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AS/400 Ethernet Type May Cause Problems

March 13th, 2015 Leave a comment Go to comments

After a core network switch reload, a strange problem occured for 5250 terminal sessions to an AS/400. Sometimes they worked and sometimes not. The same behaviour was observed using ping from within the same IP subnet.

The problem appeared to be the Ethernet framing on the AS/400. The ARP replies kept changing between type SNAP and ARPA. An ARP entry on the core routers showed the mac address of the AS/400 but using Ethernet “SNAP” encapsulation, rather than “ARPA”, the latter of which is the default encapsulation type for the Cisco 6500’s which we were using.

CORE-6500#sh ip arp | i
Internet 11 0009.6b6b.b3e2 SNAP Vlan1

Clearing the ARP cache and then pinging the AS/400 from the core (causing an arp request using ARPA encapsulation to be replied to by the AS/400 using the same ARPA encapsulation) populated the mac address with an arp entry of the correct type:

CORE-6500#sh ip arp | i
Internet 4 0009.6b6b.b3e2 ARPA Vlan1

But when the Arp cache ages out it soon reverted back:

CORE-6500#sh ip arp | i
Internet 11 0009.6b6b.b3e2 SNAP Vlan1

Apparently the AS/400 sends out both SNAP and ARPA encapsulation types (SNAP for backwards-compatibility with older systems) in Arp requests/replies.

There are two ways to fix this:

1. Change the AS/400 to ensure that it only uses ARPA
2. Configure a static ARP entry on the switch of type ARPA (not good if you ever move IP address or change the NIC or AS/400 hardware)

To add a static ARP entry of type ARPA:

CORE-6500(config)#arp 0009.6b6b.b3e2 arpa

The ARP types are:

ARPA = Standard Ethernet-style ARP (RFC 826).
SNAP = ARP packets conforming to RFC 1042.

To change the frame type on an AS/400 you can use the ETHSTD(8022) & ETHSTD(8023) commands. See page 12/13 (pages 18/19 in the .pdf):



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