1. Chapter 5 Data Link Layer – Hub, Switch
CMPT 371 Data Communications and Networking
Chapter 5 Data Link Layer – Hub, Switch
5: DataLink Layer

2. Interconnecting with hubs
Backbone hub interconnects LAN segments
Extends max distance between nodes
But individual segment collision domains become one large collision domain
hub
hub
hub
hub
5: DataLink Layer

3. switch with six interfaces
hosts have dedicated, direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link, but no collisions; full duplex
each link is its own collision domain
switching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions
switch with six interfaces
(1,2,3,4,5,6) A A’ B B’ C C’ 1 2 3 4 5 6
5: DataLink Layer

4. switch with six interfaces
Forwarding
Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?
switch with six interfaces
(1,2,3,4,5,6) A A’ B B’ C C’ 1 2 3 4 5 6
A: each switch has a switch table, each entry:
(MAC address of host, interface to reach host, time stamp)
looks like a routing table!
Q: how are entries created, maintained in switch table?
something like a routing protocol?
5: DataLink Layer

5. Self learning A switch has a switch (forwarding) table
switch learns which hosts can be reached through which interfaces
when frame received, switch “learns” location of sender: incoming LAN segment
records sender/location pair in switch table
MAC addr interface TTL A 1 60
Switch table
(initially empty)
5: DataLink Layer

6. Filtering/Forwarding
When switch receives a frame:
index switch table using MAC dest address
if entry found for destination then{
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated }
else flood
forward on all but the interface
on which the frame arrived
5: DataLink Layer

7. Switch example Suppose C sends frame to D Switch receives frame from C
address
interface
switch 1 A B E G 1 2 3 2 3
hub
hub
hub A I D F B G C H E
Switch receives frame from C
notes in switch table that C is on interface 1
because D is not in table, switch forwards frame into interfaces 2 and 3
frame received by D
5: DataLink Layer

8. Switch example Suppose D replies back with frame to C.
address
interface
switch A B E G C 1 2 3
hub
hub
hub A I D F B G C H E
Switch receives frame from from D
notes in switch table that D is on interface 2
because C is in table, switch forwards frame only to interface 1
frame received by C
5: DataLink Layer

9. Switch: traffic isolation
switch installation breaks subnet into LAN segments
switch filters packets:
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub
switch
collision domain
collision domain
collision domain
5: DataLink Layer

10. Institutional network
mail server
to external
network
web server
router
switch
IP subnet
hub
hub
hub
5: DataLink Layer

11. Summary comparison
5: DataLink Layer