Bridge Mode

[albatros]

Does anybody know what do we mean by saying "Bridge Mode"?

[Puter Padowan]

Look HERE (http://www.cisco.com/en/US/products/hw/modules/ps2706/products_configuration_example09186a00800946e0.sht ml)

Looks like the answe may be there. My knowledge of that sort of thing is ZERO at best. Perhaps someone will be able to actually explain it a bit simpler.

[Variable]

If you are talking about routers then it simply passes traffic. A router set to bridge passes traffic through it. This is the way some older DSL routers were set up. This means the internal side of the DSL connection had a public IP on the PC. Very scary.

Bridging means no routing. It passes traffic from one interface to another. With a DSL router like a cisco 800 set up to bridge the router doesn't do anything, liek for instance NAT..

[juniper]

I posted this in another thread but is relavent..

A hub is basicly a multiport repeater when a PC sends a packet to the HUB it sends the packet out all ports except the one it learned it on. As stated each computer sends a preamble before sending packets out on the wire to see if any one else is transmitting data if there is it waites a random time and tries again, If it sends data and another PC sends data at the same time the packets will collide and get corrupted this is called a collision so in ethernet they created what is known as CSMA/CD (carrier sense multi access with collision detection) this little protocol detects the collision and notifies all computers on the LAN that there was a collision and every computer goes into a randowm waite state before starting to send again. This is why you have the 5-4-3 rule in 10BaseT since your preamble will time out before it gets back to the sender if more then 4 hubs are chained.
(Im doing this off the top of my head so bear with)

As networks grew they created "Bridges" they where usualy devices with two ports seperating a collision domain (PCs on both sides have the same network address and are physicaly seperated by the bridge) so they could break the 5-4-3 rule and could increase the LAN size. A basic leason on networking is in order to understand how these work , when a computer sends data to another computer it looks at the destination IP address and compares it to its IP address if the destination IP address is on its LAN segment it will send out an ARP request it does this because NICs are layer 2 devices and dont understand IP so it needs to change the layer 3 IP address to a layer 2 MAC address ARP broadcasts out saying who has IP address x.x.x.x all computers drop the packet accept the one that has that IP address that computer sends an ARP reply it says hey I have that IP address send the packets to my MAC address of xxxx.xxxx.xxxx.xxxx then the sending computers sends the packets to that MAC. Now Bridges work by listening for these ARP requests and replies when it heres them it puts the MAC address in its ARP table and maps it to the port it heard it on so now when a computer on segment one needs to send to a computer on segment two it sends its ARP request and the bridge hears it and knows that the destination MAC is on its other port so it replies to the ARP on behalf of the real computer basicly it says Im that MAC send me the data, the sending PC sends it to the Bridge and the Bridge forwards it out the other port and changes the senders MAC to its MAC address so the recipient can reply. The bridge just seperated the LAN into 2 collision domains.

Now your saying man I didnt ask about bridges I want to know about switches well a "switch" is a multiport bridge. a PC connects to the switch and the collision domian is only between the PC and the one switch port it is connected to. The switch builds an ARP table (cisco uses the phrase CAM) that maps all the PCs MAC addresses to a port and forwards the packets on behalf of the senders this is why you can stack so many switches together. Anyway this is also great for security as the packets only go to the destination port so sniffers are very limited (unless using ARP poisong or CAM/ARP table overflow attack both attacks can be mitigated by the way but is outside the scope of this writing). As for switches and collisions I stated the only collisions can be between the PC and the switch this is can happen if the switch and PC are using "half duplex" this is because the PC and switch tranmit and recieve on the same cable pair so if the switch and PC send at the same time (rarely happens) it will cause a collision but in full duplex they use a transmit pair and a recieve pair so there can never be a collision since the wires are dedicated to each machine effectively giving double the bandwidth so 100Mb in full duplex can actualy do 200Mb, 100Mb in each direction. Are you wondering what I am? what happens when the PC wants to send to another PC like on the internet since its NIC is a layer 2 device? well If it looks at the recipients IP address and sees that it is not in its LAN it sends it to its default gateway using its MAC address the default gateway is a router it forwards the packet based on the IP information until it gets to the LAN the recipient PC is on and the reverse happens the router at the far end in the recipients LAN will send an ARP since it knows the recipient is on its LAN segement and forwards by MAC to the PC. Kool no?
Now your saying man It takes almost a minute for the link to come up on a switch, why? well a switch uses the spanning tree protocol which is basicly a layer 2 routing protocol making sure there are no loops in the network (this could be a bad thing) if you connected 2 ports on one switch to two ports on another it will shut one connection down because this would be a loop, spanning tree will monitor the link and if the running link goes down it will bring up the other link wow we just created redundancy in our switched network LOL! any way theres alot more switches can do actualy the new switches are known as multi layer switches that can manipulate packets above layer 4 application layer but that is outside this writing also). something to keep in mind switches perform the switching and routing (if multilayer) using hardware called ASICs this allows them to forward at line speed way faster then a router which uses software and its processor to make routing decisions.

Now to the router, the router is a layer 3 device it knows IP and can send data from one LAN segment to another totaly different LAN segment. example it can route traffic between PCs in network 192.168.1.0 to PCs in network 10.0.0.0. as you can see the router performs different functions then the switch and I think that was covered pretty good in the other posts. as Im getting tired of writing this Hope this helps.

[juniper]

so that explains what bridges do and basic networking. The reason they use bridging mode now is so you can put like a firewall in between two devices (like between the internet router and the switch) and not have to change your routing, packets will just passivly go through the firewall and get scanned without any real network modification.