D-Link DES-3226SM Product Manual - Page 31

DES-3226S Layer 2 Fast Ethernet Switch User's Guide Additional criteria are applied to the forwarding decision. Depending on what header information is being examined, forwarding is permitted or denied based on the packet's source or destination, its protocol classification, DiffServ code, 802.1p level or VLAN. Routine Packet Filtering Some filtering is done automatically by the Switch in the course of other routing functions. These routine functions include: • Dynamic filtering as a function of automatic learning and aging of MAC addresses. • The 802.1d and 802.1w Spanning Tree Protocols filter packets based on topology to prevent loops. • Filtering done for VLAN integrity. Some filtering requires the manual entry of information into a filtering table: MAC address filtering - the manual entry of specific MAC addresses to be filtered from the network. Packets sent from one manually entered MAC address can be filtered from the network. The entry may be specified as either a source, a destination, or both. 802.1w Rapid Spanning Tree The Switch implements two versions of the Spanning Tree Protocol, the Rapid Spanning Tree Protocol (RSTP) as defined by the IEE 802.1w specification and a version compatible with the IEEE 802.1d STP. RSTP can operate with legacy equipment implementing IEEE 802.1d, however the advantages of using RSTP will be lost. The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1d 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 function 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 two protocols is in the way ports transition to a forwarding state and the in the way this transition relates to the role of the port (forwarding or not forwarding) in the topology. RSTP combines the transition states disabled, blocking and listening used in 802.1d 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 port state discarding there is no functional difference, the port is not active in the network topology. The table below compares how the two protocols differ regarding the port state transition. Both 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 are sensitive to the status of the link. Ultimately this difference results faster detection of failed links, and thus faster topology adjustment. A drawback of 802.1d is this absence of immediate feedback from adjacent bridges. 802.1d STP 802.1w RSTP Forwarding? Learning? Disabled Discarding No No Blocking Discarding No No Listening Discarding No No Learning Learning No Yes Forwarding Forwarding Yes Yes Table 1. Comparing Port States RSTP is capable of 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 28