{"id":2176,"date":"2015-03-13T19:44:19","date_gmt":"2015-03-13T19:44:19","guid":{"rendered":"http:\/\/mccltd.net\/blog\/?p=2176"},"modified":"2015-03-13T19:47:48","modified_gmt":"2015-03-13T19:47:48","slug":"as400-ethernet-type-may-cause-problems","status":"publish","type":"post","link":"http:\/\/darenmatthews.com\/blog\/?p=2176","title":{"rendered":"AS\/400 Ethernet Type May Cause Problems"},"content":{"rendered":"<p>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.<\/p>\n<p>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 \u201cSNAP\u201d encapsulation, rather than \u201cARPA\u201d, the latter of which is the default encapsulation type for the Cisco 6500\u2019s which we were using.<!--more--><\/p>\n<blockquote><p>CORE-6500#sh ip arp | i 10.10.10.1<br \/>\nInternet 10.10.10.1 11 0009.6b6b.b3e2 SNAP Vlan1<\/p><\/blockquote>\n<p>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:<\/p>\n<blockquote><p>CORE-6500#sh ip arp | i 10.10.10.1<br \/>\nInternet 10.10.10.1 4 0009.6b6b.b3e2 ARPA Vlan1<\/p><\/blockquote>\n<p>But when the Arp cache ages out it soon reverted back:<\/p>\n<blockquote><p>CORE-6500#sh ip arp | i 10.10.10.1<br \/>\nInternet 10.10.10.1 11 0009.6b6b.b3e2 SNAP Vlan1<\/p><\/blockquote>\n<p>Apparently the AS\/400 sends out both SNAP and ARPA encapsulation types (SNAP for backwards-compatibility with older systems) in Arp requests\/replies.<\/p>\n<p>There are two ways to fix this:<\/p>\n<p>1. Change the AS\/400 to ensure that it only uses ARPA<br \/>\n2. 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)<\/p>\n<p>To add a static ARP entry of type ARPA:<\/p>\n<blockquote><p>CORE-6500(config)#arp 10.10.10.1 0009.6b6b.b3e2 arpa<\/p><\/blockquote>\n<p>The ARP types are:<\/p>\n<p>ARPA = Standard Ethernet-style ARP (RFC 826).<br \/>\nSNAP = ARP packets conforming to RFC 1042.<\/p>\n<p>To change the frame type on an AS\/400 you can use the ETHSTD(8022) &amp; ETHSTD(8023) commands. See page 12\/13 (pages 18\/19 in the .pdf):<\/p>\n<p><a href=\"https:\/\/www-912.ibm.com\/s_dir\/slkbase.NSF\/1ac66549a21402188625680b0002037e\/990a2cc52f1fb76186256665004727bf\/$FILE\/rzajy000.pdf\">https:\/\/www-912.ibm.com\/s_dir\/slkbase.NSF\/1ac66549a21402188625680b0002037e\/990a2cc52f1fb76186256665004727bf\/$FILE\/rzajy000.pdf<\/a><\/p>\n<p><a href=\"http:\/\/darenmatthews.com\/blog\/wp-content\/uploads\/2015\/03\/ethernet.png\"><img loading=\"lazy\" class=\"alignright size-full wp-image-2179\" src=\"http:\/\/darenmatthews.com\/blog\/wp-content\/uploads\/2015\/03\/ethernet.png\" alt=\"ethernet\" width=\"1134\" height=\"570\" srcset=\"http:\/\/darenmatthews.com\/blog\/wp-content\/uploads\/2015\/03\/ethernet.png 1134w, http:\/\/darenmatthews.com\/blog\/wp-content\/uploads\/2015\/03\/ethernet-300x150.png 300w, http:\/\/darenmatthews.com\/blog\/wp-content\/uploads\/2015\/03\/ethernet-1024x514.png 1024w\" sizes=\"(max-width: 1134px) 100vw, 1134px\" \/><\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>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 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[53],"tags":[80],"_links":{"self":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2176"}],"collection":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2176"}],"version-history":[{"count":4,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2176\/revisions"}],"predecessor-version":[{"id":2181,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2176\/revisions\/2181"}],"wp:attachment":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2176"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2176"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2176"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}