Copyright 2003 The Data Link Layer Logical Link Control Ethernet Token Ring
Copyright 2003 Ethernet and IEEE Frame Formats Type (Ethernet) Specifies the upper-layer protocol to receive the data after Ethernet processing. Length (IEEE 802.3) The number of bytes of data that follows this field. Data (Ethernet) Ethernet version 2 does not specify any padding. Data (IEEE 802.3) Upper-layer protocol, is defined (LLC) in the data portion of the frame. If data is insufficient to fill the frame to the minimum 64-byte size, padding bytes are added. Preamble Alternating pattern of 1s and 0s (up to 31 pairs) tells receiving station that a data frame is coming.
Copyright 2003 Memory Buffering Port-based memory buffering –Packets are stored in queues that are linked to specific incoming ports. –It is possible for a single packet to block all other packets because its destination port is busy (even if the other packets could be delivered). Shared-memory buffering –All packets use a common memory buffer. –Packets in the buffer are then linked (mapped) dynamically to the appropriate destination port. –Helps balance between 10- and 100-Mbps ports.
Copyright 2003 Redundancy and Loops Newer switched networks include redundant links to increase reliability and reduce single points of failure. With that comes the potential for data loops.
Copyright 2003 Spanning Tree Protocol Multiple paths to a destination provide redundancy and make a network robust. STP uses spanning-tree algorithm (STA). STP is a switched Ethernet network solution to deal with potential for data loops introduced by multiple paths. –Routers can manage multiple paths to avoid loops. –Hub networks would stop immediately with multiple paths. STP detects potential loops and blocks one path. –Blocked path can be quickly activated if other link fails. –Uses Layer 2 BPDUs (bridge protocol data units) to communicate between switches and detect potential loops.
Copyright 2003 Blocking –No frames forwarded, BPDUs heard Listening –No frames forwarded, listening for frames Learning –No frames forwarded, learning addresses Forwarding –Frames forwarded, learning addresses Disabled –No frames forwarded, no BPDUs heard Ports typically progress from Blocking > Listening > Learning > Forwarding. The Five STP States