topic Re: need to know whether wlc allows broadcast of traffic or it denies in Wireless

need to know whether wlc allows broadcast of traffic or it denies

rsha — Thu, 23 Jan 2025 09:09:36 GMT

Hello, I have a doubt when Ap powers up and send a dhcp discover packet and once wlc receives the dicover packet would it broadcast it or deny the broadcast of discover packet like router.

Else I have to enter relay command to let it know where the dhcp server is.

Re: need to know whether wlc allows broadcast of traffic or it denies

MHM Cisco World — Thu, 23 Jan 2025 09:23:56 GMT

Friend'

Broadcast discovery only use when both AP abd wlc in same subnet'

If AP and wlc in different subnet you can use relay in SW but cisco provides additional discovery method that not need this relay

Like use dhcp op

Use dns

So I dont prefer use relay in SW for discovery

MHM

Re: need to know whether wlc allows broadcast of traffic or it denies

Rich R — Thu, 23 Jan 2025 16:22:33 GMT

There's no reason why the WLC would be listening to DHCP discover packets unless you are running local DHCP server on the WLC.

> once wlc receives the dicover packet would it broadcast it
Never! Not even a router would broadcast a received broadcast packet - that is strictly forbidden by RFC because it would cause a broadcast storm. If the DHCP discover packet needs to be relayed then the device configured for relay will unicast the packet to the DHCP server. If the packet is received by an interface which is acting as DHCP server then the packet will be processed and DHCP OFFER or NACK will be sent to the client which is the AP in this case.

> deny the broadcast of discover packet like router
Router will ignore the packet if it is not configured as DHCP server or relay (helper). WLC would ignore the packet unless it is running as local DHCP server for that interface.

What model of WLC are you referring to and what software version are you using?
What are you trying to achieve?
Do you have a diagram showing the topology of the network?
DHCP is usually handled by the local router or switch.

Re: need to know whether wlc allows broadcast of traffic or it denies

vishalbhandari — Thu, 23 Jan 2025 17:39:47 GMT

@rsha When an AP powers up and sends a DHCP Discover packet, the packet is sent to the local network. If the WLC (Wireless LAN Controller) receives the Discover packet, it doesn’t broadcast it further like a router. Instead, the WLC only handles DHCP requests if it is configured as a DHCP server. If your DHCP server is on another network, you need to configure a DHCP relay (using the ip helper-address command on the router or switch) to forward the Discover packet to the DHCP server.

Re: need to know whether wlc allows broadcast of traffic or it denies

Rich R — Fri, 24 Jan 2025 11:15:15 GMT

@vishalbhandari all you've done is repeat my answer (mostly) but got wrong the fact that "broadcast it further like a router" - routers do not broadcast received broadcast packets!!!!!

Re: need to know whether wlc allows broadcast of traffic or it denies

rsha — Fri, 24 Jan 2025 11:21:00 GMT

We have a separate dhcp server to handle such request, but my question related solely to wlc.

When router receives a broadcast packet it would not broadcast it and when a switch receives it would broadcast to every interface except from where it came from. I just wanted to know how wlc behaves when it receives a broadcast packet.

Re: need to know whether wlc allows broadcast of traffic or it denies

MHM Cisco World — Fri, 24 Jan 2025 11:26:24 GMT

You confuse me here

If AP send broadcast discovery then check my answer above

If wifi client send broadcast then

1- broadcast for dhcp' here depend of wlc config but wlc 9800 common config is bridge the broadcast from wlan to vlan (wired)

2- it ask about mac address using arp here also depends on wlc config' most wlc use arp proxy

MHM

Re: need to know whether wlc allows broadcast of traffic or it denies

MHM Cisco World — Fri, 24 Jan 2025 11:27:35 GMT

Case1 non common and not recommend is using dhcp as relay ( like l3 router) here the wlc will relay broadcast and change it to unicast

MHM

Re: need to know whether wlc allows broadcast of traffic or it denies

Flavio Miranda — Fri, 24 Jan 2025 11:48:36 GMT

@rsha

WLC is essentially a layer2 device. As such, it will forward broadcast.

Eventually you can change this behavior. For example, by create Dynamic interfaces on AirOS WLC or interfaces vlan in IOS-XE WLCs.

Related to DHCP specifically, the WLC can act as proxy or not. While as proxy, it will not forward DHCP broadcast and will send unicast instead. If proxy is not enable, it will forward DHCP to the direct connect Layer3 device where the interface vlan and ip helper-address will move to DHCP server.

Re: need to know whether wlc allows broadcast of traffic or it denies

Rich R — Fri, 24 Jan 2025 11:54:54 GMT

From an AP (or any other device on wired network) generally 9800 will behave like a router and not forward broadcasts (certainly never back to the wired).

For a wireless client 9800 handles broadcasts as a special case. In general broadcasts are not forwarded to wireless - for example ARP broadcast is converted to unicast on wireless and best practice is to use ARP proxy on WLC so 9800 responds on behalf of the client. Same would apply to a DHCP reply from wired to wireless client.
Passive client is another special case: https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/technical-reference/c9800-best-practices.html#PassiveClients

For more general discussion of WLC and broadcasts see:
https://community.cisco.com/t5/wireless/flexconnect-and-broadcasts/td-p/2044077

https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/17-12/config-guide/b_wl_17_12_cg/m-vewlc-multicast-cg.html#wl-bcast-nip-mcast-mcast-vlan

https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/17-12/cmd-ref/b_wl_17_12_cr/configuration-commands-g-to-z.html#wp7625205920

wireless broadcast vlan

To enable broadcast support on a VLAN, use the wireless broadcast vlan command in global configuration mode. To disable Ethernet broadcast support, use the no form of the command.

But note that there is a bug with this command: https://bst.cisco.com/bugsearch/bug/CSCwc66967 (meaning do not use it unless you really know what you are doing) because broadcasts on WiFi can cause major problems for you.