Need for Routing in Ethernet switched networksDanger of LoopsSpanning TreesConcept - Bridge IDConcept - Root bridge of a networkConcept - For each bridgeConcept - For each LANConcept - Designated bridge/portSteps of Spanning Tree AlgorithmWhat we just didHow do the bridges determine the spanning tree?Short form notation for BPDUsOrdering of MessagesDetermine the Root BridgeCalculate the Root Path Cost Determine the Root PortDetermine if the bridge is the designated bridge for any of the LANs connected to its portsSelecting the Ports for the Spanning TreeBuilding the Spanning TreeAdapting to ChangesExample 1Example 2 (Practice - Solution in Exercises)Network Example (Practice - Solution in Exercises)Interesting casesExampleAnother caseSlide 26PriorityM. Veeraraghavan (originals by J. Liebeherr)1Need for Routing in Ethernet switched networks•What do bridges do if some LANs are reachable only in multiple hops ? •What do bridges do if the path between two LANs is not unique ? LAN 2Bridge 2LAN 5LAN 3LAN 1LAN 4Bridge 5Bridge 4Bridge 3dBridge 1M. Veeraraghavan (originals by J. Liebeherr)2•Consider the two LANs that are connected by two bridges.•Assume host n is transmitting a frame F with unknown destination.What is happening?•Bridges A and B flood the frame to LAN 2.•Bridge B sees F on LAN 2 (with unknown destination), and copies the frame back to LAN 1•Bridge A does the same. •The copying continuesWhere’s the problem? What’s the solution ?Danger of LoopsLAN 2LAN 1Bridge BBridge Ahost nFM. Veeraraghavan (originals by J. Liebeherr)3•IEEE 802.1 has an algorithm that builds and maintains a spanning tree in a dynamic environment.•Bridges exchange messages to configure the bridge (Configuration Bridge Protocol Data Unit, Configuration BPDUs) to build the tree.Spanning TreesM. Veeraraghavan (originals by J. Liebeherr)4Concept - Bridge ID•Each bridge has a unique identifier (8 bytes):Bridge ID = <priority level + MAC address>Priority level = 2 bytes; Note that a bridge has several MAC addresses (one for each port), but only one ID using the MAC address of the lowest numbered bridge port (port 1)•Each port within a bridge has a unique identifier (port ID).51:24:68:1f:3:40:0:1:2:3:5fe:64:96:12:1:3312Priority: 0x12:41BridgeExample above: Bridge ID = 12:41:fe:64:96:12:1:3M. Veeraraghavan (originals by J. Liebeherr)5Concept - Root bridge of a network•Root Bridge: The bridge with the lowest identifier is the root of the spanning tree.Bridge 3 with ID 0:1:34:1:21:56:19:87Bridge 1 with ID= 4:1:21:1:21:56:19:87Bridge 2 with ID= 6:4:55:4:21:56:19:87Root bridge is bridge 3since it has the smallest ID21LAN ALAN B11M. Veeraraghavan (originals by J. Liebeherr)6Concept - For each bridge•Root Port: Each bridge has a root port which identifies the next hop from a bridge to the root.•Root Path Cost: For each bridge, the cost of the min-cost path to the root•Example on previous slide: What is the root port and root path cost of bridge 1:–The root port is port 2 since it leads to the root bridge (bridge 3)–The root path cost is 1 since bridge 1 is one hop away from the root bridge (I.e., bridge 3).•Note: We assume that “cost” of a path is the number of “hops”.M. Veeraraghavan (originals by J. Liebeherr)7Concept - For each LAN•Designated Bridge, Designated Port: Single bridge on a LAN that provides the minimal cost path to the root for this LAN, and the port on this minimal cost path – if two bridges have the same cost, select the one with highest priority (lower bridge ID)– if the min-cost bridge has two or more ports on the LAN, select the port with the lowest identifier•Example on slide 25: for LAN A, the designated bridge is bridge 3 since it is the root bridge itself; port 1 is the designated port; for LAN B, the designated bridge is bridge 1 since this is closer to the root bridge than bridge 2. The designated port is port 1.M. Veeraraghavan (originals by J. Liebeherr)8Concept - Designated bridge/port•Even though each LAN is the entity that has a designated bridge/designated port, it is each bridge that determines whether or not it is the designated bridge for the LAN on each of its ports.•Example: Bridge 1 in the example on slide 25 determines whether it is the designated bridge for LAN A (to which its port 2 is connected) and for LAN B (to which its port 1 is connected).–Answer in this case is that bridge 1 is the designated bridge for LAN B, but it is not the designated bridge for LAN AM. Veeraraghavan (originals by J. Liebeherr)9Steps of Spanning Tree Algorithm1. Determine the root bridge of the whole network2. For all other bridges determine root ports3. For all bridges, determine which of the bridge ports are designated ports for their corresponding LANs•The spanning tree consists of all the root ports and the designated ports.•These ports are all set to the “forwarding state,” while all other ports are in a “blocked state.”M. Veeraraghavan (originals by J. Liebeherr)10What we just did•We just determined the spanning tree for a network of LANs and bridges in a “centralized manner.” – We knew the bridge IDs of all the bridges and the port IDs of all the ports in all the bridges.–We determined the root bridge (the bridge with the smallest ID.)–For each bridge, we determined the shortest path to the root by counting hops and thus identified the root port.–For each bridge, we determined which of its ports are designated ports for each of its LANs•However, the network of bridges determines the spanning tree in a “distributed manner” - each with limited knowledge.–This is done using messages called BPDUs.M. Veeraraghavan (originals by J. Liebeherr)11How do the bridges determine the spanning tree? With the help of the BPDUs, bridges can:•Elect a single bridge as the root bridge.•Each bridge can determine:– a root port, the port that gives the best path to the root.–And the corresponding root path cost•Each bridge determines whether it is a designated bridge, for the LANs connected to each of its ports. The designated bridge will forward packets towards the root bridge.•Select ports to be included in the spanning tree.–Root ports and designated portsM. Veeraraghavan (originals by J. Liebeherr)12Short form notation for BPDUs•Each bridge sends out BPDUs that contain the following information:root bridge (what the sender thinks it is) root path cost for sending bridgeIdentifies