Consider the scenario in the above image. If the mDNS gateway functionality is enabled on the router, then service information can be sent from one subnet to another and vice-versa. For example, the printer and fax service information being advertised in the network with IP address 192.0.2.6 are redistributed to the network with IP address 198.51.100.4. Ultimately, the printer and fax service information in the in the network with IP address 192.0.2.6 is learnt by mDNS-enabled hosts and devices in the other network.

You can filter services that you want to redistribute. While creating a service–list, the permit or deny option must be used. The permit option allows you to permit/transport specific service–list information. The deny option allows you to deny service–list information that is available to be transported to other subnets. You need to mention a sequence number when using the permit or deny option. The same service–list name can be associated with multiple sequence numbers and each sequence number will be mapped to a rule.

Note	If no filters are configured, the default action is to deny service–list information to be transported through the device or interface.

Query is another option provided while creating service–lists. You can create queries using a service–list. If you want to browse for a service, then active queries can be used. This helps to keep the records refreshed in the cache.

Note	Active queries can only be used globally and cannot be used at the interface level.

A service end–point (such as, a printer, fax, and so on) sends unsolicited announcements when a service starts up. After that, it sends unsolicited announcements whenever a network change event occurs (such as, an interface coming up or going down, and so on). The device always respond to queries.

After creating a service–list and using the permit or deny option, you can filter by using match statements (commands) based on service–instance, service–type, or message–type (announcement or query).

1.    enable


2.    configure terminal


3.    service-list mdns-sd service-list-name {deny sequence-number | permit sequence-number | query}


4.    match message-type {announcement | any | query } OR match service-instance {instance-name | any | query} OR match service-type mDNS-service-type-string


5.    exit

Device> enable

Device# configure terminal

Device(config)# service-list mdns-sd sl1 permit 3
                
              Or

Device(config)# service-list mdns-sd sl4 query

Device(config-mdns-sd-sl)# match message-type announcement

                         OR

Device(config-mdns-sd-sl)# match service-instance servInst 1

                         OR

Device(config-mdns-sd-sl)# match service-type _ipp._tcp

Device(config-mdns-sd-sl)# exit

After enabling mDNS gateway for a device, you can apply filters (apply IN-bound filtering or OUT-bound filtering) and active queries by using service-policy and service-policy-query commands respectively. You can redistribute services and service announcements using the redistribute mdns-sd command, and set some part of the system memory for cache using the cache-memory-max command.

Note	Steps 4 to 7 are optional and not meant to be used in any specific order.

1.    enable


2.    configure terminal


3.    service-routing mdns-sd


4.    match service-instance service-policy-name {IN | OUT}


5.    redistribute mdns-sd


6.    cache-memory-max cache-config-percentage


7.    service-policy-query service-list-query-name service-list-query-periodicity


8.    exit

Device> enable

Device# configure terminal

Device(config)# service-routing mdns-sd

Device(config-mdns)# service-policy serv-pol1 IN

Device(config-mdns-sd-sl)# exit

Device(config-mdns)# cache-memory-max 20

Device(config-mdns)# service-policy-query sl-query1 100

Device(config-mdns)# exit