336
Configuring MSTP
Information About Configuring MSTP
The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST region.
Remember that an MST region looks like a single switch for the CIST. The CIST external root path cost is the root
path cost calculated between these virtual switches and switches that do not belong to any region.
The CIST regional root was called the IST master in the prestandard implementation. If the CIST root is in the region,
the CIST regional root is the CIST root. Otherwise, the CIST regional root is the closest switch to the CIST root in the
region. The CIST regional root acts as a root switch for the IST.
The CIST internal root path cost is the cost to the CIST regional root in a region. This cost is only relevant to the IST,
instance 0.
Table 41 on page 336 compares the IEEE standard and the Cisco prestandard terminology.
Hop Count
The IST and MST instances do not use the message-age and maximum-age information in the configuration BPDU to
compute the spanning-tree topology. Instead, they use the path cost to the root and a hop-count mechanism similar to
the IP time-to-live (TTL) mechanism.
By using the spanning-tree mst max-hops global configuration command, you can configure the maximum hops inside
the region and apply it to the IST and all MST instances in that region. The hop count achieves the same result as the
message-age information (triggers a reconfiguration). The root switch of the instance always sends a BPDU (or
M-record) with a cost of 0 and the hop count set to the maximum value. When a switch receives this BPDU, it decrements
the received remaining hop count by one and propagates this value as the remaining hop count in the BPDUs it generates.
When the count reaches zero, the switch discards the BPDU and ages the information held for the port.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the same throughout the
region, and the same values are propagated by the region designated ports at the boundary.
Boundary Ports
In the Cisco prestandard implementation, a boundary port connects an MST region to a single spanning-tree region
running RSTP, to a single spanning-tree region running PVST+ or rapid PVST+, or to another MST region with a different
MST configuration. A boundary port also connects to a LAN, the designated switch of which is either a single
spanning-tree switch or a switch with a different MST configuration.
There is no definition of a boundary port in the IEEE 802.1s standard. The IEEE 802.1Q-2002 standard identifies two kinds
of messages that a port can receive: internal (coming from the same region) and external. When a message is external,
it is received only by the CIST. If the CIST role is root or alternate, or if the external BPDU is a topology change, it could
have an impact on the MST instances. When a message is internal, the CIST part is received by the CIST, and each MST
instance receives its respective M-record. The Cisco prestandard implementation treats a port that receives an external
message as a boundary port. This means a port cannot receive a mix of internal and external messages.
An MST region includes both switches and LANs. A segment belongs to the region of its designated port. Therefore, a
port in a different region than the designated port for a segment is a boundary port. This definition allows two ports
internal to a region to share a segment with a port belonging to a different region, creating the possibility of receiving
both internal and external messages on a port.
Table 41 IEEE standard and the Cisco Prestandard Terminology
IEEE Standard Cisco Prestandard Cisco Standard
CIST regional root IST master CIST regional root
CIST internal root path cost IST master path cost CIST internal path cost
CIST external root path cost Root path cost Root path cost
MSTI regional root Instance root Instance root
MSTI internal root path cost Root path cost Root path cost
To Next Page
Loading...
Need help?
General