Re: MAC Address [7:62251]
Sorry for the confusion. My indication to the original post was meant to say that the source mac address will change from hop to hop...and the destination mac address, the source and dest. ip address's should remain the same. As Scott says,the routers may change more than the mac address's when the packet is re-wrote, but I didn't think that level of detail was asked in the question My answer about wan issues was incorrect as Priscilla pointed out...which obviously points out my lack of day to day knowledge on the wan side. Larry Letterman Network Engineer Cisco Systems - Original Message - From: "Priscilla Oppenheimer" To: Sent: Monday, February 03, 2003 12:45 PM Subject: Re: MAC Address [7:62251] > s vermill wrote: > > > > s vermill wrote: > > > > > > Priscilla Oppenheimer wrote: > > > > > > > > s vermill wrote: > > > > > > > > > > Larry Letterman wrote: > > > > > > > > > > > > In most cases you will only re-write the source mac > > > address > > > > > > when traversing > > > > > > across a L3 device. > > > > > > > > > > I don't think that's so. > > > > > > > > Did you misplace your comment? > > > > > > No. I disagree that a source MAC re-write would be all that > > > takes place when crossing a L3 device. Host A, sending to an > > > off-subnet Host B, would use its own MAC as the source and the > > > L3 device interface MAC as the destination. The L3 device > > > strips both at ingress. If, in fact, the destination is on a > > > directly attached shared medium, the source MAC is re-writen > > to > > > that of the egress interface. The destination MAC is whatever > > > the L3 device has in the ARP cache for Host B. Both source > > and > > > destination MACs change when crossing a L3 device. Doesn't it > > > sound like Larry is saying that the source MAC is all that > > > changes and not the destination MAC? Or maybe I just took > > that > > > wrong? > > > > I think that maybe Larry was saying that the only time it would > > be *necessary* to change the source MAC is when traversing a L3 > > device. > > That's how I read it. (He was comparing it to a L2 device.) The word "only" > is an evil word that editors hate. :-) > > P. > > > He isn't necessarily saying that only the source MAC > > would change when crossing one. Sorry Larry. I think that was > > a mis-read on my part. > > > > > > > > I think his first comment is > > > > correct, but then a following one is strangely worded. See > > > below > > > > > > > > > A host will have an ARP cache entry > > > > > for its gateway. That would be the destination MAC. The > > > > > source MAC would be that of the sending host itself. > > Using > > > > its > > > > > own ARP cache, the gateway would re-write both the source > > > and > > > > > destination MAC if the destination was, in fact, directly > > > > > attached to (or reachable via) another Ethernet > > interface. > > > > > If > > > > > not, and the packet needed to cross some serial WAN link, > > > both > > > > > MACs would simply be stripped off. Every L3 device strips > > > off > > > > > source and dest. MAC at ingress. Whether or not a new > > > source > > > > > and dest. MAC is encapsulated around the IP packet depends > > > on > > > > > whether or not the destination is reachable via another > > > > > Ethernet interface. > > > > > > > > Or Token Ring, FDDI, LocalTalk. :-) > > > > > > > > > > > > > > > If you go across a layer 2 network, all > > > > > > the mac address's > > > > > > would typically be part of the same broadcast domain and > > > not > > > > > > need to be changed. > > > > > > > > > > > > If you go across a T1 or Frame it will still be mapped > > to > > > or > > > > > > have an assigned IP Address > > > > > > that constitutes a layer 3 hop and write its mac address > > > in > > > > > > the frame. > > > > > > > > Here's where he went astray. As I mentioned earlier, a > > serial > >
Re: MAC Address [7:62251]
s vermill wrote: > > s vermill wrote: > > > > Priscilla Oppenheimer wrote: > > > > > > s vermill wrote: > > > > > > > > Larry Letterman wrote: > > > > > > > > > > In most cases you will only re-write the source mac > > address > > > > > when traversing > > > > > across a L3 device. > > > > > > > > I don't think that's so. > > > > > > Did you misplace your comment? > > > > No. I disagree that a source MAC re-write would be all that > > takes place when crossing a L3 device. Host A, sending to an > > off-subnet Host B, would use its own MAC as the source and the > > L3 device interface MAC as the destination. The L3 device > > strips both at ingress. If, in fact, the destination is on a > > directly attached shared medium, the source MAC is re-writen > to > > that of the egress interface. The destination MAC is whatever > > the L3 device has in the ARP cache for Host B. Both