1、解釋：
Multicast Domain Name System (mDNS) is a protocol used for service discovery by Apple products (called Bonjour) and by Google products (called Chromecast). The mDNS service discovery enables wireless clients to access Apple services such as Apple Printer and Apple TV advertised in a different Layer 3 network.
2、標準：
mDNS performs DNS queries over IP multicast. mDNS supports zero-configuration IP networking. As a standard, mDNS uses multicast IP address 224.0.0.251 as the destination address and 5353 as the UDP destination port.
3、特定位置服務Location Specific Services（LSS）：
The processing of mDNS service advertisements and mDNS query packets support Location-Specific Services (LSS).
All the valid mDNS service advertisements that are received by the controller are tagged with the MAC address of the AP that is associated with the service advertisement from the service provider while inserting the new entry into the service provider database.
The wireless service provider database entries are filtered based on the AP-NEIGHBOR-LIST if LSS is enabled for the service.
If LSS is disabled for any service, the wireless service provider database entries are not filtered when they respond to any query from a wireless client for the service.
LSS applies only to wireless service provider database entries. There is no location awareness for wired service provider devices.
The status of LSS cannot be enabled for services with ORIGIN set to wired and vice-versa.
4、mDNS AP
The mDNS AP feature allows the controller to have visibility of wired service providers that are on VLANs that are not visible to the controller.
You can configure any AP as an mDNS AP and enable the AP to forward mDNS packets to the controller.
VLAN visibility on the controller is achieved by APs that forward the mDNS advertisements to the controller.
The mDNS packets between the AP and the controller are forwarded in capwap data tunnel that is similar to the mDNS packets from a wireless client.
Only CAPWAPv4 tunnels are supported.
APs can be in either the access port or the trunk port to learn the mDNS packets from the wired side and forward them to the controller.
You can also use this configuration to specify the VLANs from which the AP should snoop the mDNS advertisements from the wired side.
The maximum number of VLANs that an AP can snoop is 10.
If the AP is in the access port, you should not configure any VLANs on the AP to snoop. The AP sends untagged packets when a query is to be sent.When an mDNS advertisement is received by the mDNS AP, the VLAN information is not passed on to the controller.
The service provider's VLAN that is learned through the mDNS AP's access VLAN is maintained as 0 in the controller.
Restrictions for Configuring Multicast DNS
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mDNS over IPv6 is not supported.
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mDNS snooping is not supported on access points in FlexConnect mode in a locally switched WLAN and mesh access points. For locally switched WLANs, all multicast traffic including mDNS is simply bridged between the local VLAN and the SSID.
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mDNS is not supported on remote LANs.
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mDNS is not supported on Cisco AP1240 and Cisco AP1130.
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Third-party mDNS servers or applications are not supported on the controller using the mDNS feature. Devices that are advertised by the third-party servers or applications are not populated on the mDNS service or device table correctly on the controller.
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The controller prevents addition or modification of the mDNS-profile when any interface is in use by an active WLAN in an AP group. When attempting to make changes to the mDNS profile which is already linked to an active WLAN, the following error message is displayed—Interface is mapped to an AP Group.
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mDNS snooping is not necessary in order to forward mDNS multicasts, if the network is configured to forward multicast traffic. However, Apple mDNS (Bonjour) traffic is sent with time to live of 1, so without mDNS snooping, Bonjour will work within a Layer 2 broadcast domain.
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In a large campus network, if multicast forwarding is enabled, it is recommended to enable mDNS snooping, and then disable mDNS on all WLANs, except anywhere mDNS is required. This is in order to prevent Bonjour multicast traffic from overwhelming the network.
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mDNS APs cannot duplicate the same traffic for the same service or VLAN.
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LSS filtering is restricted to only wireless services.
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The LSS, mDNS AP, Priority MAC address, and origin-based discovery features can be configured only using the controller CLI and cannot be configured using the controller GUI.
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mDNS-AP feature is not supported in CAPWAP V6.
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ISE dynamic mDNS policy mobility is not supported.
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mDNS user profile mobility is not supported in guest anchors.