durga ch
durga ch
Communications
04 Mar 2012

networking titbits: ARP table and MAC address table

ok, I have been recently testing out all the questions I ask.
consider a simple set up Pc1 <-->SW1<-->R<-->SW2<-->PC2
now, given that Pc1 and Pc2 are in same vlan(default vlan 1). and Pc1 and Pc2 have default gateway as R, when Pc1 and PC2 are booted up for first time, what will be the contents of ARP and MAC tables of sw1 ans sw2?
ou really dont need to test anything, but a theorical answer would be anough.

Also, what will happen if Pc1 and Pc2 are in different IP subnets? and,, what will happen if an IP address is assigned to Sw1 ans Sw2 and pings are performed from Sw1 and Sw2.
04 Mar 2012
If PC1 and PC2 are in the same VLAN, they can find them by their names. i.e., \\PC1\ resolves in \\PC2\, but in case, if they are in different VLANs, IP address subnets would be different and they can be pinged by their IP Addresses and not by their names! 😔
durga ch

durga ch

Communications
04 Mar 2012
ok, but how are MAC and ARP tables affected on sw1??
04 Mar 2012
durga
ok, but how are MAC and ARP tables affected on sw1??
ARP Tables doesn't have the entries itself!!!
durga ch

durga ch

Communications
05 Mar 2012
Praveen,
If possible could you tell in details? many who might be reading this thread, wont guess what we are talking about??
05 Mar 2012
durga
Praveen,
If possible could you tell in details? many who might be reading this thread, wont guess what we are talking about??
Okay... Check this out...
durga ch

durga ch

Communications
17 Mar 2012
Hi praveen,

thanks for the vedio, but that is not what i was really expecting. I was expecting that we discuss how arp tables and mac tables behave in the above 2 scenarios.
As per my expectation below will happen:

scenario 1:when Pc1 and PC2 are booted up for first time, what will be the contents of ARP and MAC tables of sw1 ans sw2?
When PC1 and PC2 boot up for the first time, there arp caches are empty and so are mac address tables and arp tables of switch 1 and switch2.Since switch generally creates a single broadcast domain, when PC1 needs to ping PC2 it sends a ARP broadcast.Now ARP is an IP packet enclosed in L2 Broadcast with details like (Source MAC { source IP ---- Destination IP} Boradcast MAC)where a computer generally asks for MAC address related to a specific IP address. When the switch recives the packet on a wire (L1) , it strips the L1 header and reads the L2 header . Since this is the first time the switch is seeing the packet, it will do a source based learning and learn the detailed location of the PC1 and since destination is a L2 broadcast it sends the packets on all ports. Even a packet destined to unknown destination is broadcasted by switch. Please see that, the switch is not reading the ARP packet which is enclosed in the data feild of the L2 frame.
So at this point, the MAC table contains the details of PC1 MAC and ARP table is still empty as ARP apcekt was not opned. It is the computer PC1 which needs to learn the address not the switch, but once PC2 replies and you check arp cache of PC1 arp table is updated.

Now can anyone come up with explaintion for scenario 2 mentioned in post#1??

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