What does "bridge" mean?

[Larry David]

Hi,

I currently have two networks in two locations. They are connected by a
T1 "local loop" with a Cisco router on each end of the loop. The two
networks have an address scheme of 192.168.0.0/24 and 192.168.1.0/24
respectively.

A friend of mine has a similar situation to mine. He has two networks in
two locations connected by a T1 "local loop" with a Cisco on each end -- but
in HIS case, all computers in both locations are on the same logical
network: 192.168.112.0/24. I don't understand how this can be, but he said
that the person who configured his Ciscos set up a "bridge."

Could someone explain to me what is a bridge is conceptually? This whole
concept seems weird to me because it seems like he would end up having all
kinds of broadcast messages (arp, dhcp, netbios, etc.) going across the that
slow T1 link since all of the machines are on the same logical network.
Clearly this can't be what my friend's network engineer intended, is it?

Could someone give me the scenarios where MY configuration is preferable
and scenarios where HIS bridge configuration is preferable? I'm just asking
out of idle curiosity. I like to know things work -- even though I'm
(obviously) not a network professional. I'm funny that way! Any comments
would be appreciated.

Thanks!

 

[f/fgeorge]

Hi,

I currently have two networks in two locations. They are connected by a
T1 "local loop" with a Cisco router on each end of the loop. The two
networks have an address scheme of 192.168.0.0/24 and 192.168.1.0/24
respectively.

A friend of mine has a similar situation to mine. He has two networks in
two locations connected by a T1 "local loop" with a Cisco on each end -- but
in HIS case, all computers in both locations are on the same logical
network: 192.168.112.0/24. I don't understand how this can be, but he said
that the person who configured his Ciscos set up a "bridge."

Could someone explain to me what is a bridge is conceptually? This whole
concept seems weird to me because it seems like he would end up having all
kinds of broadcast messages (arp, dhcp, netbios, etc.) going across the that
slow T1 link since all of the machines are on the same logical network.
Clearly this can't be what my friend's network engineer intended, is it?

A bridge is where you have a router and then 2 access point(ap)s. One
of the aps is connected directly to the router, the other is connected
in a "bridge" to the first ap. This lets you plug a hub for example
into the second ap and the aps talk to each other and then the first
talks to the router. I use a bridge here because I did not want to run
a wire to the basement to connect those 11 computers directly. The aps
in bridge mode are wireless to each other, so the basement computers
are each on the net, just wireless. AND I did not have to buy 11 more
wireless cards.

 

[Ryan Hanisco]

Larry,

In this case the "bridge" your friend is referring to is not a wireless
bridge at all. It sounds like he has a VPN or Tunnel set up between the
two routers. This allows the two separate networks to act as though they
are really one. Both end points can have the same subnet, as long as the
external interfaces (the tunnel endpoints) are not in that subnet and can
route to one another. I will point out, that in this case, you will usually
keep them in different subnets and turn on routing between them -- though it
is valid to keep them in the same subnet if you want. Putting them in
different subnets make sure that broadcasts and the like do not cross your
WAN link.

Go to the Cisco site and look up:
Router VPN Tunnel
GRE Tunnel
Cisco Easy VPN Client

 

[Phillip Windell]

A friend of mine has a similar situation to mine. He has two networks in
two locations connected by a T1 "local loop" with a Cisco on each end -- but
in HIS case, all computers in both locations are on the same logical
network: 192.168.112.0/24. I don't understand how this can be, but he said
that the person who configured his Ciscos set up a "bridge."

I'll keep it simple. A bridge is what a switch does, the old bridges of
"days gone by" were effectively a regular Switch with only two ports,...now
days they build them with multiple ports and call them Switches instead and
they use them to replace the old Hubs to avoid "collisions" that is common
on Ethernet.

In the case of two networks separated by a distance,...a Router is used to
connect them when they are different subnets,...but if the two networks are
the same subnet then a Bridge is used instead. In this case it is usually
the same hardware device either way because most good routers can be set to
function as a bridge.

