Port Transition States; Edge Port; P2P Port - D-Link xStack DGS-3200-10 User Manual

xStack
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 IE EE 802.1Q-2005, t he Rapid S panning T ree P rotocol (R STP) as defined by t he IEEE 8 02.1D-2004 specification a nd 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 I EEE 802 .1D-2004 Rap id Sp anning Tree Pro tocol ( RSTP) evo lved from th e 8 02.1D-1998 STP stand ard. RSTP w as
developed in order to overcome som e lim itations of ST P that im pede the function of s ome recent switching innovations, in
particular, cert ain Layer 3 fun ctions t hat are i ncreasingly ha ndled by E thernet s witches. The basic function a nd much o f t he
terminology is th e same as STP. Mo st of the settin gs configured for STP are also used for RSTP. This section in troduces some
new Spanning Tree concepts and illustrates the main differences between the two protocols.

Port Transition States

An essen tial differen ce between th e t hree protocols is in th e way ports tran sition to a forward ing state an d in th e way th is
transition relates to t he role of the port (forwarding or no t forwarding) in the topology. MSTP and RSTP combine the transition
states di sabled, bl ocking a nd l istening used i n 802.1D-1998 a nd creat es a si ngle st ate Di scarding. In ei ther 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 pro tocols calcu late a stab le topo logy in t he same way. Ev ery segment will have a si ngle path t o t he root bridg e. 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 - i t no longer relies on timer configurations - RSTP compliant
bridges are sensitiv e to feedback from other RSTP co mpliant bridge links. Po rts do not need to w ait for the topology to stabilize
before transitioning to a fo rwarding 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 ex ample wo uld be a port con nected directly to a single workst ation. Ports th at are d esignated as edg e ports tran sition 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 wh en necessary. However, an y se gment u sing 80 2.1D-1998 STP will no t benefit from th e rap id tran sition and rap id
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.
®
DGS-3200 Series Layer 2 Gigabit Ethernet Managed Switch
802.1D-2004 RSTP 802.1D-1998 STP
Disabled Disabled
Discarding Blocking
Discarding Listening
Learning Learning
Forwarding Forwarding
Table 3 - 2. Comparing Port States
Forwarding
No
No
No
No
Yes
103
Learning
No
No
No
Yes
Yes