Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual page 1858

Complete Software Guide for Junos
1754
®
OS for EX Series Ethernet Switches, Release 10.4
Discarding—The port discards all BPDUs. This state replaces the disabled, blocking,
and learning states used by STP. 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. RSTP eliminates the listening state
that proceeds the learning state in STP because the new mechanism for re-convergence
(proposal-agreement handshake) does not require the switch to spend time listening
for the spanning tree to reconfigure.
Forwarding—The port filters and forwards frames. A port in the forwarding state is part
of the active spanning tree.
The five port roles used by RSTP are:
Root port—The port closest to the root bridge (has the lowest path cost from a bridge)
and serves as the only port that receives frames from and forwards frames to the root
bridge. The root port functions the same as in STP.
Designated port—The port that forwards traffic away from the root bridge toward a
leaf. A designated bridge has one designated port for every link connection it serves.
A root bridge forwards frames from all of its ports, which serve as designated ports. A
designated port functions the same as in STP.
Alternate port—A port that provides an alternate path toward the root bridge if the
root port fails and is placed in the discarding state. This port is not part of the active
spanning tree, but if the root port fails, the alternate port immediately takes over.
Backup port—A port that provides a backup path toward the leaves of the spanning
tree if a designated port fails and is placed in the discarding state. A backup port can
only exist where two or more bridge ports connect to the same LAN for which the bridge
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. Note that in STP,
"disabled" is a state and not a role.
STP and RSTP maintain the spanning tree differently. Both use BPDUs to communicate
the current tree topology. With STP, however, the root bridge initiates these messages
and they propagate throughout the tree every hello time interval. With RSTP, a non-root
bridge sends a BPDU with its current information every hello time interval, regardless of
receiving BPDUs from the root bridge. If an RSTP device does not receive a configuration
message from its neighbor after an interval of three hello times, it determines it has lost
a connection with that neighbor. In this way, the RSTP BPDUs serve as a "keep-alive"
mechanism that provides rapid failure detection. Note that EX Series switches configured
to use STP actually run RSTP force version 0, which is compatible with STP, so BPDU
behavior is the same.
When a root port or a designated port fails on an RSTP-enabled device, the alternate or
backup port takes over after an exchange of BPDUs called the proposal-agreement
handshake. RSTP propagates this handshake over point-to-point links, which are dedicated
links between two network nodes, or switches, that connect one port to another. If a
local port becomes a new root or designated port, it negotiates a rapid transition with
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