Local and Corporate Networks 2010
Unit 3 Multisegment LAN Networks
3.2 Types of Bridges.
Bridges examine the MAC Layer addresses of every packet that has to pass through them on a LAN
network. Determining the segment to which each MAC address belongs can be done through two
different mechanisms called 'transparent bridging' and 'source route bridging.'
Transparent or Adaptive Bridging
This method involves the use of a database forwarding route data through packets sent between
network segments. The forwarding table is initially empty and database table entries are added as
packets begin to cross the Bridge. If a packet's address is not found in the database, that packet is
relayed through all ports of the Bridge, and the packet is thus 'forwarded' to all addresses, except to
the segment of the port from which it was received. If other Bridges are encountered on the network,
they will respond to the promotional message, thus the Bridge will find out which port it can use to
access the segment where the station with the MAC address that was unknown is located.
By operation:
Transparent (802.1): Frames need no additional information
(between networks of the same type)
By routing from source (802.5): Frames have additional routing information
(Token Ring)
Translational: Connect different networks to one another
MAC Layer Bridges: interconnect homogenous networks (802.3 with 802.3)
Mixed Media Bridge: translate between different Layer 2 protocols (802.3 with 802.5)
By scope:
Local: They connect networks in the same area
Remote: They connect networks in different areas, through WAN links
Source Route Bridging
In source route bridging, two different types of packets are used in order to find a way to access the
segment containing the destination station. For known addresses, Single Route (SR) packets are
generated; whereas for unknown addresses All Route (AR) packets are generated and sent by
broadcast and it is through them that Bridges find unknown addresses and update the routes to
access the respective segments.
3.3 Transparent or Adaptive Bridge
Transparent bridges were developed by Digital Equipment Corporation in the 1980s. The outcome of
those experiments became the IEEE 802.1 Standard. It was called transparent because the
operation to discover routes was transparent to data traffic.
Transparent Bridges have been defined in IEEE 802.1 D and 'learn' routes based on packets entering
them, examining their source addresses, and generating suitable routing tables. They require no
special programs nor any configuration as they operate through data received from the network. If a
packet is sent to a destination and has not yet been delivered, the Bridge will forward the packet in
broadcast mode, seeking a reply, so that the Bridge that knows the destination address can reply and
the route to get to the destination can be determined.
The problem with the operation of transparent Bridges is one of packet overhead, with a weaker yield
in bandwidth use. Each Bridge gateway is called a Port and is usually connected to a Network
segment.
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Local and Corporate Networks 2010
Unit 3 Multisegment LAN Networks
Operates in Layer 2; at the MAC Sublayer level.
Requires both networks to have a similar addressing scheme
MAP (802.4) networks could, for example, connect with TOP (802.3) networks
Bridge Table
LAN1 LAN2 LAN3
1
2 3
To:
ADR1
ADR2
ADR3
Use:
PORT 3
PORT 2
PORT 1
Bridge
They make no noise, since they generate packets
Copies frames from one network to another; (repeats packets)
Adaptive or Learning Bridges: decide what packets to relay to each segment,
by storing the addresses of each side of the bridge in its corresponding Tables
Because bridges can receive packets simultaneously at their various ports, a bridge must have a
buffer to store incoming packets and forward them through the proper exit at the right time. Each
port operates in promiscuous mode, receiving all incoming packets and forwarding each packet
only to the necessary output port. Transparent bridges operate by means of a forwarding table
that maps MAC addresses to a particular port. When a bridge receives a packet, it searches its
table to determine which port is associated with the destination MAC address. If such an
association exists, the bridge will forward the packet through the specified output port, otherwise
the packet is forwarded to all output ports. Broadcast packets are forwarded to all output ports. In
either case, a packet is never again retransmitted through the source port.
To avoid having to manually configure the tables, the function called backward learning is
normally used. It works as follows:
1. The Bridge receives all packets of each segment (promiscuous mode).
2. The source MAC address of each packet received is stored in a cache memory (forwarding
table) along with the port where it was received.
3. Every packet that is received gets processed at the forwarding table to determine its
destination MAC address:
If the address is found in the table the packet is forwarded only to the port specified in the
table. If the output port is the same as the entry port, the packet is discarded (filtered) because it
is inside the original segment.
If the address is not found, the packet is forwarded through all ports except the port
through which said packet was received.
4. The entries in the forwarding table have a countdown timer through which the association is
discarded when this timer times out. With every new packet received through a port, the
corresponding association entry is renewed. This allows topology changes to be made without
having to manually configure the bridge. It also makes the forwarding tables smaller.
