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New Member

VLANs and routing over same link


We have 2 locations (using 6500's in hybrid mode) in our WAN that are connected our main bulding (using a 3550-12G) via fiber from Comcast. Both connections run over seperate VLANS on Comcast's network to their switches on sites. Our interfaces on their switches are access ports, so their vlans are transparent to us. They have set their MTU to 1504 to allow for 802.1Q tagging on their network. WE can pass 1500 byte packets with no difficulty.

Currently, I have seperate vlans at each site with a VLAN (499) connectinng all 3 sites. I am statically routing between all 3 sites using vlan 499. I have added another VLAN (808) to span all 3 sites for virtual server redundancy for all 3 sites.

The problem I am having is that I cannot send packets any larger than 1496 bytes across the new vlan(808). The vlan 499 does not have any problems with 1500 byte packets.

Anyone have any ideas why this is?



Hall of Fame Super Silver

Re: VLANs and routing over same link

Hello Bob,

if the link is the same from the point of view of L2 devices that becomes a trunk.

To carry your trunk your provider Comcast shoud deploy 802.1Q tunneling QinQ.

Can be vlan 499 the native vlan and this could explain why vlan 499 can use 1500 bytes IP packets and new vlan 808 only up to 1496.

If instead the new vlan has its own dedicated link ask comcast to configure as the previous ones

Hope to help


New Member

Re: VLANs and routing over same link


Thanks for the response. VLAN 499 was set as the native vlan for the connection. earlier we had found that the Comcast link was set to 1500 bytes, but it should have been 1504 because they run it across a vlan on their network and hand it off to us as an access port. Once this was changes, we were able to send other vlans across.