Thank you everyone. OK, the mystery deepens, I guess. The machine does need to support several VLAN's, it is currently on a trunkport (8021q encapsulated), it made it into the ARP table - which I specifically tested for by physically unplugging the table, clearing the ARP table and plugging it back in. The ARP table currently looks like this: hq#show arp Protocol Address Age (min) Hardware Addr Type Interface Internet 192.168.48.100 0 0025.6440.0301 ARPA Vlan48 Internet 192.168.48.101 - 001b.906a.bcc4 ARPA Vlan48 Internet 192.168.48.1 0 0025.6440.063f ARPA Vlan48 Internet 192.168.2.52 0 0025.6440.0547 ARPA Vlan2 Internet 192.168.3.1 - 001b.906a.bcc2 ARPA Vlan3 Internet 192.168.2.1 - 001b.906a.bcc1 ARPA Vlan2 Internet 192.168.7.1 - 001b.906a.bcc3 ARPA Vlan7 hq# The network config on the machine currently looks like this: it has nothing assigned to eth0, eth0.48 = 192.168.48.100/24, eth0.49 = 192.168.49.100/24, eth0.50 = 192.168.50.100/24. And - even though the ARP table seems to be OK - there is no connectivity! Boris. On Sun, Jan 25, 2015 at 11:42 AM, Les Mikesell <lesmikesell at gmail.com> wrote: > On Sun, Jan 25, 2015 at 8:38 AM, Andrew Holway <andrew.holway at gmail.com> > wrote: > > On 25 January 2015 at 15:12, Boris Epstein <borepstein at gmail.com> wrote: > > > >> OK... but why does it need to be a trunk port? > >> > > > > Because a trunk port will "trunk" the vlan. > > > > A VLAN is basically a 4 byte "tag" that gets injected into the packet > > header when the packet enters the VLAN network. When we trunk a VLAN we > say > > to the switch "pass packets on VLAN x but do not strip the tag out". > > > > You can either terminate the VLAN at the switch port (untagged) which > will > > strip out the VLAN tag or you can pass the packet containing the VLAN tag > > to the computer or other device(tagged/trunk). This device can then pull > > out the tag. On linux this mechanism is done by an 8021q VLAN interface. > > > > Hope this is useful. > > > > Just to add to that - normally if a host only needs to be on one > subnet you would use an access port on the switch to select a single > vlan and deliver those packets untagged so the host does not need to > care about tags or vlan numbers. And to that end, switches default > to treating everything as access ports on native/untagged vlan 0 > unless configured otherwise. However, if the host needs interfaces > on multiple subnets, you can do it on a single network connection by > giving it a trunk connection from the switch and letting it split out > the vlan interfaces internally. > > -- > Les Mikesell > lesmikesell at gmail.com > _______________________________________________ > CentOS mailing list > CentOS at centos.org > http://lists.centos.org/mailman/listinfo/centos >