Port Transition States; Edge Port; P2P Port - D-Link DES-3528 - xStack Switch - Stackable Reference Manual

xStack® DES-3528/DES-3552 Series Layer 2 Stackable Fast Ethernet Managed Switch Web UI Reference Guide
802.1D-2004 Rapid Spanning Tree
The Switch implements three versions of the Spanning Tree Protocol, the Multiple Spanning Tree Protocol (MSTP) as
defined by the IEEE 802.1Q-2005, the Rapid Spanning Tree Protocol (RSTP) as defined by the IEEE 802.1D-2004
specification and a version compatible with the IEEE 802.1D-1998 STP. RSTP can operate with legacy equipment
implementing IEEE 802.1D-1998; however the advantages of using RSTP will be lost.
The IEEE 802.1D-2004 Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1D-1998 STP standard. RSTP
was developed in order to overcome some limitations of STP that impede the function of some recent switching
innovations, in particular, certain Layer 3 functions that are increasingly handled by Ethernet switches. The basic
function and much of the terminology is the same as STP. Most of the settings 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 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-2004 RSTP
Disabled
Discarding
Discarding
Learning
Forwarding
802.1D-1998 STP
Disabled
Blocking
Listening
Listening
Forwarding
83
Forwarding
No
No
No
No
Yes
Learning
No
No
No
Yes
Yes