Multicast DNS (mDNS) has become a critical protocol for modern networks, enabling service discovery across network segments without the need for traditional DNS servers. With the proliferation of IoT devices, Apple products, and various other devices requiring seamless connectivity, implementing an effective mDNS gateway solution has never been more important. In this blog post, we'll explore Cisco's implementation of mDNS gateway functionality on the 9800 series wireless controllers and how it can help solve common network challenges.

Understanding mDNS and Its Challenges

Multicast DNS, commonly known as mDNS, operates using the Bonjour protocol. It allows devices to discover services like printers, Apple TVs, Chromecasts, and other network resources without manual configuration. Devices advertise their capabilities via multicast packets on the 224.0.0.251 IPv4 address using UDP port 5353.

However, mDNS has one fundamental limitation: it doesn't cross subnet boundaries by default. This creates significant challenges in enterprise networks where:

• Wireless and wired networks often exist on different subnets
• Different departments might operate on separate VLANs
• IoT devices may reside in isolated segments for security reasons

This is where Cisco's mDNS gateway functionality on the 9800 series controllers becomes invaluable.

mDNS Gateway on Cisco 9800 Controllers: An Overview

The Cisco 9800 series wireless controllers introduced a more robust and scalable implementation of mDNS gateway functionality compared to earlier controller models. This implementation allows the controller to act as a proxy for mDNS services, effectively bridging the multicast boundary issues across subnets.

Key advantages of the 9800 series mDNS gateway implementation include:

• Support for a wider range of service types
• Enhanced scalability for large enterprise deployments
• More granular control through service policies
• Integration with Cisco DNA Center for centralized management
• Better performance with dedicated hardware acceleration

Configuration and Implementation

Basic mDNS Gateway Configuration

Setting up the mDNS gateway on a Cisco 9800 controller involves several key steps:

(config)# mdns-sd gateway

This enables mDNS Globally on the Wireless controller.

(config)# mdns-sd service-list service-list-name-in IN

(config-mdns-sl-in)# match service-type _airplay._tcp.local

(config-mdns-sl-in)# match service-type _raop._tcp.local

(config-mdns-sl-in)# match service-type _ipp._tcp.local

The Inbound service list defines which mDNS services the Cisco 9800 controller will keep in its cache.

(config)# mdns-sd service-list service-list-name-out OUT

(config-mdns-sl-in)# match all

The Outbound service list defines which mDNS services the Cisco 9800 controller will keep in its cache. A wired filter list can be attached in the outbound list to filter services learned by device MAC addresses and/or VLAN tags. These filter lists are only for services learned on the wired subnets attached to the Cisco 9800 Controller.

(config)# mdns-sd service-policy service-policy-name

(config -mdns-ser-pol)# service-list service-list-name-in IN

(config -mdns-ser-pol)# service-list service-list-name-out OUT

(config -mdns-ser-pol)# location site-tag

The Service Policy ties the inbound and outbound service lists together into a policy to be applied in different areas of the Cisco 9800 controller. Location identifiers can be applied to provide filtering of services learned via wireless clients. These Location identifiers can filter based on different criteria; such as AP site-tags, ap-name, or ssid, to name a few.

(config)# wireless profile policy policy-profile-name

(config-wireless-policy)# mdns-sd service-policy service-policy-name

The Service Policy can then be applied to a WLAN to enable the policy to the wireless clients joined to said WLAN.

(config)# interface vlan svi-number

(config-int)# mdns-sd gateway

(config-if-mdns-sd)# service-policy service-policy-name

The Service Policy can also be applied to an SVI to service any mDNS services on a wired network.

Best Practices and Optimization

When implementing mDNS gateway on Cisco 9800 controllers, consider these best practices:

1. Limit Service Types: Only include service types that are actually needed in your environment to reduce unnecessary multicast traffic.

2. Strategic Service Policy Application: Apply service policies only to WLANs where mDNS services are needed.

3. Monitor Cache Size: The controller maintains a cache of discovered services; monitor its size to ensure it doesn't approach limits in large deployments.

4. Implement Rate Limiting: Use rate limiting to prevent mDNS storms:

(config)# mdns-sd rate-limit 100

Troubleshooting Common Issues

Viewing mDNS Cache

To verify that services are being discovered and cached:

# show mdns-sd cache

Checking Service Statistics

Monitor the performance of the mDNS gateway:

# show mdns-sd statistics

Common Issues and Solutions

Real-World Use Cases

University Campus Deployment

A large university successfully deployed Cisco 9800 controllers with mDNS gateway to allow:

• Students to discover printers only on their residential floor
• Faculty to access departmental resources across the campus
• Common resources like projectors to be available in meeting spaces

Healthcare Implementation

A hospital network used location-specific mDNS policies to:

• Restrict medical device discovery to specific departments
• Enable patient entertainment systems only in patient areas
• Allow staff to discover relevant printers and resources throughout the facility

Conclusion

The mDNS gateway implementation on Cisco 9800 series wireless controllers provides a robust solution for service discovery challenges in modern networks. By acting as a proxy for mDNS traffic, it effectively bridges the multicast domain gap between subnets, enabling seamless service discovery across your entire network.

Whether you're managing a campus network, healthcare facility, or enterprise environment, proper implementation of mDNS gateway functionality can significantly enhance user experience while maintaining network segmentation for security purposes.