Spring 2000John Kristoff1 LAN Bridges and Switches Computer Networks

Spring 2000John Kristoff2 Where are we?

Spring 2000John Kristoff3 Recall zLANs have physical distance limitations zPerformance suffers when LAN utilization increases zSeparate LANs may eventually want to connect to each other

Spring 2000John Kristoff4 Motivation zUsers require arbitrary distance connections yExample: 2 computers across a corporate campus are part of one workgroup zMay not want to forward all transmissions to all workgroups for performance or security reasons zMay want to avoid a single point of failure (redundancy/reliability) The books: Interconnections - Radia Perlman, The Switch Book - Rich Seifert

Spring 2000John Kristoff5 LAN Bridges/Switches zA hardware device with minimal software zConnects 2 or more similar LANs together zForwards frames between connected LANs zDoes not forward collisions, noise, beacons, etc. zExamines data link layer information zAllows each LAN to operate independently

Spring 2000John Kristoff6 Bridge/Switch Operation zListen to all LANs in promiscuous mode zOnly move frames between LANs if necessary zOnly act on layer 1/2 information

Spring 2000John Kristoff7 Connections

Spring 2000John Kristoff8 Transparent Bridging Illustrated

Spring 2000John Kristoff9 Transparent Bridging Rules z1. Watch all frames on each LAN z2. For each frame, store the source address in a cache along with the associated LAN the frame arrived on (bridge table) z3. For each frame, the cache is queried for the destination address ya. If found, the frame is forwarded to the LAN associated with the address, unless its the LAN the frame arrived on (filtered) yb. If not found, the frame is forwarded to all LAN interfaces except the one on which the frame arrive (flooding) zTransparent bridges make all the forwarding decisions, end stations don’t even know the bridge is there!

Spring 2000John Kristoff10 Will This Work?

Spring 2000John Kristoff11 Introducing Spanning Tree zAllow a path between every LAN without causing loops (loop-free environment) zBridges communicate with special configuration messages (BPDUs) zStandardized by IEEE 802.1d Note: redundant paths are good, active redundant paths are bad (they cause loops)

Spring 2000John Kristoff12 Spanning Tree Requirements zEach bridge is assigned a unique identifier yConsists of the MAC address and a priority zA group address for bridges on a LAN zA unique port identifier for all ports on all bridges

Spring 2000John Kristoff13 Spanning Tree Concepts: Root Bridge zThe bridge with the lowest bridge ID value is elected the root bridge zOne root bridge chosen among all bridges zEvery other bridge calculates a path to this root bridge

Spring 2000John Kristoff14 Spanning Tree Concepts: Path Cost zAssociated with each port on each bridge zThe cost associated with transmission onto the LAN connected to the port zCan be manually or automatically assigned zCan be used to alter the path to the root bridge

Spring 2000John Kristoff15 Spanning Tree Concepts: Root Port zThe port on each bridge that is on the path towards the root bridge zThe root port is part of the lowest cost path towards the root bridge zIf port costs are equal on a bridge, the port with the lowest ID becomes root port

Spring 2000John Kristoff16 Spanning Tree Concepts: Root Path Cost zThe minimum cost path to the root bridge zThe cost starts at the root bridge zEach bridge computes root path cost independently based on their view of the network

Spring 2000John Kristoff17 Spanning Tree Concepts: Designated Bridge zOnly one bridge on a LAN at one time is chosen the designated bridge zThis bridge provides the minimum cost path to the root bridge for the LAN zOnly the designated bridge passes frames towards the root bridge

Spring 2000John Kristoff18 Spanning Tree Concepts: Illustrated

Spring 2000John Kristoff19 Spanning Tree Concepts: Illustrated [continued]

Spring 2000John Kristoff20 Spanning Tree Algorithm: An Overview z1. Determine the root bridge among all bridges z2. Each bridge determines its root port yThe port in the direction of the root bridge z3. Determine the designated port on each LAN yThe port which accepts frames to forward towards the root bridge

Spring 2000John Kristoff21 Spanning Tree Algorithm: Selecting Root Bridge z1. Initially, each bridge considers itself to be the root bridge z2. Bridges send BDPU frames to its attached LANs ya. The bridge and port ID of the sending bridge yb. The bridge and port ID of the bridge the sending bridge considers root yc. The root path cost for the sending bridge z3. Best one wins (lowest ID/cost/priority)

Spring 2000John Kristoff22 Spanning Tree Algorithm: Selecting Root Ports zEach bridge selects one of its ports which has the minimal cost to the root bridge zIn case of a tie, the lowest uplink (transmitter) bridge ID is used zIn case of another tie, the lowest port ID is used

Spring 2000John Kristoff23 Spanning Tree Algorithm: Select Designated Bridges z1. Initially, each bridge considers itself to be the designated bridge z2. Bridges send BDPU frames to its attached LANs ya. The bridge and port ID of the sending bridge yb. The bridge and port ID of the bridge the sending bridge considers root yc. The root path cost for the sending bridge z3. Best one wins (lowest ID/cost/priority)

Spring 2000John Kristoff24 Forwarding/Blocking State zRoot and designated ports will forward frames to and from their attached LANs zAll other ports are in the blocking state

Spring 2000John Kristoff25 Configuration Messages

Spring 2000John Kristoff26 Bridge Encapsulation

Spring 2000John Kristoff27 Source Route Bridging zUsed in token ring environments zAlternative to transparent bridging zBridge loops can exist zDefined by IBM and standardized by IEEE zIntelligence moves from bridges to end stations

Spring 2000John Kristoff28 Source Routing Bridging

Spring 2000John Kristoff29 Source Route Destinations zNull - destination on the same LAN zNon-broadcast - includes a route to destination zAll routes broadcast - flooded to each LAN, bridges record route along the way zSingle route broadcast - only one frame per LAN, spanning tree used

Spring 2000John Kristoff30 Route Discovery zTransmit “all-route” broadcast to destination yDestination sends non-broadcast response to the first frame received (using that route) zTransmit “single-route” broadcast to destination yDestination sends back an all-route broadcast response ySender picks the first response received from destination Routes can also be manually configured on stations

Spring 2000John Kristoff31 Source Route Discovery: Illustrated

Spring 2000John Kristoff32 Routing Information Field zIf bit 0 of byte 0 in the source address is set to 1, then this frame is a source routed frame

Spring 2000John Kristoff33 Bridge Filters zUseful for controlling LAN traffic zExamines data link layer information zExamples yDo not forward frames from MAC address X yDo not forward Ethernet frames of type X yDo not forward broadcast frames from X yLimit source route hops to 6

Spring 2000John Kristoff34 Switches zPhysically similar to hubs zLogically similar to bridges zTakes advantage of improvements in ASIC technology zPermits full duplex operation zQuickly replacing hub/bridge technology zThe name switch is a marketing gimmick

Spring 2000John Kristoff35 Inside a Switch zConceptual operation yOne LAN segment per host yBridge interconnects each host/segment

Spring 2000John Kristoff36 Switches: Final Notes zStore and Forward zCut-through zMixing interfaces zVLANs zNetwork Management Issues yPort Mirroring ySecurity

Spring 2000John Kristoff37 Virtual LANs - An Introduction zDefines a broadcast domain on switches zOnly difference from LAN is the packaging zTo move between VLANs, you need a route (layer 3 device) zWhy have separate VLANs?

Spring 2000John Kristoff38 VLANs Illustrated