate spanning trees on an individual port-based VLAN basis.
However, until the IEEE standard for multiple spanning trees is finalized, vendors are using different methods to
support multiple spanning trees within their own products. PVST+ is an extension to PVST that enables a Cisco
device to interoperate with other devices that are running a single spanning tree (IEEE 802.1Q) while still running
a separate spanning tree in each VLAN.
PVST+ uses 802.1Q single STP BPDUs on VLAN 1 and PVST BPDUs (which have a proprietary format) for other
VLANs. In this case, the Cisco device uses devices running 802.1Q as tunnels for PVST (non-802.1Q) traffic.
The 802.1Q single STP BPDUs are addressed to the well-known STP MAC address 01-80-C2-00-00-00. The
PVST BPDUs for the other VLANs are addressed to multicast address 01-00-0C-CC-CC-CD.
The PVST+ method can require manual configuration of STP parameters on the 802.1Q devices to ensure that
traffic for the PVST VLANs is not blocked. In addition, the opportunities to adjust STP parameters to load balance
traffic on a VLAN basis are limited when using PVST+.
Using Foundry Single STP with Cisco PVST+
Since Foundry's single STP feature complies with IEEE 802.1Q (the single STP specification), you also can use a
Foundry device running single STP to interoperate with a Cisco device running PVST+. When you enable single
STP on a Foundry device, the PVST compatibility feature is not enabled, even if a port receives a PVST BPDU.
Enabling PVST/PVST+ Statically
PVST/PVST+ support is automatically enabled on a port if the port receives a BPDU in PVST/PVST+ format.
However, you can statically enable PVST/PVST+ support on a port if desired. In this case, the support is enabled
immediately and support for Foundry tagged BPDUs is disabled at the same time. To enable the PVST/PVST+
support, use the following CLI method.
NOTE: When PVST/PVST+ support is enabled on a port, support for Foundry BPDUs is disabled.
USING THE CLI
To enable PVST/PVST+ support on a port, enter commands such as the following:
12 - 24 December 2000