source > and > > destination MACs change when crossing a L3 device. Doesn't it > > sound like Larry is saying that the source MAC is all that > > changes and not the destination MAC? Or maybe I just took > that > > wrong? > > I think that maybe Larry was saying that the only time it would > be *necessary* to change the source MAC is when traversing a L3 > device. That's how I read it. (He was comparing it to a L2 device.) The word "only" is an evil word that editors hate. :-) P. > He isn't necessarily saying that only the source MAC > would change when crossing one. Sorry Larry. I think that was > a mis-read on my part. > > > > > I think his first comment is > > > correct, but then a following one is strangely worded. See > > below > > > > > > > A host will have an ARP cache entry > > > > for its gateway. That would be the destination MAC. The > > > > source MAC would be that of the sending host itself. > Using > > > its > > > > own ARP cache, the gateway would re-write both the source > > and > > > > destination MAC if the destination was, in fact, directly > > > > attached to (or reachable via) another Ethernet > interface. > > > > If > > > > not, and the packet needed to cross some serial WAN link, > > both > > > > MACs would simply be stripped off. Every L3 device strips > > off > > > > source and dest. MAC at ingress. Whether or not a new > > source > > > > and dest. MAC is encapsulated around the IP packet depends > > on > > > > whether or not the destination is reachable via another > > > > Ethernet interface. > > > > > > Or Token Ring, FDDI, LocalTalk. :-) > > > > > > > > > > > > If you go across a layer 2 network, all > > > > > the mac address's > > > > > would typically be part of the same broadcast domain and > > not > > > > > need to be changed. > > > > > > > > > > If you go across a T1 or Frame it will still be mapped > to > > or > > > > > have an assigned IP Address > > > > > that constitutes a layer 3 hop and write its mac address > > in > > > > > the frame. > > > > > > Here's where he went astray. As I mentioned earlier, a > serial > > > interface doesn't have a MAC address and the data-link-layer > > > protocols used across serial interfaces don't have MAC > > > addresses in them. > > > > > > The sentence isn't parsable, (sorry Larry!) but may indicate > > > some additional misunderstanding. The fact that the next > hop > > > has a Layer 3 address isn't of major significance when > talking > > > about forwarding traffic and the addresses that end up in > the > > > forwarded packet. The IP addresses don't change end-to-end. > > MAC > > > addresses on LANs change, hop by hop. WANs don't have MAC > > > addresses. > > > > > > Yes, routing protocols exchange next hop info using IP > > > addresses. So, if we're considering Ethernet, at some point > > the > > > source router must have found out the MAC address of the > > > destination router using ARP. The router will put its own > MAC > > > address in the source field and the destination (next hop) > > > router's MAC address in the destination field
Re: MAC Address [7:62251]
s vermill wrote: > > Priscilla Oppenheimer wrote: > > > > s vermill wrote: > > > > > > Larry Letterman wrote: > > > > > > > > In most cases you will only re-write the source mac > address > > > > when traversing > > > > across a L3 device. > > > > > > I don't think that's so. > > > > Did you misplace your comment? > > No. I disagree that a source MAC re-write would be all that > takes place when crossing a L3 device. Host A, sending to an > off-subnet Host B, would use its own MAC as the source and the > L3 device interface MAC as the destination. The L3 device > strips both at ingress. If, in fact, the destination is on a > directly attached shared medium, the source MAC is re-writen to > that of the egress interface. The destination MAC is whatever > the L3 device has in the ARP cache for Host B. Both source and > destination MACs change when crossing a L3 device. Doesn't it > sound like Larry is saying that the source MAC is all that > changes and not the destination MAC? Or maybe I just took that > wrong? I think that maybe Larry was saying that the only time it would be *necessary* to change the source MAC is when traversing a L3 device. He isn't necessarily saying that only the source MAC would change when crossing one. Sorry Larry. I think that was a mis-read on my part. > > I think his first comment is > > correct, but then a following one is strangely worded. See > below > > > > > A host will have an ARP cache entry > > > for its gateway. That would be the destination MAC. The > > > source MAC would be that of the sending host itself. Using > > its > > > own ARP cache, the gateway would re-write both the source > and > > > destination MAC if the destination was, in fact, directly > > > attached to (or reachable via) another Ethernet interface. > > > If > > > not, and the packet needed to cross some serial WAN link, > both > > > MACs would simply be stripped off. Every L3 device strips > off > > > source and dest. MAC at ingress. Whether or not a new > source > > > and dest. MAC is encapsulated around the IP packet depends > on > > > whether or not the destination is reachable via another > > > Ethernet interface. > > > > Or Token Ring, FDDI, LocalTalk. :-) > > > > > > > > > If you go across a layer 2 network, all > > > > the mac address's > > > > would typically be part of the same broadcast domain and > not > > > > need to be changed. > > > > > > > > If you go across a T1 or Frame it will still be mapped to > or > > > > have an assigned IP Address > > > > that constitutes a layer 3 hop and write its mac address > in > > > > the frame. > > > > Here's where he went astray. As I mentioned earlier, a serial > > interface doesn't have a MAC address and the data-link-layer > > protocols used across serial interfaces don't have MAC > > addresses in them. > > > > The sentence isn't parsable, (sorry Larry!) but may indicate > > some additional misunderstanding. The fact that the next hop > > has a Layer 3 address isn't of major significance when talking > > about forwarding traffic and the addresses that end up in the > > forwarded packet. The IP addresses don't change end-to-end. > MAC > > addresses on LANs change, hop by hop. WANs don't have MAC > > addresses. > > > > Yes, routing protocols exchange next hop info using IP > > addresses. So, if we're considering Ethernet, at some point > the > > source router must have found out the MAC address of the > > destination router using ARP. The router will put its own MAC > > address in the source field and the destination (next hop) > > router's MAC address in the destination field. > > > > In the case of a T1 point-to-point link, a MAC address isn't > > necessary since it's not a shared medium and there's no need > to > > identify which station should receive the frame. There is only > > one other station! > > > > Now, Frame Relay is shared "in the cloud." The DLCI would help > > the L2 switches in the cloud forward the frame correctly. > > Inverse ARP would help the router map a L3 next hop address to > > a DLCI, if I understand it correctly. > > > > Priscilla > > > > > > > > > > > > > > However if I am wrong here, Priscilla or
Re: MAC Address [7:62251]
Priscilla Oppenheimer wrote: > > s vermill wrote: > > > > Larry Letterman wrote: > > > > > > In most cases you will only re-write the source mac address > > > when traversing > > > across a L3 device. > > > > I don't think that's so. > > Did you misplace your comment? No. I disagree that a source MAC re-write would be all that takes place when crossing a L3 device. Host A, sending to an off-subnet Host B, would use its own MAC as the source and the L3 device interface MAC as the destination. The L3 device strips both at ingress. If, in fact, the destination is on a directly attached shared medium, the source MAC is re-writen to that of the egress interface. The destination MAC is whatever the L3 device has in the ARP cache for Host B. Both source and destination MACs change when crossing a L3 device. Doesn't it sound like Larry is saying that the source MAC is all that changes and not the destination MAC? Or maybe I just took that wrong? I think his first comment is > correct, but then a following one is strangely worded. See below > > > A host will have an ARP cache entry > > for its gateway. That would be the destination MAC. The > > source MAC would be that of the sending host itself. Using > its > > own ARP cache, the gateway would re-write both the source and > > destination MAC if the destination was, in fact, directly > > attached to (or reachable via) another Ethernet interface. > > If > > not, and the packet needed to cross some serial WAN link, both > > MACs would simply be stripped off. Every L3 device strips off > > source and dest. MAC at ingress. Whether or not a new source > > and dest. MAC is encapsulated around the IP packet depends on > > whether or not the destination is reachable via another > > Ethernet interface. > > Or Token Ring, FDDI, LocalTalk. :-) > > > > > > If you go across a layer 2 network, all > > > the mac address's > > > would typically be part of the same broadcast domain and not > > > need to be changed. > > > > > > If you go across a T1 or Frame it will still be mapped to or > > > have an assigned IP Address > > > that constitutes a layer 3 hop and write its mac address in > > > the frame. > > Here's where he went astray. As I mentioned earlier, a serial > interface doesn't have a MAC address and the data-link-layer > protocols used across serial interfaces