ARP Entries

Unanswered Question
Sep 19th, 2007
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I am sitting with a problem where I have a virtual server x.x.82.16 connected to a cisco router. I have 5 physical servers hanging off the virtual server. Basically the virtual server (.16) acts as a load balancer by assigning a connection to the first server it finds available.

What is happening is we need to add the mac and ip into the arp table to get connectivity to the .16 server. That works for roughly 10 minutes then we cannot get to any of the servers (x.x.82.11 to .16) Once we remove the ARP entry and re-add it things work again.

Is there something one can do to resolve this issue by making the arp entry permanent?



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Kevin Dorrell Wed, 09/19/2007 - 00:57
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I have seen something like this. It is a longshot, but I might be right ...

When you have no ARP entry in the router, and you ping the 82.16 address, what do you get in the ARP cache of the router? If my longshot is correct, then you will get nothing at all. I have seen a load balancing schemes that rely on returning a multicast MAC address in the ARP response. But the router does not believe an ARP response with a multicast MAC.

So, try pinging the 82.16 address from some other host in the VLAN, and see what ARP response you get. If it starts with 01: then it is a multicast, and that is the address that you need to put as a static ARP entry in your router.

Kevin Dorrell


willemvwyk Wed, 09/19/2007 - 21:24
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I made sure the arp entry is not in the switch's arp table. I then pinged the server from the switch and checked the arp table. The ip is in, but the mac address shows incomplete.

Kevin Dorrell Wed, 09/19/2007 - 22:37
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Is the server in the same VLAN as the management VLAN of the switch (or one of its layer-3 VLANs if it is a layer-3 switch)? If not, you will not get a response from your ping. Did the server respond?

I think you are going to have ping it from a host, e.g. a PC or another server, on the same VLAN as the server in order to see the ARP response.

Unitil we can see the MAC address that the server gives in its ARP response, we are not going to make a progress down this path.

Kevin Dorrell


cjnwodo Wed, 09/19/2007 - 01:02
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Firstly it sounds to like your virtual server does NOT respond to ARP requests!!! If it did you wouldn't need to add the static ARP entries.

Are the physical servers plugged into a switch? Does the switch have a dynamic MAC entry for x.x.82.16?

arp 0800.0900.1834 arpa -for example is what you need to add a static ARP entry on a router


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