NSX-T logical routers provide North-South connectivity, thereby enabling tenants to access public networks, and East-West connectivity between different networks within the same tenants.
A logical router is a configured partition of a traditional network hardware router. It replicates the hardware's functionality, creating multiple routing domains within a single router. Logical routers perform a subset of the tasks that can be handled by the physical router, and each can contain multiple routing instances and routing tables. Using logical routers can be an effective way to maximize router usage, because a set of logical routers within a single physical router can perform the operations previously performed by several pieces of equipment.
With NSX-T 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 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 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.