number of times a frame changes destination

[stevec90]

I've had a couple of conflicting answers on practice test (don't you love that!) and the question is "How many times does a packet get framed with a new destination MAC address as it travels from host A to host B?"

I'm not sure what Cisco is looking for here.

Please correct me if I'm wrong; switches are not counted and only routers would be counted. I spoke with a network person and they stated that switches are typically invisible to the layer 2 process, also, the practice tests have me blue in the face with the fact that: the destination MAC address of a frame when it leaves the host is always the MAC address of the router interface (when the packet that is being framed is bound for another network).

I thought I was correct (above), but I've seen answers where they count all switches as well as the routers, anyone know what the rule of thumb is?

[Fuzz]

You wouldn't include the switches for this question. The destination MAC address doesn't change for devices on the same LAN segment. Remember, for a switch to send a frame to the correct location, it must use the destination address in the received frame and lookup its CAM table for the entry matching the outgoing port to the MAC address. If the frame contained the MAC address of the switch, how would it know where to send it?

[lildeezul]

When a host wants to talk to another host on a different subnetwork, The destination mac address will be the router interface port mac address (default gateway).. This is called proxy arp. However the ip address will be the destination host ip address.


host A---------Router 1------------Router 2-------- host b



host A arps for host b mac address, and the first router replies to the arp broadcast its mac address (proxy arp) (which is saying to get to host b , send your data to me)

Router 1 looks at its routing table to determine the best path to host b ( through router 2), its strips the orignal frame, and makes the destination frame to router 2 , and the same process occurs for router 2 to arp for host b mac.

that was a high level overview.. Review todd lammle books for the full process, its in the ip routing secion. This is essential to know before you move any further.

hope this helps.

-Marcus

[stevec90]

Thanks fuzz, that makes sense now. True, it could never be any other way.

Lildeezul; thanks for the detailed description, but I think it is slightly different than you describe, as far as ARP. If host A were to ping "HostB" (remote) then an arp broadcast would be made by HostA to see if HostB is on the local subnet. But if Host A pings the IP address of Host B, then the network mask would determine that the host is not local, so the initial ARP request would (in this second case only) broadcast for the router interface. Is this how you understand it?

As far as "moving on" I am taking the test next week! I don't think the CCNA test would be this nit-picky on ARP, but I should know this anyway.

[lildeezul]

here hopefully i can clear this up.

The icmp packet, or any application packet is created. the layer 3 information is developed. The host needs to get the layer 2 data finished.. And it looks at the ip address and subnet mask and it calculates its own network address. It notices that the remote host is on a different network, therefore it must send arp for the default gateway ( if the arp table isnt populated) the router responds with its mac, and the process i mentioned below is performed.


-Marcus

[stevec90]

thanks for the confirmation