1. Chapter 8 Local Area Networks - Internetworking

2. Chapter Eight - Local Area Networks:
Internetworking
Introduction
Many times it is necessary to connect a local area network to another local area network or to a wide area network.
Local area network to local area network connections are often performed with a bridge-like device.
Local area network to wide area network connections are usually performed with a router.
A third device, the switch, can be used to interconnect segments of a local area network.

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Internetworking
Why Interconnect?
To separate / connect one corporate division with another.
To connect two LANs with different protocols.
To connect a LAN to the Internet.
To break a LAN into segments to relieve traffic congestion.
To provide a security wall between two different types of users.

4. Repeaters Repeaters extend the maximum allowed distance between nodes.
telnet
Repeaters
Repeaters repeat a signal from one port to another. A repeater forwards every frame;
Repeaters pass all traffic through without error checking or filtering..
Repeaters pass collisions, too. 4

5. Hubs Chapter Eight - Local Area Networks: Internetworking
As seen earlier, a hub interconnects two or more workstations into a local area network.
When a workstation transmits to a hub, the hub immediately resends the data frame out all connecting links.
A hub can be managed or unmanaged. A managed hub possesses enough processing power that it can be managed from a remote location.

6. Hubs Very similar to repeaters
very simple devices that pass all traffic in both directions between the LAN sections they link.
same or different cable types
pass on every message
used to connect LANs of similar technology, or to extend the distance of one LAN
Very similar to repeaters
Also operates at the physical layer
use physical layer protocols

7. Hubs Hub Hubs make it very easy to add and remove hosts on a network.
telnet
Hubs…
Hubs propagate a signal received on one port to all other ports.
Hubs propagate errors and collisions across ports, too.
Hubs simplify the addition and removal of nodes on a LAN.
Hubs are also used to connect network segments cabled with different media types. 7

8. Can also create multiple level of hierarchy
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9. Hubs Advantages :- inexpensive easy to Install
can connect different media
very little delay
Disadvantages :-
limited distance between devices
no protocol
no error detection
does not filter

10. Bridges Chapter Eight - Local Area Networks: Internetworking
A bridge (or bridge-like device) can be used to connect two similar LANs, such as two CSMA/CD LANs.
A bridge can also be used to connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN.
The bridge examines the destination address in a frame and either forwards this frame onto the next LAN or does not.
The bridge examines the source address in a frame and places this address in a routing table, to be used for future routing decisions.

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12. Bridges Bridge Connects two or more LANs at the link layer
Extracts destination address from the frame.
Looks up the destination in a table.
Forwards the frame to the appropriate LAN segment.
same or different cable types.
forward only those messages that need to go out (filtering).
“learn” whether to forward packets.
Each segment can carry its own traffic
host
host
host
host
host
host
Bridge
host
host
host
host
host
host 12 12

13. Bridges
Bridge
Bridges make it possible to segment your network into separate collision domains to minimize collisions and improve performance.
Hub
Hub
telnet
telnet
Separate Collision Domains
Bridges
Bridges provide all the functionality of a hub....
Bridges filter frames by destination MAC, and segment a LAN into multiple collision domains.
Bridges filter signal and timing errors.
Bridges can be used to connect segments operating at different speeds. 13

14. If the frame is to be forwarded, the decision must specify to port.
A bridge has a table used in filtering decisions.
It can check the destination address of a frame and decide if the frame should be forwarded or dropped.
If the frame is to be forwarded, the decision must specify to port.
A bridge has a table that maps address to ports. 14

15. A bridge connecting two LANs
the bridge can check the physical/MAC addresses contained in the frame.
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16. Chapter Eight - Local Area Networks: Internetworking
Transparent Bridges
A transparent bridge does not need programming but observes all traffic and builds routing tables from this observation.
This observation is called backward learning.
Each bridge has two connections (ports) and there is a routing table associated with each port.
A bridge observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table.
A transparent bridge is found with CSMA/CD LANs.

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18. Chapter Eight - Local Area Networks: Internetworking

19. Chapter Eight - Local Area Networks: Internetworking
Transparent Bridges
A transparent bridge can also convert one frame format to another, but this does not happen too often anymore since most networks are CSMA/CD.
Note that some people / manufacturers call a bridge such as this a gateway or sometimes a router.
The bridge removes the headers and trailers from one frame format and inserts (encapsulates) the headers and trailers for the second frame format.

20. A learning bridge and the process of learning
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21. Remote Bridges Chapter Eight - Local Area Networks: Internetworking
A remote bridge is capable of passing a data frame from one local area network to another when the two LANs are separated by a long distance and there is a wide area network connecting the two LANs.
A remote bridge takes the frame before it leaves the first LAN and encapsulates the WAN headers and trailers.
When the packet arrives at the destination remote bridge, that bridge removes the WAN headers and trailers leaving the original frame.

