NSX-T Data Center logical routers provide North-South connectivity, thereby enabling tenants to access public networks, and East-West connectivity between different networks within the same tenants. For East - West connectivity, logical routers are distributed across the kernel of the hosts.
With NSX-T Data Center it’s possible to create two-tier logical router topology: the top-tier logical router is Tier 0 and the bottom-tier logical router is Tier 1. This structure gives both provider administrator and tenant administrators complete control over their services and policies. Administrators control and configure Tier-0 routing and services, and tenant administrators control and configure Tier-1. The north end of Tier-0 interfaces with the physical network, and is where dynamic routing protocols can be configured to exchange routing information with physical routers. The south end of Tier-0 connects to multiple Tier-1 routing layer(s) and receives routing information from them. To optimize resource usage, the Tier-0 layer does not push all the routes coming from the physical network towards Tier-1, but does provide default information.
Southbound, the Tier-1 routing layer interfaces with the logical switches defined by the tenant administrators, and provides one-hop routing function between them. For Tier-1 attached subnets to be reachable from the physical network, route redistribution towards Tier-0 layer must the enabled. However, there isn’t a classical routing protocol (such as OSPF or BGP) running between Tier-1 layer and Tier-0 layer, and all the routes go through the NSX-T Data Center control plane. Note that the two-tier routing topology is not mandatory, if there is no need to separate provider and tenant, a single tier topology can be created and in this scenario the logical switches are connected directly to the Tier-0 layer and there is no Tier-1 layer.
A logical router consists of two optional parts: a distributed router (DR) and one or more service routers (SR).
A DR spans hypervisors whose VMs are connected to this logical router, as well as edge nodes the logical router is bound to. Functionally, the DR is responsible for one-hop distributed routing between logical switches and/or logical routers connected to this logical router. The SR is responsible for delivering services that are not currently implemented in a distributed fashion, such as stateful NAT.
A logical router always has a DR, and it has SRs if any of the following is true:
The logical router is a Tier-0 router, even if no stateful services are configured
The logical router is Tier-1 router linked to a Tier-0 router and has services configured that do not have a distributed implementation (such as NAT, LB, DHCP )
The NSX-T Data Center management plane (MP) is responsible for automatically creating the structure that connects the service router to the distributed router. The MP creates a transit logical switch and allocates it a VNI, then creates a port on each SR and DR, connects them to the transit logical switch, and allocates IP addresses for the SR and DR.