don't have MAC > addresses in them. > > The sentence isn't parsable, (sorry Larry!) but may indicate > some additional misunderstanding. The fact that the next hop > has a Layer 3 address isn't of major significance when talking > about forwarding traffic and the addresses that end up in the > forwarded packet. The IP addresses don't change end-to-end. MAC > addresses on LANs change, hop by hop. WANs don't have MAC > addresses. > > Yes, routing protocols exchange next hop info using IP > addresses. So, if we're considering Ethernet, at some point the > source router must have found out the MAC address of the > destination router using ARP. The router will put its own MAC > address in the source field and the destination (next hop) > router's MAC address in the destination field. > > In the case of a T1 point-to-point link, a MAC address isn't > necessary since it's not a shared medium and there's no need to > identify which station should receive the frame. There is only > one other station! > > Now, Frame Relay is shared "in the cloud." The DLCI would help > the L2 switches in the cloud forward the frame correctly. > Inverse ARP would help the router map a L3 next hop address to > a DLCI, if I understand it correctly. > > Priscilla > > > > > > > > > However if I am wrong here, Priscilla or Howard or Chuck > > > will let me know...:) > > > > > > Larry Letterman > > > Network Engineer > > > Cisco Systems > > > > > > > > > - Original Message - > > > From: "Cisco Newbie" > > > To: > > > Sent: Friday, January 31, 2003 11:42 AM > > > Subject: RE: MAC Address [7:62251] > > > > > > > > > > First, thanks for all that responded. One clarification > > > that I need address > > > > is the following: > > > > > > > > If I cross a L3 router and the outgoing interface is > > > something other than > > > > Ethernet, will the L2 frame show a new MAC address? In > > > other words, if my > > &g
Re: MAC Address [7:62251]
s vermill wrote: > > Larry Letterman wrote: > > > > In most cases you will only re-write the source mac address > > when traversing > > across a L3 device. > > I don't think that's so. Did you misplace your comment? I think his first comment is correct, but then a following one is strangely worded. See below. > A host will have an ARP cache entry > for its gateway. That would be the destination MAC. The > source MAC would be that of the sending host itself. Using its > own ARP cache, the gateway would re-write both the source and > destination MAC if the destination was, in fact, directly > attached to (or reachable via) another Ethernet interface. > If > not, and the packet needed to cross some serial WAN link, both > MACs would simply be stripped off. Every L3 device strips off > source and dest. MAC at ingress. Whether or not a new source > and dest. MAC is encapsulated around the IP packet depends on > whether or not the destination is reachable via another > Ethernet interface. Or Token Ring, FDDI, LocalTalk. :-) > > > If you go across a layer 2 network, all > > the mac address's > > would typically be part of the same broadcast domain and not > > need to be changed. > > > > If you go across a T1 or Frame it will still be mapped to or > > have an assigned IP Address > > that constitutes a layer 3 hop and write its mac address in > > the frame. Here's where he went astray. As I mentioned earlier, a serial interface doesn't have a MAC address and the data-link-layer protocols used across serial interfaces don't have MAC addresses in them. The sentence isn't parsable, (sorry Larry!) but may indicate some additional misunderstanding. The fact that the next hop has a Layer 3 address isn't of major significance when talking about forwarding traffic and the addresses that end up in the forwarded packet. The IP addresses don't change end-to-end. MAC addresses on LANs change, hop by hop. WANs don't have MAC addresses. Yes, routing protocols exchange next hop info using IP addresses. So, if we're considering Ethernet, at some point the source router must have found out the MAC address of the destination router using ARP. The router will put its own MAC address in the source field and the destination (next hop) router's MAC address in the destination field. In the case of a T1 point-to-point link, a MAC address isn't necessary since it's not a shared medium and there's no need to identify which station should receive the frame. There is only one other station! Now, Frame Relay is shared "in the cloud." The DLCI would help the L2 switches in the cloud forward the frame correctly. Inverse ARP would help the router map a L3 next hop address to a DLCI, if I understand it correctly. Priscilla > > > > However if I am wrong here, Priscilla or Howard or Chuck > > will let me know...:) > > > > Larry Letterman > > Network Engineer > > Cisco Systems > > > > > > - Original Message - > > From: "Cisco Newbie" > > To: > > Sent: Friday, January 31, 2003 11:42 AM > > Subject: RE: MAC Address [7:62251] > > > > > > > First, thanks for all that responded. One clarification > > that I need address > > > is the following: > > > > > > If I cross a L3 router and the outgoing interface is > > something other than > > > Ethernet, will the L2 frame show a new MAC address? In > > other words, if my > > > outgoing interface is say T1 PPP or even a dial-up, should > > I be seeing a new > > > MAC address? > > > > > > Is it only when I cross a L3 device AND my outgoing > > interface is a share > > > medium like Ethernet that a new MAC address will be placed > > on the frame? > > > > > > Thanks. > > > > > > > > > > > > - > > > Do you Yahoo!? > > > Yahoo! Mail Plus - Powerful. Affordable. Sign up now > > [EMAIL PROTECTED] > > > > > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62368&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: MAC Address [7:62251]
Larry Letterman wrote: > > In most cases you will only re-write the source mac address > when traversing > across a L3 device. I don't think that's so. A host will have an ARP cache entry for its gateway. That would be the destination MAC. The source MAC would be that of the sending host itself. Using its own ARP cache, the gateway would re-write both the source and destination MAC if the destination was, in fact, directly attached to (or reachable via) another Ethernet interface. If not, and the packet needed to cross some serial WAN link, both MACs would simply be stripped off. Every L3 device strips off source and dest. MAC at ingress. Whether or not a new source and dest. MAC is encapsulated around the IP packet depends on whether or not the destination is reachable via another Ethernet interface. > If you go across a layer 2 network, all > the mac address's > would typically be part of the same broadcast domain and not > need to be changed. > > If you go across a T1 or Frame it will still be mapped to or > have an assigned IP Address > that constitutes a layer 3 hop and write its mac address in > the frame. > > However if I am wrong here, Priscilla or Howard or Chuck > will let me know...:) > > Larry Letterman > Network Engineer > Cisco Systems > > > - Original Message - > From: "Cisco Newbie" > To: > Sent: Friday, January 31, 2003 11:42 AM > Subject: RE: MAC Address [7:62251] > > > > First, thanks for all that responded. One clarification > that I need address > > is the following: > > > > If I cross a L3 router and the outgoing interface is > something other than > > Ethernet, will the L2 frame show a new MAC address? In > other words, if my > > outgoing interface is say T1 PPP or even a dial-up, should > I be seeing a new > > MAC address? > > > > Is it only when I cross a L3 device AND my outgoing > interface is a share > > medium like Ethernet that a new MAC address will be placed > on the frame? > > > > Thanks. > > > > > > > > - > > Do you Yahoo!? > > Yahoo! Mail Plus - Powerful. Affordable. Sign up now > [EMAIL PROTECTED] > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62354&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: MAC Address [7:62251]
Larry Letterman wrote: > > In most cases you will only re-write the source mac address > when traversing > across a L3 device. If you go across a layer 2 network, all > the mac address's > would typically be part of the same broadcast domain and not > need to be changed. > > If you go across a T1 or Frame it will still be mapped to or > have an assigned IP Address > that constitutes a layer 3 hop and write its mac address in > the frame. A serial interface doesn't have a MAC address and the protocols used across a serial link don't have MAC addresses in their headers. If I misunderstood your point, just let me know. I'm sure you will! :-) Prisiclla > > However if I am wrong here, Priscilla or Howard or Chuck > will let me know...:) > > Larry Letterman > Network Engineer > Cisco Systems > > > - Original Message - > From: "Cisco Newbie" > To: > Sent: Friday, January 31, 2003 11:42 AM > Subject: RE: MAC Address [7:62251] > > > > First, thanks for all that responded. One clarification > that I need address > > is the following: > > > > If I cross a L3 router and the outgoing interface is > something other than > > Ethernet, will the L2 frame show a new MAC address? In > other words, if my > > outgoing interface is say T1 PPP or even a dial-up, should > I be seeing a new > > MAC address? > > > > Is it only when I cross a L3 device AND my outgoing > interface is a share > > medium like Ethernet that a new MAC address will be placed > on the frame? > > > > Thanks. > > > > > > > > - > > Do you Yahoo!? > > Yahoo! Mail Plus - Powerful. Affordable. Sign up now > [EMAIL PROTECTED] > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62318&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: MAC Address [7:62251]
