Part 9 Bridges and STP 2011-12

Bridges and spanning tree protocol1Transparent BridgesSpanning tree bridge•Forwards frames via use of look-up table•Based upon backward learning•Flooding•Tables purged regularly•Needs a spanning tree structure -no loops2Bridges•Link layer device•stores and forwards Ethernet frames•examines frame header and selectively forwards frame based on MAC dest address•when frame is to be forwarded on segment, usesCSMA/CD to access segment•transparent•hosts are unaware of presence of bridges•plug-and-play, self-learning•bridges do not need to be configuredForwardingHow do determine to which LAN segment to forward frame?•Looks like a routing problem...Self learning• A bridge has a bridge table•entry in bridge table:•(Node LAN Address, Bridge Interface, Time Stamp)•stale entries in table dropped (TTL can be 60 min) •bridges learn which hosts can be reached through which interfaces•when frame received, bridge “learns” location of sender: incoming LAN segment•records sender/location pair in bridge tableBridge exampleSuppose C sends frame to D and D replies back with frame toC.❒Bridge receives frame from from C❍notes in bridge table that C is on interface 1❍because D is not in table, bridge sends frame intointerfaces 2 and 3❒frame received by DBridge Learning: example❒ D generates frame for C, sends❒bridge receives frame❍notes in bridge table that D is on interface 2❍bridge knows C is on interface 1, so selectively forwards frame to interface 1Needed: Routing•Complex large LANs need alternative routes•Load balancing•Fault tolerance•Bridge must decide whether to forward frame•Bridge must decide which LAN to forward frame on •Routing selected for each source-destination pair of LANs •Done in configuration•Usually least hop route•Only changed when topology changesSpanning Tree•Bridge automatically develops routing table •Automatically update in response to changes •Frame forwarding•Address learning•Loop resolutionReference: StallingsFrame forwarding •Maintain forwarding database for each port•List station addresses reached through each port•For a frame arriving on port X:•Search forwarding database to see if MAC address is listed for any port except X•If address not found, forward to all ports except X•If address listed for port Y, check port Y for blocking or forwarding state•Blocking prevents port from receiving or transmitting•If not blocked, transmit frame through port YAddress Learning•When frame arrives at port X, it has come form the LAN attached to port X•Use the source address to update forwarding database for port X to include that address•Timer on each entry in database (reset whenever frame received)•Each time frame arrives, source address checked against forwarding databaseLoop of BridgesLoop Resolving•The simple learning mechanism described fails in presence of loops in the LAN•Loops may be present by mistake, or deliberately provided for redundency•This problem is resolved by running a distributed spanning tree algorithmSpanning Tree Algorithm •Creates a logical, or “active” topology that behaves like a spanning tree•Makes alternate bridges redundant•Is run periodically, so will discover failures and usealternate bridges if necessarySpanning tree algorithm •Spanning tree algorithm is a protocol used by a set ofbridges to agree upon a spanning tree for a particularextended LAN.•Essentially, this means that each bridge decides the ports over which it is and is not willing to forward packets.•Some ports (or even entire bridges) may not participate in a spanning tree•How does the bridge select the ports to include(/exclude)?Spanning Tree AlgorithmWorking: Bridges regularly exchange frames known as Bridge Protocol Data Units (BPDUs). This exchange does the following:1.Each bridge has a unique Identifier2.Bridge with highest priority and smallest ID is selected asroot bridge.3.Each bridge determines for each port, the least cost pathfrom root bridge to this port. This is the Root Path Cost(RPC) for this port.a)Select the port which has the least RPC and designate it as theRoot Port (RP). This is the port which will be used forcommunicating with the root.Algorithm...1.Once root port is determined, one bridge port isselected for each LAN segment as the designatedbridge port (DP) over which frames will be sent forthat LAN segment.a)This is a port (which is NOT a root port) which has the leastpath cost to the rootb)The ports of the root bridge are always DPs for the LANsegments connected to the root bridge2.The state of the bridge ports can be set either toforwarding or blocking.a)All ports that are either RPs or DPs are forwarding, the restare blocking.19Example: working•B1 is the root bridge•B3 and B5 are both connected to LAN A, but B5 is the designated port since it's closer to root•B5 and B7 are both connected to LAN B, but B5 is the designated port due to smaller ID (equal distance).21Topology Initialization •BPDUs are sent to a broadcast MAC address of all bridges on the LAN•All bridges initially assume they are the root bridge•Each BPDU contains (self ID, root ID, transmitting port ID, RPC of thisport)• A bridge updates its own info if it receives an update which •identifies a root with smaller id or•identifies a root with equal id but with shorter distance•The bridge adds 1 to the received RPC in the above update and saves this info.Initialization...•When a bridge determines that it's not the root bridge, it stops generating messages on its own and only forwards configuration messages from others•Also, when a bridge determines that it's the DP for that LAN, it stops sending messages on that port•When the system stabilizes, only the root is generating messages, and the messages are forwarded over only the spanning tree23Topology Change•Root bridge regularly transmits BPDUs, forwarded by all bridges on all ports•Bridges will keep timers associated with each of its forwarding ports •When timers expire, procedure similar to topology initialization is doneSome bridge features•Isolates collision domains resulting in higher total max throughput•limitless number of nodes and geographical coverage •Can connect different Ethernet types•Transparent (“plug-and-play”): no configuration necessarySpanning Tree AlgorithmSpanning tree bridge•Chose ROOT bridge•Find least cost path to ROOT -ROOT port•Find designated Bridge -designated ports26Spanning Tree AlgorithmA DESIGNATED bridge is chosen to forward frames from eachof the LAN segments. Choice is based on the least path cost to the ROOT bridge from the LAN segment.27。