how router signals hostA to send ping to hostB?

[ozcancisco]

i was trying to see each step taking in arp process in simulation mode of packet tracer using  hostA,SW1,R1,SW2, hostB diagram.

In simulation mode , everything was flowing nicely till R1's arp resolves hostB's mac address. Then suddenly hostA starts sending icmp packet.

2 question; i thought hostA already sent the icmp to R1  once resolving R1's mac. What happened the icmp packet R1 received.why R1 didnt send the icmp to hostB after resolving hostB's mac.

2.nd question; after R1'arp resolves hostB mac,  what triggers hostA send icmp packet cause in simulation mode there is no any signalling to hostA from R1 to do that.

looking forward to replies. thanks.  

[cadet alain]

Hi,

there is no signaling at all.

when host A wnts to ping host B it does this:

1° it first does a logical binary AND between its IP address and its mask

2° it performs the same AND between Host B IP and its mask

3° it compares the 2 results and finds out they are not equal so it knows that Host B is on a different subnet

4° it must then send the icmp packet to its default-gateway which is R1

5° it looks in its arp cache for the mac address of the default-gateway and if it ain't in the cache it will send an ARP request for it

6° once it receives a reply it puts it into its cache and it can send the icmp packet in a L2  ethernet frame

7° R1 receives the packet and looks at the destination IP and sees if there is a valid route in the routing table for Host B IP

8°it finds out the outgoing interface and it must reencapsulate the icmp packet with its source mac and destination mac of Host B

9° Host B receives and performs the same steps

I omitted the source learning and initial flooding on the switches.

Regards

Alain

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[ozcancisco]

Hi Cadet,

thanks a lot for replying. i think i couldnt explain well what i mean. In simulation mode  u see how packet ,frame moves from hostA -sw1-R1-sw2-hostB ,  i was following excatly what u listed there . the problem is after R1's  arp got the hostB'mac , suddenly hostA start sending icmp, what prompted hostA start sending ping . somehow hostA got prompted to send icmp . i was excepting to see R1 sends hostB's icmp reply to hostA . but it didnt happen , as soon as R1 got the hostB's mac resolved,  without any communication ,hostA start sending ping.

after R1's got hostB's mac, what is the next step ? 

thanks again for your time.