D-Link DGS-3200-10 Product Manual - Page 116

xStack® DGS-3200 Series Layer 2 Gigabit Ethernet Managed Switch 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 802.1D-2004 RSTP 802.1D-1998 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 - 2. Comparing Port States 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. 103