Solved: Network Bridge concept

iifanatic · ‎03-19-2008

Hello,

Beginner studying for CCNA exam.

I have a bit of confusion between Bridges.

"Bridges are used to connect network segments"

Do these network segments need to be on the same network?

I mean can a network with 10 hosts on a hub with ip address 172.16.1.15 communicate with another network 192.168.1.2 by connecting a bridge.

With switches, what is the major difference between a bridge and a switch not with respect to functionality but general difference in terms of frame forwarding ..

Do bridges and switches work same EXCEPT bridges have less ports and switches have more?

If someone can avoid this confusion, I would be really grateful to him.

Regards,

Francois Tallet · ‎03-19-2008

My definition of switch is "marketing term for fast bridge"... which technically means "bridge";-)

I guess the only reason why the term was introduced was to push you to upgrade your "old" bridge and buy a new cool device. I think you can challenge anyone who has a clear definition of a switch (vs. a bridge). However, if some CCNA book pretend those are two kind of devices, don't lose points for that!

Regards,

Francois

View solution in original post

meballard · ‎03-19-2008

Yes, if it doesn't already know where to send the packet (ie when the destination MAC address is not in it's table), it always floods all ports (at least all ports in the same VLAN, if you don't understand it, ignore the VLAN part for the time being about it yet).

You can have multiple MAC addresses on one port, such as if there is a switch/hub on that port, or say a user running virtual machines on a computer. As such, it can never assume it knows all possible MAC addresses on one port, so it always has to flood all ports (when it doesn't know where it is).

Note that it isn't flooding addresses, it is flooding ports, ie it doesn't flood the known addresses, when it reaches the flood stage, it doesn't care about what is already there and that table has no relevance.

View solution in original post

Francois Tallet · ‎03-19-2008

My definition of switch is "marketing term for fast bridge"... which technically means "bridge";-)

I guess the only reason why the term was introduced was to push you to upgrade your "old" bridge and buy a new cool device. I think you can challenge anyone who has a clear definition of a switch (vs. a bridge). However, if some CCNA book pretend those are two kind of devices, don't lose points for that!

Regards,

Francois

meballard · ‎03-19-2008

A switch is a multi-port bridge. A plain bridge has 2 ports, a switch has more than 2 (such as 8, 24, 48, etc).

Beyond that, there isn't an inherent difference between a bridge and a switch.

A bridge only separate collision domains, not network subnets. Both sides of a bridge are part of the same network, so 172.16.1.15 would never be able to communicate with 192.168.1.2 with a regular bridge, a router would be required for that.

iifanatic · ‎03-19-2008

Thanks guys.

One more question.

There is a concept of flooding in bridges.

If a bridge doesnt know the destination mac address it will flood all the ports.

Following is the scenario:

4 hosts connected to the bridge. There MAC addresses are as follows:

aaaaHost1 port1

bbbbHost2 port2

ccccHost3 port3

ddddHost4 port4

Host aaaaHost1 communicates with ddddHost4

As it is a new bridge, the bridge enters the entries in the bridging table:

aaaa.host1 port1

cccc.host4 port4

Now bbbbhost2 sends a unknown frame to some destination, will the bridge FLOOD all the ports including the KNOWN addresses above in the table?

PAUL TRIVINO · ‎03-19-2008

If bbbb *sends to* any address other than aaaa and dddd (assuming "cccc.host4 port4" above is a typo), it will flood it, because the only specific destinations it knows are aaaa and dddd.

Paul

meballard · ‎03-19-2008

Yes, if it doesn't already know where to send the packet (ie when the destination MAC address is not in it's table), it always floods all ports (at least all ports in the same VLAN, if you don't understand it, ignore the VLAN part for the time being about it yet).

You can have multiple MAC addresses on one port, such as if there is a switch/hub on that port, or say a user running virtual machines on a computer. As such, it can never assume it knows all possible MAC addresses on one port, so it always has to flood all ports (when it doesn't know where it is).

Note that it isn't flooding addresses, it is flooding ports, ie it doesn't flood the known addresses, when it reaches the flood stage, it doesn't care about what is already there and that table has no relevance.