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 > > > > 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=62372&t=62251 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
