You can install NSX Edge as an edge services gateway (ESG) or as a distributed logical router (DLR). The number of edge appliances including ESGs and DLRs is limited to 250 on a host.
Edge Services Gateway
The ESG gives you access to all NSX Edge services such as firewall, NAT, DHCP, VPN, load balancing, and high availability. You can install multiple ESG virtual appliances in a datacenter. Each ESG virtual appliance can have a total of ten uplink and internal network interfaces. With a trunk, an ESG can have up to 200 subinterfaces. The internal interfaces connect to secured port groups and act as the gateway for all protected virtual machines in the port group. The subnet assigned to the internal interface can be a publicly routed IP space or a NATed/routed RFC 1918 private space. Firewall rules and other NSX Edge services are enforced on traffic between network interfaces.
Uplink interfaces of ESGs connect to uplink port groups that have access to a shared corporate network or a service that provides access layer networking. Multiple external IP addresses can be configured for load balancer, site-to-site VPN, and NAT services.
Distributed Logical Router
The DLR provides East-West distributed routing with tenant IP address space and data path isolation. Virtual machines or workloads that reside on the same host on different subnets can communicate with one another without having to traverse a traditional routing interface.
A logical router can have eight uplink interfaces and up to a thousand internal interfaces. An uplink interface on a DLR generally peers with an ESG, with an intervening Layer 2 logical transit switch between the DLR and the ESG. An internal interface on a DLR peers with a virtual machine hosted on an ESX hypervisor with an intervening logical switch between the virtual machine and the DLR.
The DLR has two main components:
The DLR control plane is provided by the DLR virtual appliance (also called a control VM). This VM supports dynamic routing protocols (BGP and OSPF), exchanges routing updates with the next Layer 3 hop device (usually the edge services gateway) and communicates with the NSX Manager and the NSX Controller cluster. High-availability for the DLR virtual appliance is supported through active-standby configuration: a pair of virtual machines functioning in active/standby modes are provided when you create the DLR with HA enabled.
At the data-plane level, there are DLR kernel modules (VIBs) that are installed on the ESXi hosts that are part of the NSX domain. The kernel modules are similar to the line cards in a modular chassis supporting Layer 3 routing. The kernel modules have a routing information base (RIB) (also known as a routing table) that is pushed from the controller cluster. The data plane functions of route lookup and ARP entry lookup are performed by the kernel modules. The kernel modules are equipped with logical interfaces (called LIFs) connecting to the different logical switches and to any VLAN-backed port-groups. Each LIF has assigned an IP address representing the default IP gateway for the logical L2 segment it connects to and a vMAC address. The IP address is unique for each LIF, whereas the same vMAC is assigned to all the defined LIFs.
A DLR instance is created from the NSX Manager UI (or with API calls), and routing is enabled, leveraging either OSPF or BGP.
The NSX Controller leverages the control plane with the ESXi hosts to push the new DLR configuration including LIFs and their associated IP and vMAC addresses.
Assuming a routing protocol is also enabled on the next-hop device (an NSX Edge [ESG] in this example), OSPF or BGP peering is established between the ESG and the DLR control VM. The ESG and the DLR can then exchange routing information:
The DLR control VM can be configured to redistribute into OSPF the IP prefixes for all the connected logical networks (172.16.10.0/24 and 172.16.20.0/24 in this example). As a consequence, it then pushes those route advertisements to the NSX Edge. Notice that the next hop for those prefixes is not the IP address assigned to the control VM (192.168.10.3) but the IP address identifying the data-plane component of the DLR (192.168.10.2). The former is called the DLR "protocol address," whereas the latter is the "forwarding address."
The NSX Edge pushes to the control VM the prefixes to reach IP networks in the external network. In most scenarios, a single default route is likely to be sent by the NSX Edge, because it represents the single point of exit toward the physical network infrastructure.
The DLR control VM pushes the IP routes learned from the NSX Edge to the controller cluster.
The controller cluster is responsible for distributing routes learned from the DLR control VM to the hypervisors. Each controller node in the cluster takes responsibility of distributing the information for a particular logical router instance. In a deployment where there are multiple logical router instances deployed, the load is distributed across the controller nodes. A separate logical router instance is usually associated with each deployed tenant.
The DLR routing kernel modules on the hosts handle the data-path traffic for communication to the external network by way of the NSX Edge.