Bridging switches packets at the Layer2 level (MAC address). This is not
considered to be "routing" but is called "packet switching" (hence the name
"Switch"). They don't see the IP# and don't care about the IP#.

Routers switch packets at the Layer3 level (IP numbers).

But like I said, many of the Routers can be configure to do either one.

 

[David Carlsson]

You can describe a bridge as a switch with two (or more) interfaces of
different network technology, ethernet, token ring, T1, etc.

/David

 

[Phillip Windell]

You can describe a bridge as a switch with two (or more) interfaces of
different network technology, ethernet, token ring, T1, etc.

That would be a Protocol Gateway, but my be known by other names depending
on who's book you read. A Bridge is nothing more than a device that
switches packets at Layer2 (MAC address). Most have the same network
technology on both sides.

The most common Bridge found on a LAN now days is a simple Switch.

The second most common would be a router on a WAN link where the same Layer4
Network exists on both sides and the router is configured to "bridge"
according to Layer2 rather than route according to Layer3. We used to run
several of those here, now we are down to just one. It runs from our main
building to an office across from the Governor's Office in the State Capitol
over a 30 mile distance.

 

[curiousg]

I have a similar situation with a widows 2K server.

http://www.webservertalk.com/message904616.html

I have a DLink di-624 wireless router which provides wireless acces
and internet routing and network switching for three machines.

I also have a win2k server located too inconveniently to run cable fro
it to the di-624.

I also have some other network devices I would like to connect throug
a switch that could hookup to the Win 2k machine.

I use a wireless NIC to access the di-624 router on the Win 2K machine
I would like to use a wired NIC to connect to devices on a wired switc
in the same location as the Win 2K server. I can't seem to get the tw
to work.

Is there a way to configure the two NICs to work in conjunction wit
each other.

I know I could get a bridge and hook it up to the switch, but it woul
be great if I could just use the existing cards.

Any ideas


-
curious

 

[f/fgeorge]

I have a similar situation with a widows 2K server.

http://www.webservertalk.com/message904616.html

I have a DLink di-624 wireless router which provides wireless access
and internet routing and network switching for three machines.

I also have a win2k server located too inconveniently to run cable from
it to the di-624.

I also have some other network devices I would like to connect through
a switch that could hookup to the Win 2k machine.

I use a wireless NIC to access the di-624 router on the Win 2K machine.
I would like to use a wired NIC to connect to devices on a wired switch
in the same location as the Win 2K server. I can't seem to get the two
to work.

Is there a way to configure the two NICs to work in conjunction with
each other.

I know I could get a bridge and hook it up to the switch, but it would
be great if I could just use the existing cards.

Any ideas?

A second nic in the server would let you plug a hub or whatever into
it and then thru ICS you could share the wireless nic to the router.
This will slow down the machine quite a bit. I have your situation and
I ended up with 2 AP's, one by the server, one connected to the
router, the AP's are in Bridge mode and wireless. Works fine and no
slowdowns!

 

[Ryan Hanisco]

A bridge doesn't have to be a media converter... though they often were,
especially in the days of Token ring/ Ethernet interop.

 

[Ryan Hanisco]

Bottom line though... In the example you gave though.... it was a tunnel or
VPN that was "bridging" the two networks together.

This was a case of using the best word to make an example rather than the
best technical word.

 

[Phillip Windell]

I know I could get a bridge and hook it up to the switch, but it would
be great if I could just use the existing cards.

A bridge is a switch,...a switch is a bridge. They are synonymous. They
used to be called "Multiport Bridges" (opposed to a standard two-port
bridge) before someone got the bright idea to just call them a "Switch"
based on the term "packet switching" that describes their actual function.

Anyway, I don't fully grasp what you describe, so I hesitate to suggest
anything without understanding what you are really asking.