arp on routers

carl_townshend · ‎04-05-2008

Hi all, can anyone tell what devices routers should have as an arp entry, would it be mainly next hop routers, or anything that was pinged from the router ?

Thotsaphon Lueangwattanaphong · ‎04-05-2008

Hi carl,

Router should have every arp entries of devices that have ip connectivity to/from the router within the segment of router interfaces.

f.e. hosts send packets to router(default gateway) when the router replies those packets it needs layer3-to-layer2 mapping then send the packets back to hosts.

Hopes this helps

Thot

shiva_ial · ‎04-05-2008

hi carl,

i beleive you are aware that every pc

communicates within network through mac address associated with network card

this address is fixed and there is no change in it

assuming packet travelling from one pc to another PC interconnected with routers

PCA---router1----router2------router3----PCB

==============================================

packet travelling from PCA has mac address of this PCA along with interface address of router so that it can reach router(gets this through default gateway) now that packet reaches router, since the router uses only layer 3 information , it will figure out how to send to PCB and send to router3 using ip address which operates on layer 3,now the packet reached routers3-destination network

but it has to figure out where the host is located, so for that routers uses ARP entry

with this example routers3 will send a broadcast query to destination ip address to ask for mac address

after reply for particular ip address

(pc)router send to that mac address

-----------------------------------------

so in summary arp used for LAYER3 -to -LAYER2 MAPPING

rate if it helps

rgds

shiva