switching question

Michael2

At long last I have decided to take the ICND1. I have been reading Odom's book and watching the CBT Nuggets video. Though I have not actually read or learned this, it seems like NIC's stop sending ARP messages once a switch is connected and simply forward all requests to the switch. Is this true?

NetworkVeteran

Michael2 wrote: »

Though I have not actually read or learned this, it seems like NIC's stop sending ARP messages once a switch is connected and simply forward all requests to the switch. Is this true?

No. Ethernet switching is also known as transparent switching. Hosts generally behave the same regardless of whether or not they are connected to a transparent switch. Specifically, they send ARP messages in precisely the same way and under precisely the same circumstances.

Michael2

Thank you for your reply. What happens to the ARP when it reaches the switch?

Prasad Satav

The switch learns about the source mac address of the sender (say "A")and adds it to its mac address table and forwards the ARP request on all other ports except the one it recieved it from.. the corresponding NIC or device with the IP address same as the one in the ARP's destination IP field (Say "B") will respond to the ARP request with its MAC address and that packet will be carried back to the ARP initiator (A) through the switch..
Also the switch learns about the mac address of the "B" and adds it to the mac address table..

NetworkVeteran

Well-said, Prasad Satav.

Prasad Satav

NetworkVeteran wrote: »

Well-said, Prasad Satav.

Cheers mate

Michael2

Thank you.

Michael2

Let me clarify my question. Assume we have a switch that has already learned the MAC addresses of all connected devices. Somebody decides to replace one of the computers, or hosts, and begins using the new host immediately. Also assume that the NIC in the former host is unusable for some reason. What happens when the new host makes a request to a specific host on the LAN? Would it send out the ARP request, or would it somehow receive notification from the switch that ARPing is not necessary? If the former is true, how would the ARP request be handled and responded to? If the latter is true, how would the switch learn the MAC and IP addresses of the new host and how would that information be forwarded to the other hosts?

mapletune

Hosts, just like switches, keep arp table (mac-address-table).

So when the new switch connects to the existing LAN infrastructure, it doesn't know any mac addresses naturally. It will ARP to try to learn what mac-address belongs to the ip it's trying to reach.

ARPs are broadcasted, thus, the switch will forward it out to all ports. (even though the switch knows where to reach a specific mac, it doesn't matter. Because the host sent out an ARP broadcast, not a unicast frame)

Now, once the new host that you just connected learns the mac-address of the ip it's trying to reach, then it will start sending unicast frames. So, the switch can now forward these frames to just the appropriate port.

hope that helps.

NetworkVeteran

Michael2 wrote: »

Assume we have a switch that has already learned the MAC addresses of all connected devices.

So: PC_A

---

SWITCH

---

PC_B
The switch's CAM table would look like so--
port1 20CF-3000-0001
port2 20CF-3000-0002

Somebody decides to replace one of the computers, or hosts, and begins using the new host immediately.

So: PC_A

---

SWITCH

---

PC_C
The switch's CAM table still would look like so--
port1 20CF-3000-0001
port2 20CF-3000-0002
The second entry is now wrong, but the switch doesn't know that. It'll age out in a few minutes.

What happens when the new host makes a request to a specific host on the LAN?

Irrelevant since you've removed it.

Would it send out the ARP request

Yes.

or would it somehow receive notification from the switch that ARPing is not necessary?

PC_C does not know what MAC address PC_A is using, so ARP is essential.

The Switch is a L2 device. It understands frames. It doesn't understand ARP or IP.

If the former is true, how would the ARP request be handled and responded to?

So: PC_A

---

SWITCH

---

PC_C
The switch's CAM table still would look like so--
port1 20CF-3000-0001
port2 20CF-3000-0002

PC will send an ARP encapsulated in a L2 frame. The switch sees:
SourceAddress:20CF-3000-0003. DestinationAddress:FFFF-FFFF-FFFF. It does two things:
a) Adds to its CAM table that 20CF-3000-0003 is on port2.
b) Forwards the broadcast out all ports except the one it received it on.

PC_A received the ARP request and sends an ARP reply unicast to 20CF-3000-0003. The switch sees:
SourceAddress:20CF-3000-0001 DestinationAddress:20CF-3000-0003

PC_A

---

SWITCH

---

PC_C
If you recall, the switch's CAM table now looks like this--
port1 20CF-3000-0001
port2 20CF-3000-0002, 20CF-3000-0003

Its CAM table says 20CF-3000-0003 is on Port2 so it forwards the frame there.

If the latter is true, how would the switch learn the MAC

Per the above. It learnes based on the source addresses in frames.

and IP addresses

The switch doesn't learn IP addresses. The switch has never heard that term before.

how would that information be forwarded to the other hosts?

The switch doesn't forward or relay information. It forwards L2 frames. It uses the destination MAC address to make forwarding decisions. It uses the source MAC address to learn.

Prasad Satav

Precisely NetworkVeteran..!!
I think he answers all that Michael2
Always remember.. IP does not come into play until a router appears..!! or some high end switch which can route.