Re: Router/Bridge re-transmit frames? [7:43459]
You didn't read his example well enough. He said, 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. If the e1 interface tries to send the frame and the frame experiences a collision, the e1 interface retransmits. This is assuming the e1 interface is configured for half duplex. This is called CSMA/CD. (If the interface is configured for full duplex and its frame experiences a collision, than there's a duplex mismatch, and you have a more serious problem.) Ethernet 101. That may be on the test too, you know. Priscilla At 01:03 AM 5/7/02, Kris Keen wrote: I'm doing my written tomorrow, I've studied that retransmits are part of the Host's job, especially in a TB network. TB's are stupid, they do no error recover or anything similar. You are correct Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43472t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
Priscilla, In your reply below, you're saying that 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 (I assume you're talking about the interface on the router/bridge) re-transmits the frame. So this tells me that a router/bridge Ethernet interface is able to re-transmit a frame. Correct? Then why do you state in the next paragraph The CCIE tests expect you to know that neither a bridge nor router re-transmits if a frame experiences a bit error or gets lost somehow. Could you please clarify further? Thank you! Shawn K. -Original Message- From: Priscilla Oppenheimer [SMTP:[EMAIL PROTECTED]] Sent: Tuesday, May 07, 2002 12:16 AM To: [EMAIL PROTECTED] Subject: Re: Router/Bridge re-transmit frames? [7:43459] 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=7i=43483t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Router/Bridge re-transmit frames? [7:43459]
Kaminski, Shawn G wrote: In your reply below, you're saying that 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 (I assume you're talking about the interface on the router/bridge) re-transmits the frame. So this tells me that a router/bridge Ethernet interface is able to re-transmit a frame. Correct? Then why do you state in the next paragraph The CCIE tests expect you to know that neither a bridge nor router re-transmits if a frame experiences a bit error or gets lost somehow. Could you please clarify further? Thank you! I'm sure Priscilla will clarify her comments, but I'd like to say something. You (Shawn) correctly distinguish the two issues that Priscilla introduced: * retransmission of frames that collide * retransmission of frames are corrupted CSMA/CD Ethernet interfaces are built to detect collisions. If there is a collision, the multiple senders back off, hopefully for different time periods, and retransmit the frames that collided. On the other hand, a corrupted frame is a frame that does not collide with another frame but for some reason arrrives at the receiver with one or more bit errors. Bit errors are bits that are inserted, deleted, or toggled (0 to 1, 1 to 0). The cinematic taxonomy here is: * The Good: a frame that does not collide with another frame on the transmission medium and arrives at the receiver exactly as it was sent, with no bit errors. * The Bad: a frame that does not collide with another frame but nevertheless arrives at the receiver with one or more bit errors. * The Ugly: a frame that collides with another frame. -- TT Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43491t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
Let me try, The CCIE tests expect you to know that neither a bridge nor router re-transmits if a frame experiences a bit error or gets lost somehow. Could you please clarify further? Thank you! There is distinct difference between bit errors and collisions. Retransmitting in a case of bit error or data loss would be error recovery procedure, which, as such, does not exist on Ethernet layer 2. On Ethernet network, this is, usually, accomplished by upper layer protocols (TCP is one example). Listening to collisions on the other hand is defined by protocol that Ethernet is using (CSMA/CD) and is not considered to be error recovery procedure. Please note that this matter is covered in material needed for CCNA exam. Marko. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43495t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
You know, it really annoys me when someone makes a comment like this (Please note that this matter is covered in material needed for CCNA exam). I know this. I've been through it all and then some. Regardless of whether or not something similar to this is covered in the CCNA material, certain situations exist where things aren't cut and dry, which is why I was trying to get opinions from others on this board. Did you read the original post? Why do people need to come off as being superior to others? Geez. Shawn K. -Original Message- From: Marko Milivojevic [mailto:[EMAIL PROTECTED]] Sent: Tuesday, May 07, 2002 9:27 AM To: [EMAIL PROTECTED] Subject: RE: Router/Bridge re-transmit frames? [7:43459] Let me try, The CCIE tests expect you to know that neither a bridge nor router re-transmits if a frame experiences a bit error or gets lost somehow. Could you please clarify further? Thank you! There is distinct difference between bit errors and collisions. Retransmitting in a case of bit error or data loss would be error recovery procedure, which, as such, does not exist on Ethernet layer 2. On Ethernet network, this is, usually, accomplished by upper layer protocols (TCP is one example). Listening to collisions on the other hand is defined by protocol that Ethernet is using (CSMA/CD) and is not considered to be error recovery procedure. Please note that this matter is covered in material needed for CCNA exam. Marko. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43516t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
Shawn, You know, it really annoys me when someone makes a comment like this (Please note that this matter is covered in material needed for CCNA exam). I know this. I've been through it all and then some. Regardless of whether or not something similar to this is covered in the CCNA material, certain situations exist where things aren't cut and dry, which is why I was trying to get opinions from others on this board. Did you read the original post? Why do people need to come off as being superior to others? Geez. That was NOT my intention. I am very sorry if you felt offended by it. Applogy. Marko. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43517t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
