Failed to make mDNS work on CBS220, any idea?

[PeterWan]

Hi All,

So I am running into mDNS setup issue with my Cisco CBS220 switch that is very annoying and have no idea where to start the trouble shooting. I have 2 AP connected to the Switch with trunk port enabled and a 10G uplink trunk port to my UDM. I have enabled the mDNS repeater on UDM to repeat mDNS between different VLANs. the problem is that the mDNS advertisement from the 2 AP seems to be stuck at that exact trunk port. when my phone is connected to AP 1, it receive the advertisement from the device connected to that AP, same for AP 2. now I have enabled the mDNS snooping on the AP so at least it works between wireless device now. But say a device A is connected to AP 1 wirelessly with VLAN 2, and my PC is also connected to VLAN 2 on the switch with mode access, my PC will not receive the advertisements rom A. I have enabled forward all on my switch, nDNS snooping with IGMP v3, enabling a filter for all ports to allow all mDNS between 224.0.0.0 to 224.0.0.254 and nothing have changed. It seems like the switch simply never received that mDNS package from the AP. but since its a trunk port, it's honestly more complicated than an access port. is there anything I can do to figure it out what is stopping the mDNS package from spreading between different port on the switch? Note that my PC do receive some IGMP query from the UDM.

Best,

Peter Wan

[Flavio Miranda]

@PeterWan 

 Wireless to wireless works, right? What about two devices connected on cable?

Which access point is it?

[PeterWan]

Hi Miranda,

Thanks for the prompt response and sorry for the late response, guess I missed the email from CC. So wireless to wireless works only within the same access point, Say device A,B,C is connected to AP 1, their mDNS broadcast could not be discovered on AP2. It’s extremely weird. If the 2 device were connected to the same switch with port mode access with the same vlan, their mDNS broadcast could be discovered as expected. I’ve tweaked the mDNS snooping on switch and disabled it already. It should have been treated as a broadcast…… 

Best Regard,

peter wan 

[Flavio Miranda]

I have a suspicious that the problem is not on the swtich as wired to wired works and wireless to wireless works. I believe this could be something on the access point side.

 Something on the bridge between the wireless to wired. After all, for a wireless client to see another wireless client in different APs, they need to go wired to cross from one AP to another.

Which Access Point is it?

[PeterWan]

Hi Miranda,

Both AP are Cisco 150AX. The mesh bridge is wired, connected to the trunk port on the same switch mentioned. Note that the switch have DoS protection enabled, QoS configured and currently all snooping disabled. Thanks for the help.

Best regards,

Peter 

[KJK99]

@PeterWan 

mDNS and IGMP are two different things. Changing IGMP settings will not help you at all. Also, I don't think any CBS switch supports mDNS Snooping. IGMP Snooping, yes, but not mDNS.

mDNS propagates natively in a single VLAN (broadcast domain) so, if we talk about just a single VLAN, no special mDNS configuration is required on a switch and even on an access point. If it does not work, there must be some basic VLAN configuration issue.

If you enable and configure the mDNS reflector on your UDM, you do not even need to enable mDNS Snooping on your AP. If you need to do it to make mDNS work for your wireless devices, again, there must be some basic VLAN configuration issue.

[PeterWan]

Hi Kris K,

the manual said otherwise. According to Cisco CBS 220 Admin guide, “When enabling IGMP Snooping, the devices that monitor network flow will determine which hosts have requested to receive multicast traffic, and the switch only executes IGMP Snooping.” unless i misunderstood mDNS to be a type of multicast traffic, it should have affect on mDNS (clearly i did).

To your suggestion on VLAN configuration, both APs were connected to a TRUNK port on the switch, and all VLAN Contect works between the AP to the internet (i disabled some firewall rules during troubleshooting). This implies the trunk link between the Switch and the UDM works also. Form my understanding of Layer 2 networking, multicast should work automatically right? There is no ACL on the switch that Deny traffic. And the QoS enabled basically should not left traffic out. Dos protection reported normal and frankly speaking should not be relevant. 

Best Regards,

Peter

[KJK99]

mDNS can be confused with IGMP because of its use of multicasting and the address of 224.0. 0.251, but these are two different things. They work differently and serve different purposes.

To make mDNS work for both wired and wireless devices in a single VLAN, try to disable mDNS Snooping on your APs since it interferes in the mDNS workings. Make also sure you do not block mDNS traffic in any way. Then, if your VLAN configuration is OK, mDNS should work in each VLAN separately for any device that supports it without making any special configuration changes. mDNS is a zero-configuration service and the term zero-configuration can be taken to it literally in a single VLAN.

To make mDNS work across different VLANs, you can configure mDNS on either UDM or APs. mDNS on UDM will serve both wired or wireless devices since it acts as a mDNS reflector/repeater. mDNS on APs will serve only wireless devices since it acts as a mDNS gateway for wireless devices.

[PeterWan]

Hi Kris K,

the mDNS snooping was enabled after spotting that nothing worked. I will try disabling it again, and factory resetting the switch see if it helps. Thanks for the advice.

Best regards,

peter wan 

[PeterWan]

hi Kris K,

so I have continued my troubleshooting a few hours earlier today. I have continued to disable, STP, bonjour advertisement and Qos and nothing helped. so I moved to configure all the trunk port to become a general port (which is a trunk port encapsulated with "more fully" dot1q), and non of these helped. its getting so weird that I simply moved the 2 AP from the switch to 2 port with CPU link on the UDM, and their mdns flooded the entire switch. then I went ahead and configured port 13, 14 as the new general ports for my aps and the mdns now works perfectly. so I went ahead and disabled the Dos protection on the 2 ap port. and apparently the switch thinks mDNS traffics are DOS attacks. after disabling the DOS protection, everything works now. I have revised the log, and clearly, same as before, the suppressed traffic was not reported in the log file. thanks for all the help along the way! 

Best Regards,

Peter Wan 

[KJK99]

I'm glad to learn that you have resolved that issue. Cheers!