Re: After installing a CISCO 6509 AS 400 Arp entries dropping

eblackb · ‎05-08-2002

After installing a CISCO 6509 the AS400 entries in the ARP cache tables on the routers began dropping out or showing up incorrectly. The problem was resolved by adding permanent entries for the AS400's to the ARP tables.

CISCO says this was caused "by something the AS400 does to insure backward compatablity with older frame types" but has not provided further technical detail.

The problem only began after the 6509 came up and only affected the AS400's.

Has anyone encoutered this before? What caused this to occur?

Thanks in advance for any assistance.

Ed Blackburn CISCO 10158519

rfroom · ‎05-08-2002

The issue you see with AS400 and Cat6k is most likely the following : in some configurations AS400 sends both ARPA and SNAP encapsulated ARP packets to routers. Router always send ARPs in default encapsulation (ARPA). Its ARP entry for the AS400 first shows as being ARPA, because the router itself sends out ARP requests with ARPA encapsulation and receives ARPA ARP reply back from the AS400. But immediately after that router changes the entry to SNAP when it receives ARP reply with SNAP encapsulation from the AS400. The Cat6k MSFC runs CEF by default which does not support SNAP encapsulation. There will be a problem cef switching packets to the AS400. There are, however, a couple of things you can do to prevent this :

* best would be to make sure that AS400 sends only ARPA-encapsulated replies. Not being an AS400 expert I don't know how to do this

* another thing you can do is put a static arp entry in the router with ARPA encapsulation for the AS400. This entry will not be overridden by the SNAP ARP packets coming from the AS400.

eblackb · ‎05-08-2002

Thank you sir. Makes great sense.

Ed

glen.grant · ‎05-15-2002

this is funny , just ran into this 2 weeks ago , we ended up adding a static arp entry after initially turning off CEf for the interface which also cured the problem but we wanted to leave CEF on so we just added the static entry which overides the SNAP encapsul;ation the AS400 sends back .