Advanced Traffic Management Guide K/KA/KB.15.15

of MSTP-capable switches. However, it is not necessary to do this. You can just enable MSTP on
an MSTP-capable switch and a spanning tree instance is created automatically. This instance
always exists by default when spanning tree is enabled, and is the spanning tree instance that
communicates with STP and RSTP environments.
802.1s MSTP on Cisco switches
You must run the latest versions of IOS to support MSTP on Cisco switches (check the Cisco web
site for details). Be advised, however, that it is not an easy task tracking down the appropriate
IOS version for MSTP on individual switch models. The correct version of IOS for the Cisco 3750
switch configurations used is (IOS 12.2(25) SEC2).
What spanning-tree mode should you choose?
MSTP is the ideal choice because it is standards-based and supported by both HP and Cisco; it
converges quickly and allows load balancing of traffic on uplinks with appropriate configuration.
Operating rules
• All switches in a region must be configured with the same set of VLANs, as well as the same
MST configuration name and MST configuration number.
• Within a region, a VLAN can be allocated to either a single MSTI or to the region’s IST
instance.
• All switches in a region must have the same VID-to-MST instance and VID-to-IST instance
assignments.
• There is one root MST switch per configured MST instance.
• Within any region, the root switch for the IST instance is also the root switch for the region.
Because boundary ports provide the VLAN connectivity between regions, all boundary ports
on a region's root switch should be configured as members of all static VLANs defined in the
region.
• There is one root switch for the Common and Internal Spanning Tree (CIST). Note that the
per-port hello-time parameter assignments on the CIST root switch propagate to the ports on
downstream switches in the network and override the hello-time configured on the downstream
switch ports.
• Where multiple MST regions exist in a network, there is only one active, physical communication
path between any two regions, or between an MST region and an STP or RSTP switch. MSTP
blocks any other physical paths as long as the currently active path remains in service.
• Within a network, an MST region appears as a virtual RSTP bridge to other spanning tree
entities (other MST regions, and any switches running 802.1D or 802.1w spanning-tree
protocols).
• Within an MSTI, there is one spanning tree (one physical, communication path) between any
two nodes. That is, within an MSTI, there is one instance of spanning tree, regardless of how
many VLANs belong to the MSTI. Within an IST instance, there is also one spanning tree
across all VLANs belonging to the IST instance.
• An MSTI comprises a unique set of VLANs and forms a single spanning tree instance within
the region to which it belongs.
• Communication between MST regions uses a single spanning tree.
• If a port on a switch configured for MSTP receives a legacy (STP/802.1D or RSTP/802.1w)
BPDU, it automatically operates as a legacy port. In this case, the MSTP switch interoperates
with the connected STP or RSTP switch as a separate MST region.
• Within an MST region, there is one logical forwarding topology per instance, and each
instance comprises a unique set of VLANs. Where multiple paths exist between a pair of nodes
using VLANs belonging to the same instance, all but one of those paths will be blocked for
450 Spanning tree interoperability between HP and Cisco switches