Hierarchies

1
Hierarchies

• Ethernet Switches Must be Arranged in a Hierarchy
• Root is the top-level

Root
Ethernet Switch
2
Hierarchies

• Usually, Fastest Switches are at the Top (Root)
• Sizes given are only examples

100Base-X Building Switch
Gigabit Ethernet Campus Switch
10Base-T Workgroup Switch
3
Hierarchies

• Only a Single Possible Path (2,1,3,4) Between Any
  Two Stations

Single Possible Path
3
1
4
Ethernet Switch
B
5
2
A
4
Hierarchies

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

X
X
Ethernet Switch
5
Hierarchies

• 802.1D Spanning Tree Allows Redundant Links
• Automatically deactivated to prevent loops

Ethernet Switch
Deactivated Redundant Link
6
Hierarchies

• 802.1D Spanning Tree Allows Redundant Links
• Automatically reactivated in case of failure
• Slow and not completely effective

X
Failure
Ethernet Switch
Reactivated Redundant Link
7
Hierarchies

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

Ethernet Switch
Multiple Links
8
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
9
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

10
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
11
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
12
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