routers and switches

[saverian]

dear people one question ..
one which is not really highlighted in books..
when routers are connected to 2 seperate switches do they forward info based on ip or mac address.. ? do they strip off the source mac address replacing it with the destination mac or do they strip the ip? this might sound silly because routers are layer 3 devices but no one i know will give me a clear answer>>
cheers

 

[pansophic]

Pure routers operate on Layer 3, therefore the IP address. Now router blades in switches may operate slightly differently (I don't think that they do, but since they are integrated it is possible).

That said, they definitely strip the MAC addresses (source and destination) because they have to do so in order to get the packet to move. The source address must always be the MAC of the device that sent the ethernet packet, and the destination MAC must always be the MAC of the device receiving the packet (for this particular hop). In other words, the MACs change at every device that operates above layer 2.

The router may or may not alter the IP addresses (if they are doing NAT they will, otherwise they will not).


pansophic