ARP entry refresh?

[adil.javid]

Hello,

I have a sup720 65k. Today we had a server which had a NIC failure, and so we used another spare nic from the same server to get it back online. The same IP details were configured on the new NIC. This took about 30 minutes.

The problem was that when we connected the new nic to the switch the server was not pingable. When i checked the arp table it still had the old arp entry in its table for the faulty nic. I then cleared the arp entry for this IP and everything started working ok. The arp table correctly refreshed to point to the new nic mac-address.

Is this how arp process should work ? What if I had a dual connected server using only 1 IP for redundancy, would we still have to clear the arp entry before the failover worked ?

Your help will be greatly appreciated.

Thank you.

[MUHAMMAD SHAHEEN]

Hi,

Rick has already explained the reason. The PC needs to talk to Switch VLAn1 SVI to have its ip displayed agin in the Switch's ARP table.

When you clear the ARP in step 6, the entry is gone.

if you ping PCs IP address from the switch or VLAN1's IP from the PC, you will see ARP entry re-appear again. Switch 2950 is a L2 switch and will behave as explained by Rick.

but you will always see Mac-address of the PC in the switch's mac-address table for all that time when PC is active. This is the responsibility of a L2 Switch and keeping ARP table up-todate is the responsibility of L3 Switch/Router.

HRH

Shaheen

[amit.kumar0404]

I don't think that there would be a packet loss and neither the router would discard outgoing packets while it waits for an ARP response.

The router would follow the conventional mechanism of learning the next hop's mac-address. This is by sending an ARP request broadcast and the next hop device or receiver would reply back with its own mac-address as an unicast ARP reply. This is the same way the dynamic arp entry was learnt and the same was populated in the arp cache for the first time. The router would repeat the same process next time as well when the destination mac-address is unknown.

Explaining in deep.

When the routed packet hits the router, it would strip out the layer 2 header and check the destination IP address residing on the IP header. Then the router would check its IP routing table to look whether it knows the packet destination (the destination might be directly connected network to the router or next-hop router towards the destination). Let's suppose that the destination is one hop away and the next hop router's mac-address is unknown to the first router (just assume that the first router doesn't have an arp entry with next hop's router mac-address and IP address). Here the first router would send an ARP broadcast to learn the next hop router's mac-address so that the Layer-2 header is constructed completely once the next hop-router replies back with its own mac-address as ARP reply which is unicast. Note that Layer 2 header should be complete in order to forward the routed packet (encapsulated IP packet into a Layer 2 ethernet frame).