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23. Spanning Tree Algorithm
Chapter Eight - Local Area Networks: Internetworking
Spanning Tree Algorithm
What happens if you have many LANs interconnected with multiple bridges, such as shown in the next slide?
Data that leaves one workstation could travel to a bridge, across the next network, into the next bridge, and back onto the first network.
A packet may continue to cycle like this forever!

26. Spanning Tree Algorithm
Chapter Eight - Local Area Networks: Internetworking
Spanning Tree Algorithm
How do we stop this from happening?
Disconnect one of the bridges? Maybe we want bridge redundancy in case one bridge fails.
How about applying the spanning tree algorithm.
How is the algorithm applied?

27. Spanning Tree Algorithm
Chapter Eight - Local Area Networks: Internetworking
Spanning Tree Algorithm
Step 1: Designate a Root Bridge
Step 2: Mark one port of each bridge as the Root Port. The root port is the port with the least-cost path from that bridge to the root bridge. The root ports are denoted with an asterisk in Figure 8-7b.
Step 3: The next step is to select a designated bridge for each LAN. A designated bridge has the least-cost path between that LAN and the root bridge. Mark the corresponding port that connects that LAN to its designated bridge with two asterisks (Figure 8-7b)

28. Spanning Tree Algorithm
Chapter Eight - Local Area Networks: Internetworking
Spanning Tree Algorithm
Step 4: If a port has no asterisks, that port is redundant and can be “removed”. Keep all ports with one or two asterisks. The resulting configuration is shown in Figure 8-7c.
Note there is now only one way to get to any LAN or bridge from any other LAN or bridge.

30. Example Extended LAN with LOOPSB B9 B7 B5 C F D K B2 B1 J E G H B B4 I

31. Defining a Spanning Tree
Basic Rules
Bridge with the lowest ID is the root
For a given bridge
A port in the direction of the root bridge is the root port
For a given LAN
The bridge closest to the root (or the bridge with the lowest ID to break ties) is the designated bridge for a LAN
The corresponding port is the designated port
Bridges with no designated ports and ports that are neither a root port nor a designated port are not part of the tree.

32. Spanning Tree AlgorithmB
Root D B9 B7 B5
D – designated port R C F D D K B2 B1 B1
R –
root port J E G H B B4 I

33. Using a Spanning Tree: Forwarding
Each bridge forwards frames over each LAN for which it is the designated bridge or connected by a root port B4 B7 B1 B5 B2 A K J I H G F E D C B

34. Switches Chapter Eight - Local Area Networks: Internetworking
A switch is a combination of a hub and a bridge.
It can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns.
When a frame arrives at a switch, the switch examines the destination address and forwards the frame out the one necessary connection.
Workstations that connect to a hub are on a shared segment.
Workstations that connect to a switch are on a switched segment.

35. Switches Chapter Eight - Local Area Networks: Internetworking
The backplane of a switch is fast enough to support multiple data transfers at one time.
A switch that employs cut-through architecture is passing on the frame before the entire frame has arrived at the switch.
Multiple workstations connected to a switch use dedicated segments. This is a very efficient way to isolate heavy users from the network.
A switch can allow simultaneous access to multiple servers, or multiple simultaneous connections to a single server.

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38. Switches
Switch
Switches are similar to bridges, but offer multiple parallel communication channels across ports for improved performance.
telnet
telnet
Switches
Switches provide all the functionality of a bridge ....
Switches typically offer more ports than bridges.
Switches allow for multiple, parallel channels of communication between ports. 38

39. Virtual LANs Chapter Eight - Local Area Networks: Internetworking
A virtual LAN, or VLAN, is a logical subgroup within a local area network that is created via switches and software rather than by manually moving wiring from one network device to another
Even though the employees and their actual computer workstations may be scattered throughout the building, LAN switches and VLAN software can be used to create a “network within a network.”

40. Virtual LANs Chapter Eight - Local Area Networks: Internetworking
A relatively new standard, IEEE 802.1Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LAN
Instead of sending a technician to a wiring closet to move a workstation cable from one switch to another, an 802.1Q-compliant switch can be remotely configured by a network administrator

41. Switches Cut-through switches Store-and-forward switches
use circuit-switching to immediately connect the port with the incoming message to the correct outgoing port
very fast as decisions are done in hardware
outgoing packet is lost if port is in use
Store-and-forward switches
copy the incoming packet to memory prior to processing the destination address -- transmit it when the outgoing port is ready

42. Isolating Traffic Patterns with Switches
Chapter Eight - Local Area Networks: Internetworking
Isolating Traffic Patterns with Switches
Whether shared or dedicated segments are involved, the primary goal of a switch is to isolate a particular pattern of traffic from other patterns of traffic or from the remainder of the network
Switches, because of their backplane, can also allow multiple paths of communications to simultaneously occur

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45. Isolating Traffic Patterns with Switches
Chapter Eight - Local Area Networks: Internetworking
Isolating Traffic Patterns with Switches
Using a pair of routers, it is possible to interconnect to switched segments, essentially creating one large local area network

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Full Duplex Switches
A full duplex switch allows for simultaneous transmission and reception of data to and from a workstation.
This full duplex connection helps to eliminate collisions.
To support a full duplex connection to a switch, at least two pairs of wires are necessary - one for the receive operation and one for the transmit operation. Most people install four pairs today, so wiring is not the problem.

