Here is the set up:
branch office c3750 (switchC), two switchports in vlan 40, each connected to a LES 10. One LES 10 goes to head office 3750 (switchA) which has the one switchport in vlan 40 and EIGRP routing, the other LES 10 goes into switchB, which has no intervlan routing and which effectively passes all traffic on to switchA SwitchB has the one switchport in vlan 40. Vlan40 is a /29 with 3 interfaces, one on each switch.
On switchC I can see that all traffic is passing normally through vlan 40 interface, but when I exam the 2 physical ports, traffic is only going via the one connected to switchB. This is confirmed by looking at the physical interface on switchA, which passes no traffic - all traffic goes via switchB.
How do I get the 2 links to load balance? Do they need to be in separate vlans? IP Cef is enabled.
Any help appreciated.
Your topology is hard to follow but it looks like you have a L2 network loop. STP will block one of the two.
do a sh spanning-tree and look if you have a port in BLK state.
The way around this is to use routed interface between the switch.