SNAP type in the "show arp" command

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Apr 20th, 2011
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hello to everyboby,

I have a strange situation on my 6509 (IOS 12.2(18)SXF17a).

When I enter the "show arp" command, the output for only two hosts says "SNAP" in the type field instead the usual "ARPA".

For all other hosts id "ARPA".

What does it mean?

The odd behavior is that this "SNAP" changes randomly. Sometime in the type field there is "ARPA" and other times there is "SNAP".

When "SNAP" is present there are some traffic problems like packets loss.

All this problem is present for those hosts.

Had someone the same problem?

Thank you for any answer.


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Yogesh Ramdoss Mon, 04/25/2011 - 04:17
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Hello Fabio,

Cat6500 uses ARPA as default encap type.

The ARP cache will show SNAP when the end-host responds with SNAP encapsulation.

As you may be aware, CEF in 6500 do not support SNAP and as a result you may experience slowness or intermittent packet loss to these devices.

Please let us know if you had chance to look into these end devices.

I remember, I have seen this issue with IBM Mainframe devices. May be seen with other devices also.

Known workaround is to configure a static ARP entry in Cat6500.



fabio.giosi Tue, 04/26/2011 - 06:25
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Hi Yogesh,

thank you for your answer.

I checked and I found out that the end-hosts are a couple of  IBM mainframe, but I cannot manage them.

A last little question; what kind of L2 ancapsulation I need to add to the end of the ARP static command when I configure the 6500 (ARPA, SNAP,SAP)? (the L3 interface is an interface VLAN).

Thank you very much again.



Yogesh Ramdoss Tue, 04/26/2011 - 08:27
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It will be ARPA.

Sample Command:

Switch(config)#arp ARPA




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