Part 1: VMware NSX Replication Modes

VMware NSX Replication Modes

It is important to understand the replication modes in NSX-V and NSX-T environment. Replication modes are defined at Segment level in NSX-V and NSX-T. (VXLAN/GENEVE) Replication modes determine how BUM (Broadcast, Unknown Unicast and Multicast) traffic is routed.

VMware NSX/NSX-T Replication mode
When talking about the replication modes in VMware NSX environment, it is of 3 types- Multicast mode; Unicast mode; Hybrid mode while in the VMware NSX-T environment, the modes are Headend Replication and Two tier replication.

Here in this article, we will talk about VMware NSX replication mode, we will talk about the replication mode for NSX-T in our next article

Unicast mode:
In the unicast mode, Source VTEP sends all the BUM traffic to all the intended VTEPs on the same VTEP subnet instead of encapsulating the BUM packets received from the residing VM.

Fig 1.1- Unicast Mode

A unicast packet is sent to the Proxy VTEP on the respective VTEP subnet for hosts on those subnets. This Proxy VTEP then sends the BUM packets to the intended hosts on that subnet (also unicast).

Multicast mode:
BUM information is sent once (i.e. each copy of BUM traffic is sent) by a source VTEP on a multicast address (IGMP, IGMP snooping and PIM are used in the underlying infrastructure). This mode consumes least overhead of traffic and resource consumption.

The Source VTEP is accountable for sending BUM packets to all the VTEPs on the same VTEP subnet. For hosts on an entirely different VTEP subnet, the Proxy VTEP is responsible for sending the packets to that host.

In this case the Proxy VTEP is responsible for sending all BUM packets to all of the intended hosts in that subnet (also done in unicast). Here we have VM 3 on the host. Therefore, we reduce the load on the Source VTEP.

Hybrid mode:
There is a replication mode that lies between multicast and unicast. Here, within the VTEP subnet, BUM packets are sent as multicast. This means the switches need to be configured with multicast protocols like IGMP, IGMP snooping. 

The Layer 3 devices, that is, the router, have not been configured with multicast. Instead, the Source VTEP is sending a unicast copy of the traffic to the Proxy VTEP, which is in the other subnet. 

In Hybrid mode, Proxy VTEP is also called MTEP. It also replicates that BUM packet so it is multicast on the new subnet.