Apology accepted. I'm probably a bit touchy today because my transmission went out in my vehicle. Only 34,000 miles on it. Luckily, it was still under warranty, but it's just another hassle I don't have time for. Good thing Cisco builds their routers and switches better than cars and trucks these days (although I'm sure some of you may disagree), otherwise, this whole message board would be PO'ed all the time. :-) Shawn K. -Original Message- From: Marko Milivojevic [mailto:[EMAIL PROTECTED]] Sent: Tuesday, May 07, 2002 11:36 AM To: 'Kaminski, Shawn G'; [EMAIL PROTECTED] Subject: RE: Router/Bridge re-transmit frames? [7:43459] Shawn, You know, it really annoys me when someone makes a comment like this (Please note that this matter is covered in material needed for CCNA exam). I know this. I've been through it all and then some. Regardless of whether or not something similar to this is covered in the CCNA material, certain situations exist where things aren't cut and dry, which is why I was trying to get opinions from others on this board. Did you read the original post? Why do people need to come off as being superior to others? Geez. That was NOT my intention. I am very sorry if you felt offended by it. Applogy. Marko. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43520t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
At 11:45 AM 5/7/02, Kaminski, Shawn G wrote: Apology accepted. I'm probably a bit touchy today because my transmission went out in my vehicle. Only 34,000 miles on it. Luckily, it was still under And it doesn't do a retransmission? ;-) Sorry, couldn't resist. warranty, but it's just another hassle I don't have time for. Good thing Cisco builds their routers and switches better than cars and trucks these days (although I'm sure some of you may disagree), otherwise, this whole message board would be PO'ed all the time. :-) I resemble that remark. (That's a stupid American idiom, in case anyone doesn't know.) Your question was a good one, although the answer should be clear if you know basic Ethernet. However, I can see that it might not seem cut and dry. Here's another wrinkle! What if it's a Cisco cut-through switch? Say the switch is switching a frame from port e0 over to e1 and it encounters a collision while sending the frame out port e1. Does it retransmit? It's cut-through, remember. Does it even still have the frame to retransmit? Stay tuned for an answer. I will answer my own question if nobody else does. ;-) Priscilla Shawn K. -Original Message- From: Marko Milivojevic [mailto:[EMAIL PROTECTED]] Sent: Tuesday, May 07, 2002 11:36 AM To: 'Kaminski, Shawn G'; [EMAIL PROTECTED] Subject: RE: Router/Bridge re-transmit frames? [7:43459] Shawn, You know, it really annoys me when someone makes a comment like this (Please note that this matter is covered in material needed for CCNA exam). I know this. I've been through it all and then some. Regardless of whether or not something similar to this is covered in the CCNA material, certain situations exist where things aren't cut and dry, which is why I was trying to get opinions from others on this board. Did you read the original post? Why do people need to come off as being superior to others? Geez. That was NOT my intention. I am very sorry if you felt offended by it. Applogy. Marko. Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43532t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
Sure, it's retransmit if there's a collision. Cut-through switching will begin forwarding as soon as the MAC is read, but it must still keep a copy in memory in case of collision. I guess I don't know for certain, but I would assume... Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43537t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Router/Bridge re-transmit frames? [7:43459]
You're right. A bridge is not going to retransmit any frame that failed to reach it's destination. That will be up to the appropriate protocol on the originating host. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43461t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Router/Bridge re-transmit frames? [7:43459]
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=7i=43466t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Router/Bridge re-transmit frames? [7:43459]
I agree with you transparent bridges are just that, transparent. Any retransmittal of corrupt or lost frames would need to be done by the end station AFAIK, (with ethernet) even if a device receives a corrupt frame, at layer 2, it simply discards it it doesn't request retransmittal as that is left to higher protocols to correct. Mike W. Kaminski, Shawn G wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... 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. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43465t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Router/Bridge re-transmit frames? [7:43459]
If an Ethernet device receives a damaged frame, it silently discards it. That is true. But a half-duplex Ethernet sender knows when a collision occurs with a frame that is sending and retransmits. That's the CD part of CSMA/CD. If a frame got damaged for some other reason, say noise or crosstalk or whatever, the Ethernet sender wouldn't know, however. Very few protocols have any sort of method for explicitly telling a sender that a packet got damaged. The sender simply figures out that a frame got lost because it never gets ACKed. This usually happens at an upper layer, such as TCP. There are some exceptions to this implicit behavior. LLC2 and LAPB have an explicit REJ and FRMR, for example. LLC2 is usually end-to-end, but it can be router-to-router in DLSW+, for example. And, then there's BISYNC. It has a NAK and a WAK! Priscilla At 12:11 AM 5/7/02, Michael L. Williams wrote: I agree with you transparent bridges are just that, transparent. Any retransmittal of corrupt or lost frames would need to be done by the end station AFAIK, (with ethernet) even if a device receives a corrupt frame, at layer 2, it simply discards it it doesn't request retransmittal as that is left to higher protocols to correct. Mike W. Kaminski, Shawn G wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... 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=7i=43468t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Router/Bridge re-transmit frames? [7:43459]
I'm doing my written tomorrow, I've studied that retransmits are part of the Host's job, especially in a TB network. TB's are stupid, they do no error recover or anything similar. You are correct Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=43469t=43459 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
