I am in need of some assistance with a Cisco 3750e switch passing through Q in Q traffic. Here is how our network is setup:
We have a Adtran TA5000 DSLAM connected to a Cisco 3750 switch which is then uplinked to a 7200 series router gig E port. The Cisco 7200 series router is then uplinked to our core network. The TA5000 DSLAM is configured to "double tag" all traffic upstream with a inner and outer VLAN tag. The 3750 has only the outer tag VLAN configured, and the ports to the TA5000 and the 7200 are setup in 802.1Q trunk mode. The 7200 has a sub-interface setup for every customer(Because of some limitations on the TA5000 every customer has there own VLAN). The sub-interface in the 7200 is setup as follows:
encapsulation dot1Q 401 second-dot1q 1707
ip address X.X.X.X 255.255.X.X
With the network setup like this we can get online using a DSL line off of the TA5000 and everything ALMOST works. The problem we run into is we can only get to certain websites(I.E. we can get to google.com but cannot get to yahoo.com). If we avoid the 3750 and plug in the TA5000 directly to the 7200, we can get to all websites without a issue. Any suggestions?
probably an MTU problem on C3750.
try to increase system MTU
the additional 802.1Q tag count for 4 bytes extra
problems like this happens also when you introduce MPLS in a network you need to review MTU settings of lan switches
Hope to help