Regardless of whether a router is configured for bridging or routing, it must send an Ethernet frame successfully, without a collision. A half-duplex Ethernet interface (whether on a bridge, switch, router, server, or PC) monitors for a collision while sending. If a collision occurs, the interface retransmits the frame. This happens at the Media Access Layer, and has to do with accessing the medium successfully and nothing more. The station listens while sending and retransmits if a collision occurs. That's basic CSMA/CD. Every Ethernet interface (that is in half-duplex mode) must do CSMA/CD.
This doesn't mean that a router or bridge retransmits in most cases. The CCIE tests expect you to know that neither a bridge nor a router retransmits if a frame experiences a bit error or gets lost somehow. Retransmitting is up to the end station. A recipient bridge or router doesn't send back any sort of message to a sending bridge or router to report a problem. It's up to the end station to know that a packet didn't get ACKed. A router could send an ICMP message. In general, those go back to the end station though. An intermediate router has no way to know if a problem occurred and retransmit. A few other exceptions to the rule that a router doesn't retransmit are Binary Synchronous Communication Protocol (BISYNC) and LAPB. Priscilla At 11:27 PM 5/6/02, Kaminski, Shawn G wrote: >I've always known routers to "route" and bridges to "learn, filter, forward, >and flood". A co-worker said that if a router is configured with transparent >bridging, it can re-transmit a frame. He said that he heard this somewhere. >I'm pretty sure he's wrong because this just isn't something that a >router/bridge is meant to do. I also searched CCO but came up empty-handed. > >For example, say you have two segments connected to a router; one segment >off of e0 and one segment off of e1. If a host on the e0 segment sends a >frame to a host on the e1 segment and a collision occurs on the e1 segment >before reaching the destination host, then I believe that the host on e0 is >responsible for re-transmitting the frame, not the router/bridge. > >Has anyone heard of a router configured with transparent bridging >re-transmitting frames? I just can't see how this could happen. However, >I've seen stranger things happen, so I just wanted to get the opinions of >others on this group. > >Shawn K. ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=43466&t=43459 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
