It's PCA in both cases. How do you think B1 (running Ethernet) would even know that there was a line hit on the frame? It wouldn't. Ethernet is a best-effort, connectionless protocol, with no ACKs, sequence numbers, etc.
You may be confusing media access control with reliability. Here's an answer I sent offline to someone who was wondering about collisions and retransmissions. "Sending a frame, monitoring for a collision and retransmitting if one occurs, are fundamental jobs of every data-link-layer Ethernet interface. An analogy would be getting the token on a Token Ring network. It's as fundamental as that. The only device that is absolved from this fundamental job is a repeater since it works at the physical layer. The bridge in the example is forwarding a frame from some other device, but that doesn't matter. It still must make sure it can really transmit the frame without the frame getting damaged by a collision. A router interface must do this also, as must every Ethernet NIC. There's no way PCA would even know a collision occurred. Supposedly the bridge could send back a jam signal to let PCA know, but then you would have devices on opposite sides of the bridge being affected by each other's collisions. That means the bridge wouldn't be dividing collision domains, which is one of its main features. You seem to be trying to answer some other question related to addressing. Data-link layer addresses change at a router, whereas they don't change at a bridge. A bridge (switch) just forwards frames as is. A router decapsulates the data-link header, looks at the network-layer destination and checks the routing table to see how to send to that destination, and re-encapsulates the packet in a new data-link header, (with the router's source address). That has nothing to do with collision detection, though." So, if you had asked about collisions, instead of line hits, the answer would be different. But sending a frame without collision is just a basic part of transmitting. As I said, it's like getting the token on Token Ring. It doesn't mean that Ethernet is a reliable protocol. It's still just best-effort. Priscilla At 09:08 AM 11/12/01, Todd Carswell wrote: >Here's the setup for my 2 questions... > >PCA-------B1-----------B2--------PCB > >Bridge 1 and Bridge 2 are running Transparent Bridging between them. > >Question 1: There's a SERIAL connection between B1 and B2. B1 and B2 are >configured for transparent bridging. If PCA sends a packet to PCB and the >frame is errored somehow, who takes care of the retransmission? I assume >it's PCA because it's a serial connection. Am I right? > >Question 2: There's an ETHERNET connection between B1 and B2. The bridges >are still using Transparent Bridging and PCA sends a packet to PCB. Again, >the frame has an error. Will B1 be the device to handle the retransmission? > >Thanks, guys! > >Todd ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=25970&t=25928 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
