LevelOne ProCon GSW-1290, User Manual

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LevelOne GSW-1290 Gigabit Layer2 SNMP Switch
configured for STP are also used for RSTP. This section introduces some new Spanning Tree concepts and
illustrates the main differences between the two protocols.
Port Transition States
An essential difference between the two protocols is in the way ports transition to a forwarding state and
the in the way this transition relates to the role of the port (forwarding or not forwarding) in the topology.
RSTP combines the transition states disabled, blocking and listening used in 802.1d and creates a single
state
Discarding . In either case, ports do not forward packets; in the STP port transition states disabled,
blocking or listening or in the RSTP port state discarding there is no functional difference, the port is not
active in the network topology. The Comparing Port States table below compares how the two protocols
differ regarding the port state transition.
Both protocols calculate a stable topology in the same way. Every segment will have a single path to the
root bridge. All bridges listen for BPDU packets. However, BPDU packets are sent more frequently – with
every Hello packet. BPDU packets are sent even if a BPDU packet was not received. Therefore, all links
between bridges are sensitive to the status of the link. Ultimately this difference results faster detection of
failed links, and thus faster topology adjustment. A drawback of 802.1d is this absence of immediate
feedback from adjacent bridges.
802.1d STP 802.1w RSTP Forwarding? Learning?
Disabled Discarding No No
Blocking Discarding No No
Listening Discarding No No
Learning Learning No Yes
Forwarding Forwarding Yes Yes
Table 4- 1. Comparing Port States
RSTP is capable of more rapid transition to a forwarding state – it no longer relies on timer
configurations – RSTP compliant bridges are sensitive to feedback from other RSTP compliant bridge links.
Ports do not need to wait for the topology to stabilize before transitioning to a forwarding state. In order to
allow this rapid transition, the protocol introduces two new variables: the edge port and the point-to-point
(P2P) port.
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 will 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, all ports operating in full-duplex mode are considered to be P2P ports, unless manually overridden
through configuration.
802.1d/802.1w Compatibility
RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to
802.1d format when necessary. However, any segment using 802.1 STP will not benefit from the rapid
transition and rapid topology change detection of RSTP. The protocol also provides for a variable used for
migration in the event that legacy equipment on a segment is updated to use RSTP.
Switch Spanning Tree Settings
The Spanning Tree Protocol (STP) operates on two levels: on the Switch level, the settings are globally
implemented. On the port level, the settings are implemented on a per user-defined group of ports basis.
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