about CAPWAP data flow

[jwu5188]

As I know,  a WLC receives a user packet from AP via CAPWAP, it will remove CAPWAP header, translate L2 header and forward it to a GW. Here are my questions and do appreciate any input from you. Thanks!

Q1. If a WLC uses trunk port to connect to L3 switch, how does it decide which GW to send?

Q2. If WLC doesn't change source IP in user packet, how does it make the return traffic pass by it?

[Leo Laohoo · Accepted solution]

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@jwu5188 wrote:
Q1. If a WLC uses trunk port to connect to L3 switch, how does it decide which GW to send?

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That depends entirely on the design of the SSIDs.  

If the wireless network is one, big flat network, then it is a no-brainer.  The WLC will send all the data down a single VLAN. 

If the SSID is chopped into different SSID, then the WLC will send the data down each specified VLAN (tied to the SSID).  

In the case of Dot1x, it is exactly the same thing.  ISE will tell the WLC which VLAN to punt the data into.  

[jwu5188]

Dear Laohoo,

Thanks for your prompt response. Do you have any idea about my Q2?

[Leo Laohoo]

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@jwu5188 wrote:
Q2. If WLC doesn't change source IP in user packet, how does it make the return traffic pass by it?

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If the WLC does not change the source IP address, then you've got a problem.  Potentially, a bug.

[Rich R]

It does not change the client source IP and return traffic is routed back to the client just the same as any client traffic replies.  The reverse process takes place - packet is translated from ethernet to 802.11 and sent back down the CAPWAP tunnel to the AP and the wireless client.  ARP and the WLC internal client database maps the IP to the client MAC address.