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Local and Corporate Networks 2010
Unit 3 Multisegment LAN Networks
The IEEE 802.1D Standard can be used in any type of LAN
Bridges must operate in ‘promiscuous’ mode to be able to read all addresses
The bridge maps the stations that it has on every side, and only forwards frames that:
Are being sent to a station on another side
Have an address unknown to the bridge
Have a (broadcast or multicast) group address
Do not appear as source addresses and hence do not appear in the Bridge’s table
The frame is forwarded without changes (in Transparent Bridges)
Each Bridge interface has its own MAC address, but these addresses are not used
in the frames that are sent
In routing bridges (token ring), a MAC address may be changed
Since stations can change their location on a network and connect to various segments, bridges
must regularly refresh the information in their forwarding tables. Among the problems of the
choice of maximum time allowed without refreshing (after which the table entry is deleted) are
high values that affect mobility, when addresses are kept in 'old' segments and low values mean
that many packages have to be unnecessarily transmitted, after the table entry is cleared when
data transmission is infrequent. The choice depends on the type of traffic that stations will send.
Learning Addresses in (two-segment) Bridges:
A frame is received from Seg 1 with source address A and destination address B
A is found or included in the Seg 1 address table
The Seg 1 address table is searched for B;
If it is found, the frame is discarded; otherwise, it is sent to Seg 2
Source addresses are used only for updating tables
The address is included in the Table only when a Station transmits
After a while, the tables include addresses for most of the active stations
in the Segments connected directly or indirectly
MAC table entries have a time-out period (typically 5 minutes) to allow mobility
Tables are kept in RAM memory, and have a limited size (1000- 8000 addresses)
The tables are comprehensive; there is no mechanism to summarize or group addresses
by range, as they are normally not related to one another
3.4 Operation of Transparent Bridges
The figure illustrates two different packets: packet 1 and packet 2. Packet 1 is sent by a Station in
Segment 1 and has an address belonging to Segment 3; packet 2, meanwhile, is sent by a Station in
Segment 3 and has a destination address belonging to Segment 3.
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Unit 3 Multisegment LAN Networks
Bridges function in a more complex manner than repeaters; and have the capacity
to filter and retransmit packets to the next segment
The Bridge determines where to forward an incoming frame, if it belongs to its own segment
or to the next segment
Packet 1
going to
Segment 3
Table
Seg 1
Table
Seg 2 and 3
Segment 1
Bridge 1
Table
Seg 1 and 2
Segment 2
Bridge 2
Table
Seg 3
Packet 2
going to
Segment 3
Segment 3
There are Bridges for both Ethernet Networks and for Token Ring Networks
The network is busy when either node is transmitting. A server node can centralize
a large part of the traffic, especially when its concentrates resources of a department
Types of Bridges
The transparent bridge does not alter the MAC frame
The translational bridge changes the MAC but not the LLC
Transparent (Ethernet)
Source Routing Bridge (Token Ring)
Translational (between different networks)
Operation of Packet 1:
When a packet gets to Bridge 1, its destination address is checked in the Segment 1 Table to
determine whether the packet should be sent to the bridge exit. If the destination address belongs to
Segment 1, that packet is discarded, since the Receiving Station is located in Segment 1. As the
destination address in this case belongs to Segment 3, Packet 1 goes to the Bridge 1 exit port, and is
thus sent in Segment 2. Note: The Bridge in this case has only one 'entry' and one 'exit' and the
packet therefore can only be sent 'forward.' If the Bridge had several exits, another table would have
to be consulted to determine to which output packet 1 should be sent.
Once bridge 1 can forward the packet in segment 2, (it may conflict with another packet that is being
sent at that time in that segment), the packet will get to Bridge 2, where the procedure is repeated,
and when no destination address is found in the entry table for the bridge, packet 1 will be moved to
the bridge exit, and its is transmitted through Segment 3. Once packet 1 is transmitted in Segment 3,
it will get to the destination station located in that segment.
With regard to Packet 2 coming from a segment 3 station and headed to another segment 3 station,
the packet will access the destination station directly, but it will also get to the entry port of Segment 3
on Bridge 2. That Bridge will check whether the destination address is found in the table for the Bridge
entry segment, and, since it is found in the table, packet 2 will be discarded, because that packet has
already been delivered to the destination station located in the entry segment itself.
The operation sequence for transparent bridges is very simple, and includes a learning phase and a
forwarding phase where packet addresses are unknown. A packet to be sent should only be
discarded when no bridge knows the destination, either because the address is incorrect or because
the destination station has not yet been activated and no bridge knows its address.
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Unit 3 Multisegment LAN Networks
Each interface can have a different speed
The bridge processes whole frames and can adapt to different speeds
Packet 1
going to
Segment 3
Table
Seg 1
Table
Seg 2
Segment 2
Segment 1
Bridge 1
100 Mb/s
10 Mb/s
Table
Seg 1 and 2
Segment 3
10 Mb/s
Table
Seg 3
When the bridge has obtained all the station addresses the
packets travel between the Segments involved
A Segment 1 to 3 frame does not use Segment 2
Frame received
without error at
port x
Is destination
address found in
database?
Forwarding
Yes
No
Sí
Exit port
= x?
Forward frame
to all
ports except x
No
Forward frame
via exit port
Is source address
found in the
database?
Learning
Yes
Update address
and timer
End
CITEL04382E04
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No
Add source address
to data base
(with number of port
and timer)