Hierarchies Ethernet Switches Must be Arranged in a Hierarchy –Root is the top-level Ethernet Switch Root

Hierarchies Usually, Fastest Switches are at the Top (Root) –Sizes given are only examples Gigabit Ethernet Campus Switch 100Base-X Building Switch 10Base-T Workgroup Switch

Hierarchies Only a Single Possible Path (2,1,3,4) Between Any Two Stations Single Possible Path Ethernet Switch A B 2

Hierarchies Vulnerable to Single Points of Failure –Switch or Link (trunk line between switches) –Divide the network into pieces X X Ethernet Switch

Hierarchies 802.1D Spanning Tree Allows Redundant Links –Automatically deactivated to prevent loops Deactivated Redundant Link Ethernet Switch

X Hierarchies 802.1D Spanning Tree Allows Redundant Links –Automatically reactivated in case of failure –Slow and not completely effective Reactivated Redundant Link Ethernet Switch Failure

Hierarchies Link Aggregation Protocol Allows Multiple Links Between Stations –If one link fails, others continue –Switch failures or cuts of all links still fatal Multiple Links Ethernet Switch

Hierarchies Single Possible Path Simplifies Switch Forwarding Decisions –When frame arrives, only one possible output port (no multiple alternative routes to select among) –Switch sends frame out that port Simple Forwarding Decision Ethernet Switch

Hierarchies Router networks are meshes, allowing multiple alternative routes to the destination host –Each alternative route is represented by a row in the router forwarding table (Ch. 3) –Router must evaluate each row for each packet –For each row, may have to compute match length, and metric –After looking at all rows, must choose the best alternative route

Hierarchies Switches allow only a single path for each MAC destination address –Associated with a single port on each switch –So switch forwarding table has one and only one row for each MAC address Ethernet Switch Address A3.. B2.. Port 3 5

Hierarchies Ethernet switch only has to find the single row that matches the destination MAC address –Only has to examine half the rows on average; less if the table is alphabetized –Comparison at each row is a simple match of the frame and row MAC addresses; much less work than row comparison in routers –Overall, this is much less work than routers must do Address A3.. B2.. Port 3 5

Hierarchies Overall, then, Ethernet switch forwarding is much simpler than router forwarding (Ch. 3) –So Ethernet switches are both cheaper and faster than routers Simple Forwarding Decision Ethernet Switch