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New Member

packet flow in inter-vlan routing

I have a question on how packets move between machines in the following scenario:

Switch A has IP routing enabled and has two vlans defined with an ip address assigned to each. Lets say those vlans are 10

and 20.

Switch B is trunked to switch A. On switch B I have two servers: one on vlan 10, one on vlan 20. Their gateway is

set to the IP address of their vlan on Switch A.

What I want to know is when I transfer stuff between the two servers on switch B, do all packets have to travel

through switch A? Do the packets stay within switch B? Do only the first few packets go to switch A to get routed

and then the rest stay within switch B?


If in case it matters, we're talking about TCP and UDP packets, maybe even some multicast traffic.



VIP Super Bronze

Since you are transfering

Since you are transfering between 2 servers that are connected to switch B and the 2 servers are in the same vlan, the traffic never gets to switch A.

It is all local to switch B.


New Member

The two servers are not in

The two servers are not in the same vlan. One is on vlan 10, the other on vlan 20. Their gateway is the IP address set on their respectively vlans on switch A.




VIP Super Bronze

In that case, the

In that case, the copmunication betweem the vlans will always go through their default gateways which is switch A.


Super Bronze

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Normally, it's as Reza describes in his 2nd post.  However, back in days of yore, the Catalyst 5000/5500 series supported multilayer switching with an external router.  When active, the first packet would go to the router, and it would route it between the subnets/VLANs, but the switch would then note what it did, and for subsequent packets of the same flow, L3 switch locally between the VLANs.