1. 371-1-0291: An Introduction to Computer Networks
Homepage
Handout #10: Interconnecting LANs
Additional Reading
Text book: Chap. 3.2
Lect-10-1: Bridging
Computer Networks

2. Interconnecting LANs
Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router.
By partitioning a large LAN into multiple smaller networks, there are fewer collisions, and more parallel communications.
It is now common for the port of an Ethernet switch to connect to just a small number of hosts.
Lect-10-1: Bridging
Computer Networks

3. An Ethernet Network Problem: Shared network limits throughput.
Router
Outside
world
Problem:
Shared network limits throughput.
Lots of collisions reduces efficiency.
Lect-10-1: Bridging
Computer Networks

4. Ethernet Switching Benefits:
Router
Outside
world
Benefits:
Number of collisions is reduced. If only one computer per
port, no collisions can take place (each cable is now a
self-contained point-to-point Ethernet link).
Capacity is increased: the switch can forward multiple
frames to different computers at the same time.
Lect-10-1: Bridging
Computer Networks

5. One Ethernet Switch
Lect-10-1: Bridging
Computer Networks

6. Ethernet Switching Examines the header of each arriving frame.
If the Ethernet DA is in its table, it forwards the frame to the correct output port(s).
If the Ethernet DA is not in its table, it broadcasts the frame to all ports (except the one through which it arrived).
The table is learned by examining the Ethernet SA of arriving packets.
Lect-10-1: Bridging
Computer Networks

7. Ethernet Switching Learning addressesB C D E F G H 1 2 3 4 5 6
Ethernet
Switch
Ethernet
Switch J F
Router
Outside
world Q
Switch learns that ‘F’ is reachable through port 5 I J K L M N O P
Ethernet
Switch
Ethernet
Switch
Lect-10-1: Bridging
Computer Networks

8. Ethernet Switching Learning addresses
Ethernet address
Port A 1 B 2 C 3 D 4
E, F, G, H, Q 5
I, J, K, L, M, N, O, P 6
+ Timeout
Ethernet Switching Learning addresses A B C D E F G H 1 2 3 4 5 6
Ethernet
Switch
Ethernet
Switch
Router
Outside
world Q
Q: How do we prevent loops? I J K L M N O P
Ethernet
Switch
Ethernet
Switch
Lect-10-1: Bridging
Computer Networks

9. Preventing loops Spanning Tree Protocol
The network of bridges is a graph.
The Spanning Tree Protocol finds a subgraph that spans all the vertices without loops.
Spanning => all bridges are included.
Tree => the topology has no loops.
The distributed protocol runs:
To determine which bridge is the root of the tree, and
Each bridge turns off ports that are not part of the tree.
Lect-10-1: Bridging
Computer Networks

10. Example Spanning Tree B8 Protocol operation: B3 B5 B7 B2 B1 B6 B4
Picks a root
For each LAN, picks a designated bridge that is closest to the root.
All bridges on a LAN send packets towards the root via the designated bridge. B3 B5 B7 B2 B1 B6 B4
Lect-10-1: Bridging
Computer Networks

11. Spanning Tree Protocol
Each bridge periodically sends a “send-thru-me” message on every port. Message = (Neighbor ID, Root ID, Distance from Neighbor to root).
Initially, every bridge claims to be root and sends a distance field of 0.
Bridges keep sending the same message (periodically) until getting a “better” message. “Better” means (IDs are unique):
A root with a smaller ID
A root with equal ID, but with shorter distance
The root ID and distance are the same, but neighbor bridge has a smaller ID.
Once getting a “better” message, it stops generating its own messages, and just forwards ones that it receives (adding 1 to the distance).
A bridge receiving a message on a port from a neighbor bridge that is closer to the root (or equally far but with smaller ID), it stops sending messages on that port.
Eventually:
Only the root bridge generates configuration messages,
Other bridges forward these messages on ports for which they are the designated bridge.
Lect-10-1: Bridging
Computer Networks

12. Example Spanning Tree Spanning Tree: Root Superfluous B8 B3 B5 B7 B2
Lect-10-1: Bridging
Computer Networks

13. Broadcast and Multicast
To support broadcast, each bridge forwards a frame with a destination broadcast address on each active port but on the one it has been received.
Multicast can be done similarly, but each host on a LAN decides if the accept the frame.
For cases where some LANs have no host that are members of a particular multicast group:
The spanning tree is used to forward multicast frames of group M to networks containing relevant hosts
A bridges learns if it needs to forward frames from group M on port p if it receives frames group M on that port.
Hosts whishing to receive messages on group M must periodically send frames with group M in the SA header field
Lect-10-1: Bridging
Computer Networks

14. Virtual LANs
When a network of interconnected LANs is beyond the size of a typical administrative organization (department) it becomes useful to partition it into Virtual LANs W X
VLAN 100
VLAN 100 B1 B2
VLAN 200
VLAN 200 Y Z
Each VLAN has an ID and packets can travel between LAN segments only if segments have the same VLAN ID.
Bridges enforce the policy after port configuration. Each port is associate with a VLAN Id.
Lect-10-1: Bridging
Computer Networks

15. Virtual LANs When a is sent by X, B2 observe it belongs to VLAN 100.
It inserts a VLAN header between the Ethernet header and its payload
The packet will be sent to B1 but not on the port to VLAN 200
B1 learns the originating VLAN ID from the VLAN header
One may change the logical topology w/o re-wiring or address changing – Merely by bridge configuration
Lect-10-1: Bridging
Computer Networks

16. Remarks on Bridging
Bridges use the frame header and therefore are limited to connect LAN segments of the same type (Ethernet and Token Ring don’t mix)
Bridges may develop congestion unexpected to applications programmed to work on LAN – dangerous since bridges are added transparently to the hosts.
may cause to dropped frames and high latencies
Frames may be reordered
Lect-10-1: Bridging
Computer Networks