Terms and Definitions

Spanning Tree Terms and Definitions lists terms and definitions used in Spanning Tree configuration.

Spanning Tree Terms and Definitions

Term	Definition
Alternate port	Acts as an alternate path to the root bridge than that provided by the root port.
Backup port	Acts as an backup for the path provided by a designated port toward the leaves of the Spanning Tree. Backup ports can exist only where two ports are connected together in a loopback mode or bridge with two or more connections to a shared LAN segment.
BID	Bridge identification, which is derived from the bridge‘s MAC address and bridge priority. The bridge with the lowest BID becomes the root bridge.
BPDU	Bridge Protocol Data Unit messages. Used by STP to exchange information, including designating a bridge for each switched LAN segment, and one root bridge for the Spanning Tree.
Bridge	Switching device.
Bridge priority	Assigns the bridge‘s relative priority compared to other bridges.
CIST	Common and Internal Spanning Tree created by MSTP to represent the connectivity of the entire network. This is equivalent to the single Spanning Tree used for STP and RSTP. Communications between MST regions occurs using the CIST.
CST	A Spanning Tree defined in the IEEE 802.1q standard that assumes one Spanning Tree instance for the entire bridged network, regardless of the number of VLANs.
Designated port	A forwarding port within an active topology elected for every switched LAN segment.
Edge port	Port on the edge of a bridged LAN.
FID	Filter Identifier. Each VLAN is associated to a FID. VLANs are mapped to SIDs using their FID association.
Forward delay	Time interval (in seconds) the bridge spends in listening or learning mode before it begins forwarding BPDUs.
Hello time	Time interval (in seconds) at which the bridge sends BPDUs.
ISL	Inter-Switch Link.
IST	A Spanning Tree instance that extends the CST inside the MST region and represents the entire MST region as a single CST virtual bridge to the outside world.
Loop Protect	Prevents or short circuits loop formation in a network with redundant paths by requiring ports to receive type 2 BPDUs (RSTP/MSTP) on point-to-point inter-switch links (ISLs) before their states are allowed to become forwarding.
Master port	The MSTI port whose connecting CIST port is root port for an entire MST region.
Max age	Maximum time (in seconds) the bridge can wait without receiving a configuration message (bridge “hello”) before attempting to reconfigure.
MST region	An MSTP group of devices configured together to form a logical region. The MST region presents itself to the rest of the network as a single device, which simplifies administration.
MSTI	Multiple Spanning Tree Instance. See Spanning Tree Port Default Settings for MSTI support per platform.
Path cost	Sum of the port costs in the best path to the root bridge.
Port cost	Value assigned to a port based on the speed of the port. The faster the speed, the lower the cost. This helps to determine the quickest path between the root bridge and a specified destination. The segment attached to the root bridge normally has a path cost of zero.
Port priority	Assigns a port‘s priority in relation to the other ports on the same bridge.
Restricted TCN	Restricts the propagation of Topology Change Notices on a specified port when set to true.
Restricted Role	Disallows root role on a specified port when set to true.
Root bridge	Logical center of the Spanning Tree, used by STP to determine which paths to block and which to open.
Root port	Port in an active topology through which the root bridge can be reached.
SID	Spanning tree identifier. By default, SID 0 is assumed. VLANs are mapped to SIDs using their FID association.
SpanGuard	Prevents Spanning Tree respans that can occur when BPDUs are received on user ports and notifies network management that they were attempted.
TCN	Topology Change Notification.

Published May 2016prev | next

Email this topic

Print this page Print this page

Leave feedback Feedback