Mstp And Switch Stacks; Interoperability With Ieee 802.1D Stp; Rstp Overview - Cisco Catalyst 2960-XR Configuration Manual

MSTP and Switch Stacks

MSTP and Switch Stacks
A switch stack appears as a single spanning-tree node to the rest of the network, and all stack members use
the same bridge ID for a given spanning tree. The bridge ID is derived from the MAC address of the stack
master.
If a switch that does not support MSTP is added to a switch stack that does support MSTP or the reverse, the
switch is put into a version mismatch state. If possible, the switch is automatically upgraded or downgraded
to the same version of software that is running on the switch stack.
When a new switch joins the stack, it sets its switch ID to the switch ID. If the newly added switch has the
lowest ID and if the root path cost is the same among all stack members, the newly added switch becomes
the stack root. A topology change occurs if the newly added switch contains a better root port for the switch
stack or a better designated port for the LAN connected to the stack. The newly added switch causes a topology
change in the network if another switch connected to the newly added switch changes its root port or designated
ports.
When a stack member leaves the stack, spanning-tree reconvergence occurs within the stack (and possibly
outside the stack). The remaining stack member with the lowest stack port ID becomes the stack root.
If the stack master fails or leaves the stack, the stack members elect a new stack master, and all stack members
change their switch IDs of the spanning trees to the new master switch ID.

Interoperability with IEEE 802.1D STP

A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate with
legacy IEEE 802.1D switches. If this switch receives a legacy IEEE 802.1D configuration BPDU (a BPDU
with the protocol version set to 0), it sends only IEEE 802.1D BPDUs on that port. An MSTP switch also can
detect that a port is at the boundary of a region when it receives a legacy BPDU, an MSTP BPDU (Version
3) associated with a different region, or an RSTP BPDU (Version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives IEEE 802.1D
BPDUs because it cannot detect whether the legacy switch has been removed from the link unless the legacy
switch is the designated switch. A switch might also continue to assign a boundary role to a port when the
switch to which this switch is connected has joined the region. To restart the protocol migration process (force
the renegotiation with neighboring switches), use the clear spanning-tree detected-protocols privileged
EXEC command.
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are RSTP
BPDUs. Therefore, MSTP switches send either a Version 0 configuration and TCN BPDUs or Version 3
MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch of which is
either a single spanning-tree switch or a switch with a different MST configuration.

RSTP Overview

The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree.
Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default
settings in the IEEE 802.1D spanning tree).
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
52
Configuring Multiple Spanning-Tree Protocol
OL-29424-01