Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual page 1856

Complete Software Guide for Junos
1752
®
OS for EX Series Ethernet Switches, Release 10.4
branch devices, which is based upon port speed or user configuration. STP 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.
Switches that forward frames through the spanning tree are called designated bridges.
The ports of a designated bridge function in one of three roles:
Root port—The root port is the port closest to the root bridge and serves as the only
port that receives frames from and forwards frames to the root bridge. A switch has
only one root port.
Designated port—A designated port forwards traffic away from the root bridge toward
a leaf. A switch has one designated port for every link connection it serves.
Blocked port—A blocked or "non-designated" port is not part of the spanning tree. Any
switch ports not serving as the root or a designated port are blocked.
In addition to assigning a bridge's ports to one of three roles in the spanning tree, STP
also places the ports of a designated bridge in one of five states:
Disabled—The port cannot receive or send any frame and is not part of the spanning
tree.
Blocking—The port does not forward frames but listens for BPDUs to determine if it
should become active in the spanning tree and to ensure its neighbor switches are still
working.
Listening—The port receives BPDUs but does not forward traffic or learn addresses.
This state is the first of two through which the port transitions to the forwarding state.
The port waits for information indicating that it should return to the blocking state. If
it doesn't receive this information before the forwarding delay timer expires (default
15 seconds), then it transitions to the learning state.
Learning—The port prepares to forward traffic by examining received frames for location
information in order to build its address table. At the end of a second forwarding delay
timer (default 15 seconds), the port transitions to the forwarding state.
Forwarding—The port filters and forwards frames. A port in the forwarding state is part
of the active spanning tree.
The spanning tree converges when the STA identifies 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 communicate the current tree topology. When a bridge receives a hello BPDU, it
compares the information to that already stored for the receiving port. If the data matches,
the bridge resets a timer called "max age" to zero and then forwards a new BPDU with
the current active topology information to the next bridge in the spanning tree.
The default interval for the max age timer is 20 seconds. This timer controls how long a
bridge saves configuration BPDU information for the receiving port. As long as the bridge
receives consistent hello BPDUs within this interval, the spanning tree remains unchanged.
If the max age timer expires, which indicates a link failure, the port re-enters the listening
state. After waiting the time allotted for the forwarding delay, the port transitions to the
learning state and again waits. At the end of a second forwarding delay, the port transitions
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