Cisco Catalyst 2960 Series, Configuration Manual

Page: 332 / 2288

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 boundary roles currently exist:
•
The boundary port is the root port of the CIST regional root
—
When the CIST instance port is proposed
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.
•
The boundary port is not the root port of the CIST regional root
—
The MSTI ports follow the state and
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.
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 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.
The port BY is fixed in a boundary, and no load balancing is possible between A and B. The same problem
exists on segment X, but B might transmit topology changes.
Figure 13: Standard and Prestandard Switch Interoperation
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
250
Information About MSTP