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