Port States Determine How A Port Processes A Frame; Edge Ports Connect To Devices That Cannot Be Part Of A Spanning Tree; Bpdus Maintain The Spanning-Tree; Using Rstp - Juniper EX9200 Features Manual

Port States Determine How a Port Processes a Frame

Edge Ports Connect to Devices That Cannot Be Part of a Spanning Tree

BPDUs Maintain the Spanning-Tree

Copyright © 2017, Juniper Networks, Inc.
serves as the designated bridge. A backup port for a designated port immediately takes
over if the port fails.
Disabled port—The port is not part of the active spanning tree.
Each port has both a state and a role. A port's state determines how it processes a frame.
RSTP places each port of a designated bridge in one of three states:
Discarding—The port discards all BPDUs. A port in this state discards all frames it
receives and does not learn MAC addresses.
Learning—The port prepares to forward traffic by examining received frames for location
information in order to build its MAC address table.
Forwarding—The port filters and forwards frames. A port in the forwarding state is part
of the active spanning tree.
RSTP also defines the concept of an edge port, which is a designated port that connects
to devices that are not STP-capable, such as PCs, servers, routers, or hubs that are not
connected to other switches. Because edge ports connect directly to end stations, they
cannot create network loops and can transition to the forwarding state immediately.
You can manually configure edge ports, and a switch can also detect edge ports by noting
the absence of communication from the end stations.
The edge ports themselves do send BPDUs to the spanning tree. If you have a good
understanding of the implications on your network and want to modify RSTP on the edge
port interface.
Spanning-tree protocols use frames called bridge protocol data units (BPDUs) to create
and maintain the spanning tree. A BPDU frame is a message sent from one switch to
another to communicate information about itself, such as its bridge ID, root path costs,
and port MAC addresses. The initial exchange of BPDUs between switches determines
the root bridge. Simultaneously, BPDUs are used to communicate the cost of each link
between branch devices, which is based upon port speed or user configuration. RSTP
uses this path cost to determine the ideal route for data frames to travel from one leaf
to another leaf and then blocks all other routes. If an edge port receives a BPDU, it
automatically transitions to a regular RSTP port.
When the network is in a steady state, the spanning tree converges when the
spanning-tree algorithm (STA) identifies both the root and designated bridges and all
ports are in either a forwarding or blocking state. To maintain the tree, the root bridge
continues to send BPDUs at a hello time interval (default 2 seconds). These BPDUs
continue to communicate the current tree topology. When a port receives a hello BPDU,
it compares the information to that already stored for the receiving port. One of three
actions takes place when a switch receives a BPDU:
Chapter 1: Using RSTP
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