Host Ping - A Question..

Unanswered Question
May 28th, 2010

Hello guys,
Can you please answer me a simple question?

Imagine a host A, host B with an IP addresses A and B. Host B is not sending any packets (except replying to ping/ARP). There is one switch as well they are both connected to.Host A pings host B.

Switch has no MAC addresses recorder yet. What will happen - switch will send ARP broadcast for host B, it will reply, switch will record it and forward traffic here. Simple. But now, what will happen if the host B will change MAC address after that initial successful ping?

Will host A get ping timeout response? Or will host A manage to ping the host B? Is a mac-address clearing needed? Etc..

Please share your thoughts!

Thanks

I have this problem too.
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Jon Marshall Fri, 05/28/2010 - 14:03

Todd

If host B changes it's mac-address then the pings will fail because the destination mac-address of the ping will not match host B anymore.

There is such a thing as gratuitous arp where if the mac of a host changes it can notify the switch/router it has a new mac-address. This is generally used for server clustering where the IP might failover to another server and the server updates the mac to IP mapping.

Jon

Todd Bren Fri, 05/28/2010 - 14:06

Jon,

Thank you, that was exactly what I was thinking about. Is that gratuitous MAC update a configuration feature of switch/router or a server?

Jon Marshall Fri, 05/28/2010 - 14:08

nightbass wrote:

Jon,

Thank you, that was exactly what I was thinking about. Is that gratuitous MAC update a configuration feature of switch/router or a server?

Todd

End devices such as servers/clients etc. are generally the ones that send gratuitous arps.


Jon

Todd Bren Fri, 05/28/2010 - 14:20

And to add an explanation to this question.

Few days ago we did upgrade from HSRP to GLBP on our company topology. And we experienced outage there.

Basically two distribution L3 switches running HSRP with redundant connections to two downstream access switches.
When we changed HSRP to GLBP on distribution switches we cut connection to all hosts connected to access switches.

Now I see that this old and incorrect MAC address could be a problem -  hosts were still pointing to a default gateway with a MAC of HSRP while switches had already been set up with GLBP and GLBP MAC address on. Therefore, no distribution switch was serving those requests.

What do you think, is it plausible? If yes - would a simple MAC clearing on access switches help?

Regards, Todd

Jon Marshall Fri, 05/28/2010 - 14:25

nightbass wrote:

And to add an explanation to this question.

Few days ago we did upgrade from HSRP to GLBP on our company topology. And we experienced outage there.

Basically two distribution L3 switches running HSRP with redundant connections to two downstream access switches.
When we changed HSRP to GLBP on distribution switches we cut connection to all hosts connected to access switches.

Now I see that this old and incorrect MAC address could be a problem -  hosts were still pointing to a default gateway with a MAC of HSRP while switches had already been set up with GLBP and GLBP MAC address on. Therefore, no distribution switch was serving those requests.

What do you think, is it plausible? If yes - would a simple MAC clearing on access switches help?

Regards, Todd

Todd

Yes, the virtual mac-addresses used by GLBP will indeed be different from the HSRP ones. You should indeed have cleared the arp tables on the switches but also the hosts if possible because they will have the wrong mac-address to IP mapping in their arp caches.

Jon

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