Why does changing the physical socket on your router cause delays?

My question involves the delays involved with changing which physical socket your ethernet cable is connected to.

I am aware that if you are connected to a router on a network and then change which physical socket on that router you are using you will gain very small additional delays initially. However I am curious as to what causes these delays. I originally thought it was to do with the infromation stored in the routing table and whether that was allocated to a specific socket on the router or not. Although, if your IP address is the same then I don't understand why there would be delays because I would of assumed that any information within the router was linked to an IP address rather than a physical socket.

Josh Browning

Posted 2013-10-28T22:41:14.440

Reputation: 31

1Although your external IP address is the same regardless of socket (i.e. the Internet doesn't care whether you're in Socket 1 or Socket 2) internally the signals transmitted are not the same. This assumes that the router routs - different physical sockets may have different information passing through. The small delay you may get may be due to the internal processing and redirecting of the original signal to the new socket. – Doktoro Reichard – 2013-10-28T22:56:14.023

http://en.wikipedia.org/wiki/OSI_model – Ƭᴇcʜιᴇ007 – 2013-10-28T23:05:56.710

Answers

When your router sees a link indication on a port, it has no idea what that port might be connected to. If that port connected to a switch that was already connected to the network, forwarding traffic on that port might form a loop, which could be disastrous.

To prevent this, the router transitions the port into a testing mode where it only sends BPDUs on that port. Once the router is convinced that the port does not form a loop, it will begin forwarding normal traffic onto that port.

David Schwartz

Posted 2013-10-28T22:41:14.440

Reputation: 58 310

With the BPDUs, I have read they all have a unique MAC address of 01:80:C2:00:00:00. What do these do in the case the router is connected to a switch and forms a loop. Does the switch recognise this MAC address as a BPDU and halt communication with the router? Or does the BPDU send a request in which the reply tells the router what it just sent its BPDU to? – Josh Browning – 2013-10-29T00:34:00.950

It either recognizes its own BPDU or recognizes a foreign BPDU, depending on exactly what the connection goes to. In either case, it does the right thing based on following the spanning tree protocol. Typically, if it receives no BDPUs at all in 30 seconds, it switches the port to the forwarding state. – David Schwartz – 2013-10-29T01:30:44.370

Your home router is acting as a layer 2 Ethernet switch.

The main difference between a switch and a hub is that a switch has knowledge of what is connected to each port. A hub does not, so it must take all incoming data and broadcast it out on every port. A switch learns which MAC addresses are attached to each port, and directs incoming data to the port where it needs to go.

When a switch first comes online, it does not know anything yet, so it has no choice but to take each incoming Ethernet frame and broadcast it to every port. But as it does so, it remembers which port sent it that frame, and what the MAC address was on it. This way, it can "learn" what MAC addresses are connected to each port.

Now let's think about what happens if we move the cables around. Now the switch remembers the old port for the MAC address. Any incoming data is sent to the old port, but the device is no longer there, so it will not get the data! When the device sends some data out, then the switch will see this MAC address is now on the new port, and it remembers that. So it does not function properly until after some data is sent out from the device on the new port. The switch has to set itself up again.

Learn more on this page: http://ciscoiseasy.blogspot.com/2010/08/lesson-9-bridgingswitching-learning.html

Kevin Panko

Posted 2013-10-28T22:41:14.440

Reputation: 6 339

Thank you very much, this explained it very clearly and helped me understand fully whats going on – Josh Browning – 2013-10-28T23:57:56.717

1This is not true. Any decent switch will forget everything it knows about a port the second the link goes down. Many things would break if this were not true. – David Schwartz – 2013-10-29T00:04:32.360