In most cases you will only re-write the source mac address when traversing across a L3 device. If you go across a layer 2 network, all the mac address's would typically be part of the same broadcast domain and not need to be changed. If you go across a T1 or Frame it will still be mapped to or have an assigned IP Address that constitutes a layer 3 hop and write its mac address in the frame. However if I am wrong here, Priscilla or Howard or Chuck will let me know...:) Larry Letterman Network Engineer Cisco Systems - Original Message - From: "Cisco Newbie" To: Sent: Friday, January 31, 2003 11:42 AM Subject: RE: MAC Address [7:62251] > First, thanks for all that responded. One clarification that I need address > is the following: > > If I cross a L3 router and the outgoing interface is something other than > Ethernet, will the L2 frame show a new MAC address? In other words, if my > outgoing interface is say T1 PPP or even a dial-up, should I be seeing a new > MAC address? > > Is it only when I cross a L3 device AND my outgoing interface is a share > medium like Ethernet that a new MAC address will be placed on the frame? > > Thanks. > > > > - > Do you Yahoo!? > Yahoo! Mail Plus - Powerful. Affordable. Sign up now [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62306&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: MAC Address [7:62251]
Cisco Newbie wrote: > > First, thanks for all that responded. One clarification that I > need address is the following: > > If I cross a L3 router and the outgoing interface is something > other than Ethernet, will the L2 frame show a new MAC address? > In other words, if my outgoing interface is say T1 PPP or even > a dial-up, should I be seeing a new MAC address? Well, the old MAC address is definitely gone. It was stripped off on ingress. T1 PPP and dial-up don't use MAC addresses, so there won't be a new one as the packet traversse that link. But the packet will end up on a router or access server at the other end probably, which will output the packet to Ethernet or some other LAN probably and put in new MAC addresses. Priscilla > > Is it only when I cross a L3 device AND my outgoing interface > is a share medium like Ethernet that a new MAC address will be > placed on the frame? > > Thanks. > > > > - > Do you Yahoo!? > Yahoo! Mail Plus - Powerful. Affordable. Sign up now > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62286&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: MAC Address [7:62251]
Cisco Newbie wrote: > > First, thanks for all that responded. One clarification that I > need address is the following: > > If I cross a L3 router and the outgoing interface is something > other than Ethernet, will the L2 frame show a new MAC address? > In other words, if my outgoing interface is say T1 PPP or even > a dial-up, should I be seeing a new MAC address? > > Is it only when I cross a L3 device AND my outgoing interface > is a share medium like Ethernet that a new MAC address will be > placed on the frame? > > Thanks. > > > > - > Do you Yahoo!? > Yahoo! Mail Plus - Powerful. Affordable. Sign up now > > Yep. If the IP packet is destined for a non-Ethernet WAN interface, the appropriate L2 header is encapsulated around it. In the case of T1 PPP, an "HDLC-like" header is used. There no longer is any source or destination MAC address to be found. They are both stripped off at the ingress Ethernet interface of the router. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62280&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: MAC Address [7:62251]
First, thanks for all that responded. One clarification that I need address is the following: If I cross a L3 router and the outgoing interface is something other than Ethernet, will the L2 frame show a new MAC address? In other words, if my outgoing interface is say T1 PPP or even a dial-up, should I be seeing a new MAC address? Is it only when I cross a L3 device AND my outgoing interface is a share medium like Ethernet that a new MAC address will be placed on the frame? Thanks. - Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62275&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: MAC Address [7:62251]
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]
RE: MAC Address [7:62251]
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. 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=62264&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
MAC Address [7:62251]
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 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62251&t=62251 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
