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When does the router update its ARP cache?

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Vive l'amour

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Apr 14, 2000, 3:00:00 AM4/14/00
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Hi, I have a question regarding router's ARP cache. Would you please tell
me what may happen in this configuration?

Here is the situation:

Suppose we have two hosts (A and B), each connects to a switch port with a
100Mb Fast Ethernet adapter. So A and B are in different collision domains.
and the switch connects to a router port.

Suppose we config A and B to use the SAME IP address (say, a.b.c.d) and we
only let A to respond to ARP queries for this IP. (ie. type "ifconfig eth0
-arp" on host B to close its ARP reply)

Now, if the router sends ARP request for ip a.b.c.d and get a response from
host A with A's MAC address. What will happen if it receive a packet send
from B's NIC? Will router update its arp cache accordingly? Or it only
updates it arp cache only when receiving an ARP reply?

Is this implementation dependent or has already been defined in any
standard?

Would you please send replies to my emailbox directly?
Thanks in advance.


Vive l'amour

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Apr 14, 2000, 3:00:00 AM4/14/00
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: Suppose we config A and B to use the SAME IP address (say, a.b.c.d) and we

: host A with A's MAC address. What will happen if it receive a packet send


: from B's NIC? Will router update its arp cache accordingly? Or it only

I mean what will happen if the router receives a packet with its source IP
a.b.c.d but the source MAC address is not as recorded in router's ARP cache?
would the router update its ARP cache accordingly?

Neiby

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Apr 15, 2000, 3:00:00 AM4/15/00
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I ran into this recently and can attest that the router will NOT update its ARP
cache simply because a packet arrives on an interface. If an IP packet arrives
with with an known IP address but a different MAC address, the router won't
even notice.

However, you will not be able to route to B in this situation, but it sounds
like that's your goal.


>Here is the situation:
>
>Suppose we have two hosts (A and B), each connects to a switch port with a
>100Mb Fast Ethernet adapter. So A and B are in different collision domains.
>and the switch connects to a router port.
>

>Suppose we config A and B to use the SAME IP address (say, a.b.c.d) and we

>only let A to respond to ARP queries for this IP. (ie. type "ifconfig eth0
>-arp" on host B to close its ARP reply)
>
>Now, if the router sends ARP request for ip a.b.c.d and get a response from

>host A with A's MAC address. What will happen if it receive a packet send
>from B's NIC? Will router update its arp cache accordingly? Or it only

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