Hop Count & MAC Address question

binaryhat

I'm trying to get this straight...

The MAC Address changes at each hop as a packet traverses across network(s). What I need clarification on is:

The MAC Address changes as it enters and exists each interface? For example:

PC1

---

>R1

---

R2

---

>PC2

Where R1 has a Fa0/0 and S0/0 interface and R2 has the same interfaces.

If I'm correct the SRC and DST MAC Address will change 5 times?

Zartanasaurus

Well, in your example the MAC would only change once.

What is the purpose of a MAC address?
On what media types are MAC addresses used?

smcclenaghan

No matter how many switches between, I believe (though I'll defer to one of the pros here to check my work), that

1. PC1 notices PC2 is in a different subnet, so sends a frame with source (MAC of PC1), dest (MAC of R1's Fa0/0) to R1, PC1's default gateway (possibly ARPing for that MAC first).

2. R1 strips the frame headers and checks its routing table to see that the packet should go to R2. R2 makes a new frame with source (MAC of R1's se0/0), dest (MAC of R2's se0/0).
(edit - oops... R1 <-> R2 is serial... no MACs used here)...

3. R2 receives the frame and strips the header, noticing the dest IP is PC2, it checks its ARP table (possibly arping if no entry), and after it has the MAC of PC2, it creates a third and final frame with source (MAC of R2's Fa0/0), dest (MAC of PC2).

So my math comes up with three (edit... two) different frames.

smcclenaghan

Crap. I missed that the connection between R1 and R2 is serial. No MAC addresses there.

Since R1 <-> R2 is serial, Zartanasaurus is right. The frame only changes once.

Zartanasaurus

smcclenaghan wrote: »

Crap. I missed that the connection between R1 and R2 is serial. No MAC addresses there.

Since R1 <-> R2 is serial, Zartanasaurus is right. The frame only changes once.

An excellent answer regardless. You obviously understand the process, just a little more attention to detail. Don't feel bad, I missed the serial part at first too when I was skimming.

smcclenaghan

Thanks! I'm figuring out that attention to detail is more and more critical the further you go with Cisco certs

binaryhat

Zartanasaurus wrote: »

Well, in your example the MAC would only change once.

What is the purpose of a MAC address?
On what media types are MAC addresses used?

MAC Address is a physical identifier hard coded on a NIC. A layer 2 switch is the media that filters MAC Addresses....

So on a serial connection MAC Addresses don't change...why?

Verdict: MAC Address only changes once?

networker050184

binaryhat wrote: »

So on a serial connection MAC Addresses don't change...why?

I think the more important question is are MAC addresses used on serial connections?

binaryhat

networker050184 wrote: »

I think the more important question is are MAC addresses used on serial connections?

No? Because it's a dedicated link?

smcclenaghan

Remember that a MAC address is specific to Ethernet. No other network type will use a MAC address.

NetworkVeteran

smcclenaghan wrote: »

MAC address is specific to Ethernet. No other network type will use a MAC address.

The statement above is false.

MAC address - Wikipedia, the free encyclopedia

NetworkVeteran

binaryhat wrote: »

No? Because it's a dedicated link?

If there's only one other person in the room, when I say "Do the dishes.", do I need to clarify who I'm speaking to?

(Assume there are no poltergeists, hallucinations, schizophrenia, voice-activated machinations, etc.)

binaryhat

I don't follow...can we get closure on why MAC Addresses do not chang on a serial connection

networker050184

Because MAC addresses aren't used on serial links.

binaryhat

networker050184 wrote: »

Because MAC addresses aren't used on serial links.

Ahhh...ok.

smcclenaghan

NetworkVeteran wrote: »

The statement above is false.

My mistake. I made an assumption. I forgot about wireless and token ring. (I'm going to make quite a few mistakes.. I'm hoping folks will consider my post count before relying too heavily on my advice).