s vermill wrote: > > Cisco Newbie wrote: > > > > I have a question that has been bothering me. If a packet > > traverses a > > > > L3 devices, does the sorce MAC changes? When does/doesn't the > > source MAC address changes? > > > > thanks. > > > > > > > > --------------------------------- > > Do you Yahoo!? > > Yahoo! Mail Plus - Powerful. Affordable. Sign up now > > > > > > The source MAC changes everytime the IP packet moves through a > L3 device.
Yes, and that's also true for an AppleTalk, IPX, DECnet, Banyan packet, for what it's worth. Not a whole lot, I suppose, although it may help one understand a router's behavior. A router takes in a frame on an input interface, decapsulates it from the L2 header, figures out the output interface, and deals with the relevant L2 issues for the type of L2 protocol on the output interface, including puttting on a new L2 header. For example, if the output interface is Ethernet, the router does CSMA and makes sure the frame is transmitted without encountering a (legal) collision. If it were Token Ring or FDDI, the router would make the output interface could get a token and attach the frame. If it's Frame Relay, it doesn't have to do much, since that's not a shared medium. The router would not, however, in most cases, monitor whether the frame arrived intact. With most L2 protocols, it has no way of knowing that. Priscilla Even in Multilayer Switching (MLS), where an > Ethernet switch moves the packet across L3 boundaries on behalf > of the router, it re-writes the source MAC to that of the > router so it looks as if it traversed the router. A L2 network > is entirely self-contained. There is no significance of a MAC > from on L2 network to another. > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62271&t=62251 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
