Ieee 802.1S Implementation; Port Role Naming Change; Interoperation Between Legacy And Standard Switches - Cisco Catalyst 2960-XR Configuration Manual

IEEE 802.1s Implementation

like a single virtual switch by sending a consistent sender switch ID to neighboring switches. In this example,
switch C would receive a BPDU with the same consistent sender switch ID of root, whether or not A or B is
designated for the segment.
IEEE 802.1s Implementation
The Cisco implementation of the IEEE MST standard includes features required to meet the standard, as well
as some of the desirable prestandard functionality that is not yet incorporated into the published standard.

Port Role Naming Change

The boundary role is no longer in the final MST standard, but this boundary concept is maintained in Cisco's
implementation. However, an MST instance port at a boundary of the region might not follow the state of the
corresponding CIST port. Two cases exist now:
• The boundary port is the root port of the CIST regional root—When the CIST instance port is proposed
• The boundary port is not the root port of the CIST regional root—The MSTI ports follow the state and

Interoperation Between Legacy and Standard Switches

Because automatic detection of prestandard switches can fail, you can use an interface configuration command
to identify prestandard ports. A region cannot be formed between a standard and a prestandard switch, but
they can interoperate by using the CIST. Only the capability of load-balancing over different instances is lost
in that particular case. The CLI displays different flags depending on the port configuration when a port
receives prestandard BPDUs. A syslog message also appears the first time a switch receives a prestandard
BPDU on a port that has not been configured for prestandard BPDU transmission.
Assume that A is a standard switch and B a prestandard switch, both configured to be in the same region. A
is the root switch for the CIST, and thus B has a root port (BX) on segment X and an alternate port (BY) on
segment Y. If segment Y flaps, and the port on BY becomes the alternate before sending out a single prestandard
BPDU, AY cannot detect that a prestandard switch is connected to Y and continues to send standard BPDUs.
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
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and is in sync, it can send back an agreement and move to the forwarding state only after all the
corresponding MSTI ports are in sync (and thus forwarding). The MSTI ports now have a special master
role.
role of the CIST port. The standard provides less information, and it might be difficult to understand
why an MSTI port can be alternately blocking when it receives no BPDUs (MRecords). In this case,
although the boundary role no longer exists, the show commands identify a port as boundary in the type
column of the output.
Configuring Multiple Spanning-Tree Protocol
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