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then port2. Depending on configured priority levels, packets from ports with lower priority level are always
forwarded only after packets from ports with higher priority level have all been forwarded; However in WRR QoS
mode, if you specify weight values: High=7; Low=1, then when the 2 ports simultaneously transmit packets to one
port, the receiving port will forward packets according to traffic ratio of 7:1.
3.5.4 STP
1. STP Overview
The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet
local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results
from them. On Ethernet, only a single active path at a time can be maintained between any two network nodes to
avoid broadcast storm. However, spare (redundant) links are indispensable to ensure reliability. Spanning tree
allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails,
without the danger of bridge loops, and disable those that are not part of the spanning tree, leaving a single active
path between any two network nodes. This is accomplished in the STP. A STP-enabled switch can perform the
following teaks:
Discover and generate an optimum STP topology
Discover and repair failures on the network; automatically update the network topology for future use. Local
topology is generated by computing bridge configurations made by a network administrator. Thus, if configured
properly, an optimum topology tree can be generated.
2. RSTP Overview
RSTP provides significantly faster spanning tree convergence after a topology change, introducing new
convergence behaviors and bridge port roles to do this. RSTP was designed to be backwards-compatible with
standard STP. RSTP is typically able to respond to changes within 3 × Hello times (default: 3 times 2 seconds) or
within a few milliseconds of a physical link failure while STP can take 30 to 50 seconds to respond to a topology
change.
RSTP delivers fast transition to forwarding status without relying on timer settings. A RSTP bridge is responsive to
other RSTP bridge's link status. The port does not need to wait for the topology to become stable. Edge port and
P2P port are introduced to the protocol for faster transition. Below explains what an Edge port and a P2P port is and
does.
Edge Port
The edge port is a configurable designation used for a port that is directly connected to a segment where a loop
cannot be created. An example would be a port connected directly to a single workstation. Ports that are designated
as edge ports transition to a forwarding state immediately without going through the listening and learning states.
An edge port loses its status if it receives a BPDU packet, immediately becoming a normal spanning tree port.
P2P Port
A P2P port is also capable of rapid transition. P2P ports may be used to connect to other bridges. Under
RSTP/MSTP, all ports operating in full-duplex mode are considered to be P2P ports, unless manually overridden
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