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49. Network Servers Chapter Eight - Local Area Networks: Internetworking
Network servers provide the storage necessary for LAN software.
They are usually the focal point for the network operating system.
Increasingly, network servers are functioning as bridges, switches, and routers. By adding the appropriate card, a server can assume multiple functions.

50. Routers Chapter Eight - Local Area Networks: Internetworking
The device that connects a LAN to a WAN or a WAN to a WAN.
A router accepts an outgoing packet, removes any LAN headers and trailers, and encapsulates the necessary WAN headers and trailers.
Because a router has to make wide area network routing decisions, the router has to dig down into the network layer of the packet to retrieve the network destination address.

51. Routers Chapter Eight - Local Area Networks: Internetworking
Thus, routers are often called “layer 3 devices”. They operate at the third layer, or OSI network layer, of the packet.
Routers often incorporate firewall functions.
An example of a router’s operation is shown on the next slide.

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53. Routers
connect two or more LANs that use the same or different data link protocols, but the same network protocol.
same or different cable types
operate at the network layer
forward only messages that need to go out
routers use the internetwork address
internal routing tables
only processes messages addressed to it

54. Routers Advantages can mix-in-match protocols and convert them
enable all ports to work at the same time
can be used as an extra layer of security
configurable
high speed
Disadvantages
hard to configure and manage
access lists must be kept current
high maintenance/high training costs
very expensive

55. Switch vs Router vs Hub vs Bridge Vs Repeater Vs Wireless Access Point
Comparison and properties of a Hub
 Hub is the simplest of these devices out of the five compared.
Hubs cannot filter data so data packets are sent to all connected devices/computers. The device has to make decision if it needs the packet. This can slow down the network overall.
Hubs do not have intelligence to find out best path for data packets. This leads to inefficiencies and wastage.
Pretty much repeat signal on one end to another.
Hubs are used on small networks where data transmission is not very high.

56. Comparison and properties of a Bridge
A bridge is more complex than hub.
A bridge maintains a MAC address table for both LAN segments it is connected to.
Bridge has a single incoming and outgoing port.
Bridge filters traffic on the LAN by looking at the MAC address.
Bridge looks at the destination of the packet before forwarding unlike a hub.It restricts transmission on other LAN segment if destination is not found.
Bridges are used to separate parts of a network that do not need to communicate regularly, but need to be connected.

57. Comparison and properties of a Switch
A switch when compared to bridge has multiple ports.
Switches can perform error checking before forwarding data.
Switches are very efficient by not forwarding packets that error-ed out or forwarding good packets selectively to correct devices only.
Switches can support both layer 2 (based on MAC Address) and layer 3 (Based on IP address) depending on the type of switch.
Usually large networks use switches instead of hubs to connect computers within the same subnet.

58. Comparison and properties of a Router
A router, like a switch forwards packets based on address.
A router uses the IP address to forward packets. This allows the network to go across different protocols.
Routers forward packets based on software while a switch (Layer 3 for example) forwards using hardware called ASIC (Application Specific Integrated Circuits)
Routers support different WAN technologies but switches do not.
Wireless Routers have Access Point built in.
The most common home use for routers is to share a broadband internet connection. The router has a public IP address and that address is shared with the network. When data comes through the router it is forwarded to the correct computer.

59. Comparison and properties of a wireless access point
Wireless Access Point bridges wireless and wired traffic.
Wireless Access Point allows devices/computers to connect to LAN in a wireless fashion.
Wireless Access Point allows wired and wireless devices work to communicate with each other.
Comparison and properties of a Repeater
Repeaters are built into the hubs or switches. Repeaters clean, amplify and resend the signals that have been weakened due to long cables traveling large distances.

60. LAN Internetworking In Action: A Small Office Revisited
Chapter Eight - Local Area Networks: Internetworking
LAN Internetworking In Action: A Small Office Revisited
Recall the In Action example from Chapter Seven.
A small office with 20 workstations in one room and 15 workstations in another room were connected to a server via 100BaseTX.
One hub was kept in a closet near the 20 workstations while a second hub was near the server.

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62. LAN Internetworking In Action: A Small Office Revisited
Chapter Eight - Local Area Networks: Internetworking
LAN Internetworking In Action: A Small Office Revisited
Now Hannah wants to connect the LAN to the Internet.
She adds a router next to the server and connects it to the hub.
She connects the router to a high-speed telephone line such as a T-1 service.
She will also have to program the router to perform IP addressing and firewall functions.

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64. LAN Internetworking In Action: A Small Office Revisited
Chapter Eight - Local Area Networks: Internetworking
LAN Internetworking In Action: A Small Office Revisited
Now network usage is so high that Hannah must consider segmenting the network.
She decides to install a database server near the original server and replace both hubs with switches.

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