Port Transition States; Edge Port; P2P Port - D-Link xStack Reference Manual

xStack® DGS-3120 Series Layer 2 Stackable Managed Switch Web UI Reference Guide

Port Transition States

An essential difference between the three protocols is in the way ports transition to a forwarding state and in the way
this transition relates to the role of the port (forwarding or not forwarding) in the topology. MSTP and RSTP combine
the transition states disabled, blocking and listening used in 802.1D-1998 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/MSTP port state discarding, there is no functional difference, the port is not active in the network topology. Table
7-3 below compares how the three protocols differ regarding the port state transition.
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-1998 is this absence of immediate feedback from adjacent bridges.
802.1Q-2005 MSTP
Disabled
Discarding
Discarding
Learning
Forwarding
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-1998/802.1D-2004/802.1Q-2005 Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to
802.1D-1998 format when necessary. However, any segment using 802.1D-1998 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.
802.1D-2004 RSTP
Disabled
Discarding
Discarding
Learning
Forwarding
802.1D-1998 STP
Disabled
Blocking
Listening
Learning
Forwarding
80
Forwarding
No
No
No
No
Yes
Learning
No
No
No
Yes
Yes