https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864952#M92343 <P>found logs in non-persistent storage</P><P>&nbsp;</P> Thu, 02 Sep 2021 12:26:47 GMT rostislav_gartz 2021-09-02T12:26:47Z https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864936#M92341 <P>Hello, All</P><P>we are having problem reconnecting two esx5.5 hosts from 5.5 vcenter to 6.5.&nbsp; this is the error:</P><P>Cannot contact the specified host (esxihost). The host may not be available on the network, a network configuration problem may exist, or the management services on this host may not be responding.</P><P>tcpdump during connection shows that bidirectional communication is going, but host is failing to connect.</P><P>here is the part of tcpdump output</P><LI-CODE lang="markup"> esxihost.https &gt; vcenter_IP.39982: Flags [P.], cksum 0xe709 (incorrect -&gt; 0xdd79), seq 0:1446, ack 1, win 128, length 1446 10:32:06.302107 IP (tos 0x0, ttl 64, id 34853, offset 0, flags [DF], proto TCP (6), length 1486) esxihost.https &gt; vcenter_IP.44320: Flags [P.], cksum 0xe709 (incorrect -&gt; 0xecad), seq 0:1446, ack 1, win 128, length 1446 10:32:57.322334 IP (tos 0x0, ttl 64, id 35341, offset 0, flags [DF], proto TCP (6), length 1486) esxihost.https &gt; vcenter_IP.39982: Flags [P.], cksum 0xe709 (incorrect -&gt; 0xdd79), seq 0:1446, ack 1, win 128, length 1446 10:33:10.312389 IP (tos 0x0, ttl 64, id 35434, offset 0, flags [DF], proto TCP (6), length 1486) esxihost.https &gt; vcenter_IP.44320: Flags [P.], cksum 0xe709 (incorrect -&gt; 0xecad), seq 0:1446, ack 1, win 128, length 1446</LI-CODE><P>&nbsp;</P><P>could you please suggest smth here?</P><P>thanks in advance</P><P>&nbsp;</P> Thu, 02 Sep 2021 10:36:10 GMT https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864936#M92341 rostislav_gartz 2021-09-02T10:36:10Z https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864948#M92342 <P>logs on both hosts in that cluster are not being updated for some time already. esxi version 5.5 9919047</P><P><span class="lia-inline-image-display-wrapper lia-image-align-inline" image-alt="rostislav_gartz_0-1630582299121.png" style="width: 400px;"><img src="https://communities.vmware.com/t5/image/serverpage/image-id/90889iAB4C2D8030D30FF4/image-size/medium/is-moderation-mode/true?v=v2&amp;px=400" role="button" title="rostislav_gartz_0-1630582299121.png" alt="rostislav_gartz_0-1630582299121.png" /></span></P><P>&nbsp;</P> Thu, 02 Sep 2021 11:34:11 GMT https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864948#M92342 rostislav_gartz 2021-09-02T11:34:11Z https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864952#M92343 <P>found logs in non-persistent storage</P><P>&nbsp;</P> Thu, 02 Sep 2021 12:26:47 GMT https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864952#M92343 rostislav_gartz 2021-09-02T12:26:47Z https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864955#M92344 <P>nevermind. problem solved with lowering MTU <img class="lia-deferred-image lia-image-emoji" src="https://communities.vmware.com/html/@7651DD0E8772B3B5D93ADA9ABA2E067C/emoticons/1f642.png" alt=":slightly_smiling_face:" title=":slightly_smiling_face:" /></P> Thu, 02 Sep 2021 12:43:51 GMT https://communities.vmware.com/t5/vCenter-Server-Discussions/can-t-connect-esxi5-5-to-6-5-vcenter/m-p/2864955#M92344 rostislav_gartz 2021-09-02T12:43:51Z