132 BES50 fundamentalsIEEE 802.1D bridgeThe switch supports IEEE 802.1D transparent bridging. The addresstable facilitates data switching by learning addresses and then filtering orforwarding traffic based on this information. The address table supportsup to 8K addresses.Store-and-forward switchingThe switch copies each frame into its memory before forwarding the framesto another port. This process ensures that all frames are a standardEthernet size and are verified for accuracy with the cyclic redundancy check(CRC), thus preventing bad frames from entering the network and wastingbandwidth.To avoid dropping frames on congested ports, the switch provides 8 MB forframe buffering. This buffer can queue packets awaiting transmission oncongested networks.Spanning Tree AlgorithmThe switch supports these spanning tree protocols:• Spanning Tree Protocol (STP, IEEE 802.1D)—This protocol providesloop detection and recovery by allowing two or more redundantconnections to be created between a pair of LAN segments. When thereare multiple physical paths between segments, this protocol chooses asingle path and disables all others to ensure that only one route existsbetween any two stations on the network. This prevents the creation ofnetwork loops. However, if the chosen path fails for any reason, analternate path is activated to maintain the connection.• Rapid Spanning Tree Protocol (RSTP, IEEE 802.1w)—This protocolreduces the convergence time for network topology changes to about3 to 5 seconds, compared to 30 seconds or more for the older IEEE802.1D STP standard. This protocol is intended as a completereplacement for STP but can still interoperate with switches running theolder standard by automatically reconfiguring ports to STP-compliantmode if they detect STP protocol messages from attached devices.The Spanning Tree Algorithm (STA) can be used to detect and disablenetwork loops and to provide backup links between switches, bridges, orrouters. Using an STA allows the switch to interact with other bridgingdevices (that is, an STA-compliant switch, bridge, or router) in your networkto ensure that only one route exists between any two stations on thenetwork and provide backup links, which automatically take over when aprimary link goes down.The following figure illustrates Spanning Tree Protocol loops.SMBUsing the Nortel Business Ethernet Switch 50 SeriesNN47924-301 01.01 Standard1.00 October 2006Copyright © 2006, Nortel Networks Nortel Networks Confidential.
