arp cache

[Mike Duffy_1]

Good afternoon,

Try this one!

I ping a device and it responds.
I check the arp cache and the ip address just pinged does not appear!

Thanks in advance.

[Bharat Katkar]

Try pinging with IP Address and then see
#arp -a

it shows.

Regards,

[Jeff_Traigle]

Not always. I've seen this before many times on both HP-UX and Windows. In fact, I just did it on a test system:

biltest1# ping 192.168.233.21
PING 192.168.233.21: 64 byte packets
64 bytes from 192.168.233.21: icmp_seq=0. time=6826. ms
64 bytes from 192.168.233.21: icmp_seq=1. time=6666. ms
64 bytes from 192.168.233.21: icmp_seq=2. time=6690. ms
64 bytes from 192.168.233.21: icmp_seq=3. time=6744. ms
64 bytes from 192.168.233.21: icmp_seq=4. time=6784. ms
64 bytes from 192.168.233.21: icmp_seq=5. time=6817. ms
64 bytes from 192.168.233.21: icmp_seq=6. time=7047. ms

----192.168.233.21 PING Statistics----
14 packets transmitted, 7 packets received, 50% packet loss
round-trip (ms) min/avg/max = 6666/6796/7047
biltest1# arp -a
172.16.16.3 (172.16.16.3) at 0:10:7b:66:13:0 ether
192.168.75.38 (192.168.75.38) at 0:d:9d:5d:b3:d5 ether
172.16.16.14 (172.16.16.14) at 0:10:5a:e4:c9:21 ether
172.16.16.15 (172.16.16.15) at 0:30:6e:2e:65:34 ether
172.16.16.5 (172.16.16.5) at 0:b:cd:9c:7d:ca ether
172.16.16.20 (172.16.16.20) at 0:b:cd:9d:20:8e ether
biltest1#

[Mike Duffy_1]

I am pinging with the IP address.

[Jeff_Traigle]

Are you trying to ping through a router? My test above was and it didn't show up. When I ping a system on the same subnet, however, it showed up in the arp cache. I don't recall the behavior I witnessed at my previous employer... we had three subnets on our LAN.

[Ron Kinner · Accepted solution]

You only get an entry in the arp table when the address is local - on the same subnet as one of the NICs. If the address is remote - reached via a gateway then the gateway's mac will show up. Exception would be the case where you do not have a gateway but rely on proxy arp (where you point the default gateway to your own address).

Ron

[Sergejs Svitnevs]

If source device wants to send an IP packet to destination device on different subnet, then source device needs MAC address of router to be used as the forwarding gateway.
So "arp -a" shows the MAC router.

Regards,
Sergejs

[rick jones]

To add to Ron's response - you would only see an ARP entry for a pinged destination if that destination is local, or if it is remote and you have configured one of your local IPs as the gateway and are using proxy ARP.

In the proxy ARP case, the MAC address in the ARP cache will be that of the router rather than the remote destination.

[rick jones]

and then to try to remove the foot from my mouth, rereading ron's post shows it already addressed that... must be time to go home for dinner if i'm chewing on my foot...