routers and broadcast packets

MrXpert

Hi,
I have learnt that layer 3 devices such as routers cannot forward on broadcasts(by default) but they can send out broadcast arps when wanting to know the mac of the next hop router.
But i want to know is it the same the other way around?
for example i logged into my router via console port and then pinged a dns server at ip 192.168.3.35(which is connected up to a switch which in turn is connected to the router i mentioned). Now the router didn't know the mac address of the dns server before i pinged 192.168.3.35. So did the router send out a broadcast arp asking "who is 0090.0CE2.A2D1?"
The dns server replies back in a uni cast frame and says "thats me here's my MAC". The router caches this (as i saw when i typed arp). The router then is able to use the mac of the dns server when it constructs the packet and uses the souce mac as its own. Am i right so far?
the router then sends it to 192.168.3.35 but does it pass by the switch? and if it does does the switch then look at it and alter the source mac addresses also?

i hope i am making sense.

networker050184

You got it down. The switch doesn't change the source address though.

SdotLow

This is based off the following topology, correct?


HOSTA <

---

> (FA 0/0) ROUTERA (FA 0/1)<

---

> DNS SERVER

Router's shouldn't be using ARP out of an serial interface to my knowledge.

hiddenknight821

I got a link that can reinforce your understanding. You can actually run a simulation of how ARP works by running the Ping. Check this out.

Just make sure you use Wireshark heavily when dealing with understanding how framing works on the LAN side. I find it to be very helpful. You may want to examine the ARP frame.

MrXpert

networker050184 wrote: »

You got it down. The switch doesn't change the source address though.

thanks, but you said the switch doesn't change the source address(i assume you mean the MAC). Why is this?

MrXpert

hiddenknight821 wrote: »

I got a link that can reinforce your understanding. You can actually run a simulation of how ARP works by running the Ping. Check this out.

Just make sure you use Wireshark heavily when dealing with understanding how framing works on the LAN side. I find it to be very helpful. You may want to examine the ARP frame.

thats an awesome simulation and yes it does help clarify.Thanks!

SdotLow

MrXpert wrote: »

thanks, but you said the switch doesn't change the source address(i assume you mean the MAC). Why is this?

Switches don't change/alter mac addresses. They Flood / Filter / Forward them.

pham0329

MrXpert wrote: »

Hi,
I have learnt that layer 3 devices such as routers cannot forward on broadcasts(by default) but they can send out broadcast arps when wanting to know the mac of the next hop router.
But i want to know is it the same the other way around?
for example i logged into my router via console port and then pinged a dns server at ip 192.168.3.35(which is connected up to a switch which in turn is connected to the router i mentioned). Now the router didn't know the mac address of the dns server before i pinged 192.168.3.35. So did the router send out a broadcast arp asking "who is 0090.0CE2.A2D1?"
The dns server replies back in a uni cast frame and says "thats me here's my MAC". The router caches this (as i saw when i typed arp). The router then is able to use the mac of the dns server when it constructs the packet and uses the souce mac as its own. Am i right so far?
the router then sends it to 192.168.3.35 but does it pass by the switch? and if it does does the switch then look at it and alter the source mac addresses also?

i hope i am making sense.

Router asks who is 192.168.3.35, not the MAC of the DNS. ARP resolves IP to MAC.

The switch doesn't alter any information of the frame, it just forwards it on.