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 > > > > 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. 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