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EOS: How to configure multicast mac to stop flooding of NLB server traffic via slow path

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TitleEOS: How to configure multicast mac to stop flooding of NLB server traffic via slow path
Microsoft Network load balancing or similar proprietary load balancing technologies, comprised of multiple physical machines responding to a single “virtual” IP address, expect the switch to forward its traffic to all connected ports on the destination VLAN. The flooded traffic uses soft forwarding path, subject to it’s rate limiters, instead of the device hardware forwarding path. This traffic will also compete for the slow path resources and the first packets from other new flows. We need to configure the switch to take the hardware path and prevent the flooding.
  • S-Series
  • N-Series
  • Securestack
  • Microsoft Network Load Balancing (NLB)
To force the virtual server packets to take a hardware switch path, configure a MAC address static entry in the Filter Database (FDB). If the destination MAC is multicast (the Group bit is set), use the set mac multicast command, optionally specifying a port-list that further scopes the flooding, to force the forwarding traffic to use the hardware path.

Network Load Balancing – Multicast Mode
  • Create a Static MAC Address Entry for multicast mac tied to ports and vlan
set mac multicast 03-bf-xx-xx-xx-xx <vlan> ge.1.1-2 

So a packet with a destination MAC address of 03:xx:xx:xx:xx:xx is forwarded and replicated to the ports defined in the ‘set MAC multicast’ command. All cluster servers receive exact duplicates of this packet. Since static entries are performed in hardware and directed to specific ports, performance is increased by removing unnecessary flooding to all group ports.

N.B. If there are switches in the layer 2 path between the users and the server cluster then the mac must be configured on all uplinks that egress towards the server.
  • ​The router must have a static ARP entry binding the Virtual-IP to the desired Multicast MAC address.
arp arpa 
Alternately, you can just turn on multicast arp learning under the VLAN interface:
interface vlan.0.xx
ip multicast-arp-learning

Network Load Balancing – Unicast Mode

The unicast as multicast feature causes unicast searches in the filter data base to match on statically configured multicast entries using hardware forwarding. When unicast as multicast is enabled on the device, a lookup is performed to determine if the unicast address has also been configured for multicast on the device. If a multicast address is found, packets are hardware forwarded out the configured VLAN and port(s) as defined in the static multicast configuration by extending the search phase of the Layer 2 lookup to match an unlearned destination MAC address against static multicast MAC entries. The unicast as multicast feature is configured by:
  1. Use the set mac multicast command to specify the MAC address to be treated as a multicast address, specifying the VLAN and egress port(s) to use.
  2. Using the set mac unicast-as-multicast command, in any command mode, to enable static unicast MAC addresses to be treated as multicast addresses on this device

Support for unicast NLB in hardware extends the “search” phase of the Layer 2 lookup in an attempt to match an unlearned destination MAC address against static multicast MAC entries. In order to accomplish this, the address will be temporarily modified for the lookup by OR’ing in the MAC multicast bit. This means that a cluster address of “02-03-04-05-06-07” (unicast address) will be configured in the switch as “03-03-04-05-06-07” (multicast address).
set mac unicast-as-multicast enable
set mac multicast 02:bf:01:01:01:01 1 ge.2.2
Warning: Unicast address converted to multicast 03-BF-01-01-01-01

N.B. The unicast mac is not learned in the switch.
Additional notes
See article How-to-configure-IP-Multicast-ARP-Learning for details of a new feature that allows the ARP cache to dynamically populate unicast IP addresses each bound to a MAC address with the group bit set to 1, representing a multicast MAC address.



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