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Info about Bridging

Posted: Wed Apr 16, 2014 11:12 pm
by qwertysqwerty
Hello. I have several questions. I'm not following what bridging is all about. I get that it is a L2 concept and allows the L2 connection between (often geographically separated) LANs, but I'm missing "the whole point" completely. If you want to pick and choose which questions to answer this will be much more valuable to me than no response at all, so please do :)

Can somebody explain:
  • the reason for the default RouterOS configuration whereby all Ethernet ports are configured as a single bridge?
  • Why is it difficult to configure networking on the RouterOS without bridging all interfaces, i.e. without bridging all interfaces just pinging another host on another Ethernet port is difficult.
  • Does bridging all interfaces mean that packets are not switched by the ASIC, but by the CPU?
  • Why would I bridge interfaces?
  • If I set Ethernet ports 2-5 to be a slave of say for instance ether3 and I then assign ether3 to a bridge do I have to sniff traffic on ether3 to see traffic RX/TX one of the slave ports? Is wire-speed switching and then assigning the master port to a bridge a common approach? If not, what is?
  • What is the best approach for DHCP server with and without bridging (i.e. if not including any ports in a bridge where is the DHCP residing?... DHCP server port-by-port?)
  • What benefit is offered by a bridge from a networking perspective and communication with the RouterOS system?
  • What disadvantage does bridging have?
  • Is bridging more a necessity when configuring QoS?
I make no excuses for "dumb" questions. Silly questions are the best kind!

Thanks.

Re: Info about Bridging

Posted: Thu Apr 17, 2014 2:39 pm
by CelticComms
Bridging does indeed join two interfaces at layer 2. It operates on layer 2 frames - not layer 3 packets. The name comes from earlier devices called "repeaters" and "bridges". When implemented in hardware a bridge is essentially a 2 port switch - or a switch is essentially a multi-port bridge. However, when implemented in software (as in RouterOS) a bridge uses the CPU to move the Ethernet frames around - a key disadvantage.

Many RouterBoards now provide one or more switch chips. If you can meet your needs using those then that is preferable to using bridges. Sometimes it may also be necessary to bridge one or more ports in order to place a group of ports in the one layer 2 domain - e.g. make a group of ports "LAN" ports.

If you are using a bridge you should place any IP address and DHCP server on the bridge itself and not on the member ports.

Other specifics really depend on the RouterBoard model and which options you have.

Re: Info about Bridging

Posted: Thu Apr 17, 2014 2:46 pm
by cdiedrich
I'd like to add something historical :-)
A bridge is also a (virtual) device which can interconnect different types of interfaces (e.g. ethernet, wireless, FDDI, nowadays tunnels, etc).
For example a bridge is the only way to add the integrated wlan interface to a given network without routing.

Re: Info about Bridging

Posted: Thu Apr 17, 2014 6:03 pm
by Zorro
there was several reasons, historically.
firstly its about performance and reliance on switch chip, caps, sometimes.
secondly as noticed, its to reduce necessary configuration amounts/steps, at expense of security and manageability.
and in third place - to mimic notorious SOHO devices, configuration and thus - reduce culture shock of new customers and prevent riots nor suicidal tendencies among them.
so in short: bridging sucks. NEVER use it, unless you had to do(which is sucks too, btw).
as for global-scale L2 traffic and networks - check Wiki about "VPLS" and "L2TP" topics, for start.

Re: Info about Bridging

Posted: Thu Apr 17, 2014 6:42 pm
by qwertysqwerty
...its to reduce necessary configuration amounts/steps, at expense of security and manageability [and] to mimic notorious SOHO devices, configuration and thus - reduce culture shock of new customers... [In] short: bridging sucks. NEVER use it...
I'm assuming the alternative L2 approach is to configure Ethernet ports 1-5 with ether3 as master (for example) and stick the DHCP server on ether3? In regards to RouterOS what alternative configuration approaches are there to have your network/LAN up and running without bridging all interfaces?

If you see my original questions I put:
  • What is the best approach for DHCP server with and without bridging (i.e. if not including any ports in a bridge where is the DHCP residing?... DHCP server port-by-port?)

Re: Info about Bridging

Posted: Thu Apr 17, 2014 6:48 pm
by nwr
Well, about bridging: try to connect two at least 1Gbps devices(PC or NAS) with "bridging" and then with "routing", move some HD movie or large archive from one device to another, open "System"->"Resources" on your router and watch. You will clearly understand difference betwen "bridging" and "routing".
And, instead of previous message, I suggest to use "routing" only when you can't reach your goals with "bridging" or "switching".

Re: Info about Bridging

Posted: Thu Apr 17, 2014 6:51 pm
by CelticComms
I'm assuming the alternative L2 approach is to configure Ethernet ports 1-5 with ether3 as master (for example) and stick the DHCP server on ether3? In regards to RouterOS what alternative configuration approaches are there to have your network/LAN up and running without bridging all interfaces?
Yes - on a smaller RouterBoard you can easily take one port on the switch as the "WAN" port - make that port independent (neither master nor slave). Then select on of the remaining ports as the "master" for the "LAN" and the others should be slaved to it. All of your LAN settings )IP, DHCP etc.) are assigned to the "master" "LAN" port.

Re: Info about Bridging

Posted: Thu Apr 17, 2014 7:01 pm
by qwertysqwerty
Thank you for your reply.