Today's lesson focus on Bridges and Data Link Layer Switching.
There are several ways of interconnecting multiple networks.
When two or more networks are interconnected under the physical layer,
the type of device is usually called a Repeater or Hub.
When interconnected to the data link layer,
it is called a Bridge.
When interconnected at the network layer, is called Routers.
When interconnected at higher layers,
it's usually called Gateway.
When range or extension is the only problem for interconnection,
repeaters may solve the problem as long as
a maximum distance between two stations is not exceeded.
Hub is the simplest repeater in Ethernet local area networks.
Because the collision domains of LANs connected as repeaters is the entire network,
all traffic appear in both networks.
But if LANs are connected to a Bridge with the MAC address filtering,
local traffic stays in its local LAN.
In general, bridges operating at
a data link layer imply capability to work with multiple network layers.
In this case, a bridge either connects one Ethernet and the other token ring.
But a bridge is needed to deal with difference in MAC formats,
in maximum frame length, in buffering,
timers and security requirements.
In a common case a bridge interconnects LANs of
the same type to have a frame filtering capability.
A bridge has to monitor the MAC address of each frame.
Two types of bridges are widely used: transparent bridges and source routing bridges.
Transparent bridges are typically used in Ethernet while source
routing bridges are typically used in token ring and FTTI networks.
We focus on transparent bridges.
The basic process works as follows: it creates a lookup table based on backward learning.
The table associates each station with a port number.
The bridge observes source address of arriving frames.
It discards a frame if source and destination are in the same LAN.
Forward a frame if source and destination are in different LANs.
Use flooding if the destination is unknown.
Let's look at an example where three LANs are
interconnected by two bridges when empty lookup tables initially.
Suppose station S1 sends a frame to station S5,
the frame carries the MAC address of S5 as a destination address,
the MAC address of S1 as the source address.
When bridge B1 receives the frame,
it finds the empty table and adds as
one source address and is a port number on which the frame arrived.
As the destination address is not found in table,
the frame is forwarded to port 2 and transmitted on LAN2.
The bridge B2 perform the same process,
adding the source address into its lookup table and forwarding their frame to LAN3.
S5 eventually receives the frame destination suite.
Both bridges have learned the location of S1 by the backward learning process.
Next station S3 sends a frame to your station S2.
Both bridges, B1 and B2,
receive the frame, see if they are connected to the same LAN as a suite.
But B1 cannot find the address S3 in its lookup table,
so it adds S3 and port 2 into its table.
It then forwards the frames to port 1 which S2 finally receive.
Bridge B2 also doesn't find the source address.
It adds the new information in its table and forward the frame on LAN3.
So forwarding is a wasted.
Now assume that S4 sends a frame to S3.
Bridge B2 records the address of S4 and a port number in which the frame arrived.
Since the record of S4 is not found in the table,
then B2 checks the station address of the frame in the forwarding table.
It matches one of the entries.
So the bridge forwards the frame to the port indicated in the entry,
which is port 1.
When Bridge B1 receives the frame,
it adds the source address and there's
a port number in which is a frame arrive into the table.
The bridge, however, finds the destination address because
the port number in the entry is the same as on which the frame arrived.
The frame is discarded and another transmitted to LAN1.
Therefore, the traffic is confined to LAN2 and LAN3 only.
Now assume, S2 sends a frame to S1,
bridge B1 first adds the address of S2 in its forwarding table.
See, if the bridge has to learn the address of S1 it discards
the frame after finding out that S1 is already connected to the same port.
Therefore, the traffic is completely isolated in LAN1.
Please note that in this case,
bridge B2 cannot learn the address of S2 because the frame is not transmitted to LAN2.
In a static network,
tables eventually store all station addresses and learning stops.
In practice, stations are added and moved all the time.
Adaptive learning introduced timer to age
each entry and force it to be relearned periodically.
The learning process works fine as long as no loops in the interconnected network.
To remove loops in a network,
our committee specified an approach called spanning tree algorithm.
Xiaobo ZhouProfessor, Interim Dean
