Part 1 – How Gateways provide Scaling
How Gateways and tunnelled WLANs provide scaling, relates to how session state sync messages are handled during a roam.
For bridged WLANs, when a STA roams the session state sync request is broadcasted over the user VLAN and is seen by all the APs.
In a busy network with up to 500 APs and a large number of roams, the quantity of broadcast messages can be significant.
For tunneled WLANs, each STA is anchored to a single Gateway in the cluster (i.e. UDG) based on the published bucketmap for the cluster. The UDG Gateway knows which AP each STA roamed from and can therefore suppress the flooding and will only forward the session state sync request to the origin AP. So when we state that Gateway can provide higher scaling, this is what we mean. Broadcast / multicast frames received on Gateway uplink ports can be filtered preventing flooding across the user VLANs.
Part 2 – How BCMC traffic to STAs can be optimized
This deals with BCMC traffic distribution and optimization for STAs. This can be broken down into three parts:
- Options on Gateways to control which BCMC traffic makes it to the APs
- WLAN options on the APs to control which received BCMC traffic makes it over the air.
- WLAN options on the APs to influence the rates at which BCMC traffic is forwarded over the air.
Gateway Controls:
When tunneled WLANs are deployed, Gateways can influence which flooded BCMC traffic makes it to the APs. The primary option that can be enabled is BCMC optimization which can be optionally enabled per user VLAN. This parameter was commonly enabled for RAP deployments and when enabled will help control flooding.
Config: //On Gateways -> Interface -> VLANs
Broadcast multicast optimization
WLAN Options:
As the APs perform the encryption / decryption, the APs are responsible for determining which BCMC traffic is forwarded over the air and how the traffic is forwarded. The broadcast filter option selected for a WLAN profile influences which BCMC traffic is forwarded over the air and ARP requests are converted to unicast. Each filtering option is well documented and should be understood:
• All – The AP drops all broadcast and multicast frames except DHCP and ARP, IGMP group queries, and IPv6 neighbor discovery protocols.
• ARP – The AP drops broadcast and multicast frames except DHCP and ARP, IGMP group queries, and IPv6 neighbor discovery protocols. Additionally, it converts ARP requests to unicast and sends frames directly to the associated clients. By default, the AP is configured to ARP mode.
• Unicast ARP Only – This option enables AP to convert ARP requests to unicast frames thereby sending them to the associated clients.
• Disabled – The AP forwards all the broadcast and multicast traffic is forwarded to the wireless interfaces.
Broadcast filtering applies to both bridged and tunneled WLANs. For tunneled WLANs features such as BCMC optimization enabled on the user VLAN on the Gateways determines which BCMC traffic makes it to the APs. The broadcast filtering option effectively determines which of the received BCMC traffic makes it over the air. For bridged WLANs, we don’t have any controls on the APs uplink ports to filter ingress BCMC traffic and have to rely on the access switching layer to perform filtering.
Config: //On APs -> General -> Advanced Settings -> Broadcast/Multicast
Broadcast filtering: Disabled / ARP / ALL / Unicast ARP Only
Data Rates:
For BCMC traffic that is received and allowed, additional WLAN profile options such as Multicast Transmission Optimization (MTO) and Dynamic Multicast Optimization (DMO) influence how BCMC is forwarded over the air. This applies to both bridged and tunneled WLANs. By default, permitted BCMC traffic destined to STAs is forwarded out the multicast BSS at the lowest configured rates for each radio:
• MTO – When enabled, BCMC traffic destined to STAs is forwarded out the multicast BSS up to a rate of 24 Mbps.
• DMO – When enabled and active on a BSS, IP multicast traffic destined to each receiver is forwarded at each receiver’s unicast rate. The destination multicast MAC is replaced with each receiver’s MAC address. When enabled and inactive on a BSS, IP multicast traffic is forwarded out the multicast BSS at either the default rate or up to 24 Mbps if MTO is enabled.
The broadcast filtering option controls which BCMC traffic is forwarded out the BSS while MTO influences the rate at which permitted BCMC traffic is forwarded out the multicast BSS. DMO is specific to IP multicast and when enabled and active will forward IP multicast groups at each receivers unicast rate.
Config: //On APs -> General -> Advanced Settings -> Broadcast/Multicast
Multicast Transmission Optimization (MTO) – Enabled
Dynamic Multicast Optimization (DMO) – Enabled
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