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CEF Switching accross VLANs


I am looking for some clarification of CEF switching behavior when using the following topology.

2 layer 3 switches, A and B, directly connected via a dot1q trunk and 2 hosts, C and D, both connected to switch B. Host C is in VLAN 10, Host D is in VLAN 20, and Switch A is the default gateway for both VLANs.

In the event that Host C needs to communicate with Host D the traffic must be sent accross the trunk link to the VLAN 10 interface on Switch A (default router) and routed back accross the same trunk link before reaching Host D in VLAN 20. Can this be avoided by configuring a L3 interface in VLANs 10 and 20 on switch B. I do not want to make switch B the default gateway for the hosts, only to add a connected L3 route on switch B. In this case would CEF add the connected route to the FIB and switch all traffic locally or will inter VLAN traffic still be routed via Switch A?


Re: CEF Switching accross VLANs


Unfortunately, even if you do what you proposed, all traffic between the VLANs will be routed via Switch A. One option you might want to consider is to create L3 interfaces on Switch A for the two VLANs and then run HSRP with Switch B. The virtual IP address that you use for HSRP can be the same address currently used as the default gateway by your hosts.

Hope that helps - pls rate the post if it does.


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