Spanning Tree Protocol Required Elements Each bridge has a unique Bridge ID unique device ID priority number Within each bridge each port has a unique ID Spanning Tree Protocol 2 Bridge 1 1 Bridge 2 B 2 1 EE 122 Intro to Communication Networks Fall 2007 WF 4 5 30 in Cory 277 3 1 A C Bridge 3 Vern Paxson 2 TAs Lisa Fowler Daniel Killebrew Jorge Ortiz 1 http inst eecs berkeley edu ee122 2 2 Bridge 5 Slides by Lisa Fowler 1 Bridge 4 D 1 4 Spanning Tree Protocol Overview Required Elements Networks need redundancy Provide redundant paths between end stations to enable the Bridged LAN to continue to provide the Service in the event of component failure of bridge or segment Assign a cost to the segments between any two bridges can be custom unlike in lecture where we used uniform cost Loops can cause major problems 2 Bridge 1 Ethernet does not provide any notion like TTL in order to eventually remove looping frames 1 cBB Bridge 2 2 1 cA Reduce bridge topology to a single spanning tree cC 3 1 cAA Trees have no cycles loop free Automatically reconfigure the spanning tree in a reproducible and predictable manner C cC Bridge 3 2 1 cA cD 2 Bridge 5 cC cD 2 1 cDD Bridge 4 2 5 Spanning Tree Protocol Network Diagram 1 Elect a root bridge a Bridge 1 LAN B Bridge 2 Compare Bridge ID compare priority number then compare unique ID 2 Each bridge selects a root port the port with the least cost path to the root 2 Bridge 1 LAN A Bridge 5 Bridge 3 LAN D Costs cA 100 cB 19 cC 4 cD 100 LAN C 2 cA cC 3 1 cAA C cC Bridge 3 2 Bridge 5 3 Bridge 2 1 1 Bridge 4 1 cBB cA cD 2 cC cD cDD 2 1 Bridge 4 6 1 Spanning Tree Protocol Spanning Tree Protocol 2 Each bridge selects a root port the port with the least cost path to the root 4 If there s still a tie for any of the above pick the lowest port a Break ties by picking neighbor w lowest ID Costs cA 100 cB 19 cC 4 cD 100 Bridge Bridge 1 2 1 cBB Costs cA 100 cB 19 cC 4 cD 100 Bridge 2 2 1 cC 1 cAA Root Port 3 C cC Bridge 3 Bridge 2 1 2 1 3 3 4 2 5 1 1 1 Bridge 5 2 2 Root Port 1 cDD Bridge 4 Bridge 1 2 1 B Bridge 2 2 1 Desgn Port 1 A 3 2 1 2 2 1 2 3 3 2 4 2 5 1 C Bridge 3 1 Bridge 5 2 2 1 D Bridge 4 7 10 Spanning Tree Protocol Spanning Tree Protocol 3 All bridges on the network segment collectively determine least cost path from network segment to root 5 Now any time frames require broadcasting a bridge only accepts them from a designated or a root port a The port that connects the network segment to the bridge that provides this path is called the designated port Costs cA 100 cB 19 cC 4 cD 100 Bridge Root Port 1 Bridge 1 2 Desgn Port 1 3 3 4 2 5 1 2 Bridge 2 2 1 1 cAA 3 C cC Bridge 3 Bridge 2 1 2 2 1 cBB 6 and only floods to other root or designated ports Costs cA 100 cB 19 cC 4 cD 100 1 1 Bridge 5 2 2 Root Port 1 cDD Bridge 4 Bridge 1 2 Desgn Port 2 1 2 3 3 2 4 2 5 1 Bridge 2 2 1 1 A 3 C Bridge 3 2 1 2 8 1 B 1 Bridge 5 2 2 1 D Bridge 4 11 Spanning Tree Protocol a The port that connects the network segment to the bridge that provides this path is called the designated port i Costs cA 100 cB 19 cC 4 cD 100 Bridge Root Port 1 Break ties by picking bridge w lowest ID Bridge 1 2 1 cBB Desgn Port 1 cAA 1 2 3 3 2 4 2 5 1 3 C cC Bridge 3 2 1 2 2 Bridge 2 2 1 1 Bridge 5 2 2 cDD 1 Bridge 4 9 2
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