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DGS-3400 Series Layer 2 Gigabit Ethernet Managed Switch
143
All three 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, each link between bridges is sensitive to the status of the link. Ultimately
this difference results in faster detection of failed links, and thus faster topology adjustment. A drawback of 802.1D is this absence
of immediate feedback from adjacent bridges.
802.1s MSTP 802.1w RSTP 802.1D STP Forwarding Learning
Disabled Disabled Disabled No No
Discarding Discarding Blocking
No No
Discarding Discarding Listening
No No
Learning Learning Learning
No
Yes
Forwarding Forwarding Forwarding Yes Yes
Table 3- 3. Comparing Port States
RSTP is capable of a 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 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 through configuration.
802.1D/802.1w/802.1s Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to 802.1D format
when necessary. However, any segment using 802.1D STP will not benefit from the rapid transition and rapid topology change
detection of MSTP or 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 or MSTP.
The Spanning Tree Protocol (STP) operates on two levels:
1. On the switch level, the settings are globally implemented.
2. On the port level, the settings are implemented on a per user-defined group of ports basis.
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