NSX-T Edge clusters are pools of capacity for NSX-T service router and load balancing functions.

North - South Routing

The routing design considers different levels of routing in the environment, such as number and type of NSX-T gateways, dynamic routing protocol, and so on. At each level, you apply a set of principles for designing a scalable routing solution.

Routing can be defined in the following directions:

  • North-south traffic is traffic leaving or entering the NSX-T domain, for example, a virtual machine on an overlay network communicating with an end-user device on the corporate network.

  • East-west traffic is traffic that remains in the NSX-T domain, for example, two virtual machines on the same or different segments communicating with each other.

As traffic flows north-south, edge nodes can be configured to pass traffic in an active-standby or an active-active model, where active-active can scale up to 8 active nodes. NSX-T service routers (SRs) for north-south routing are configured an active-active equal-cost multi-path (ECMP) mode that supports route failover of Tier-0 gateways in seconds.

Table 1. Features of Active-Active and Active-Standby SRs

Design Component




Bandwidth per node



Bandwidth per node is the same because it is independent of the Tier- 0 gateway failover model.

Total aggregate bandwidth



  • The active-active mode can support up to 8 NSX-T Edge nodes per northbound SR.

  • The active-standby mode is limited to a single active NSX-T node.



With up to 8 active-active NSX-T Edge nodes, availability can be as high as N+7, while for the active-standby mode it is N+1.

Failover Time

Both are capable of sub-second failover with use of BFD, only when using Bare Metal Edge form factor.

Routing Protocol Support


The active-active mode requires BGP for ECMP failover.

Figure 1. Dynamic Routing in a Single Availability Zone

Table 2. Design Decisions on the High Availability Mode of Tier-0 Gateways

Decision ID

Design Decision

Design Justification

Design Implication


Deploy an active-active Tier-0 gateway.

Supports ECMP north-south routing on all Edge nodes in the NSX-T Edge cluster.

Active-active Tier-0 gateways cannot provide stateful services such as NAT.

Table 3. Design Decisions on Edge Uplink Configuration for North-South Routing

Decision ID

Design Decision

Design Justification

Design Implication


To enable ECMP between the Tier-0 gateway and the Layer 3 devices (ToR switches or upstream devices), create two VLANs.

The ToR switches or upstream Layer 3 devices have an SVI on one of the two VLANs and each NSX-T Edge node in the cluster has an interface on each VLAN.

Supports multiple equal-cost routes on the Tier-0 gateway and provides more resiliency and better bandwidth use in the network.

Additional VLANs are required.


Assign a named teaming policy to the VLAN segments to the Layer 3 device pair.

Pins the VLAN traffic on each segment to its target Edge node interface. From there the traffic is directed to the host physical NIC that is connected to the target top of rack switch.



Create a VLAN transport zone for NSX-T Edge uplink traffic.

Enabled the configuration of VLAN segments on the N-VDS in the Edge nodes.

Additional VLAN transport zones are required if the edge nodes are not connected to the same top of rack switch pair.

Table 4. Design Decisions on Dynamic Routing

Decision ID

Design Decision

Design Justification

Design Implication


Use BGP as the dynamic routing protocol.

  • Enables the dynamic routing by using NSX-T. NSX-T supports only BGP for dynamic routing.

In environments where BGP cannot be used, you must configure and manage static routes.


Configure the BGP Keep Alive Timer to 4 and Hold Down Timer to 12 between the top of tack switches and the Tier-0 gateway.

Provides a balance between failure detection between the top of rack switches and the Tier-0 gateway and overburdening the top of rack switches with keep-alive traffic.

By using longer timers to detect if a router is not responding, the data about such a router remains in the routing table longer. As a result, the active router continues to send traffic to a router that is down.


Do not enable Graceful Restart between BGP neighbors.

Avoids loss of traffic.

On the Tier-0 gateway, BGP peers from all the gateways are always active. On a failover, the Graceful Restart capability increases the time a remote neighbor takes to select an alternate Tier-0 gateway. As a result, BFD-based convergence is delayed.



Enable helper mode for Graceful Restart mode between BGP neighbors.

Avoids loss of traffic.

During a router restart, helper mode works with the graceful restart capability of upstream routers to maintain the forwarding table which in turn will forward packets to a down neighbor even after the BGP timers have expired causing loss of traffic.



Enable Inter-SR iBGP routing.

In the event that an NSX-T Edge node as all of its northbound eBGP sessions are down, north-south traffic will continue to flow by routing traffic to a different NSX-T Edge node.


This design assumes that physical network does not support Bidirectional Forwarding Detection (BFD). Enable BFD if the network supports and is configured for BFD.

Intra-SDN Routing

Gateways are needed to provide routing between logical segments created in the NSX-T based SDN. Logical segments can be connected directly to a Tier-0 or Tier-1 gateway.

Table 5. Design Decisions on Tier-1 Gateway Configuration

Decision ID

Design Decision

Design Implication

Design Justification


Deploy a Tier-1 gateway and connect it to the Tier-0 gateway.

Creates a two-tier routing architecture.

A Tier-1 gateway can only be connected to a single Tier-0 gateway.

In cases where multiple Tier-0 gateways are required, you must create multiple Tier-1 gateways.


Deploy a Tier-1 gateway to the NSX-T Edge cluster.

Enables stateful services, such as load balancers and NAT, for SDDC management components.

Because a Tier-1 gateway always works in active-standby mode, the gateway supports stateful services.



Deploy a Tier-1 gateway in non-preemptive failover mode.

Ensures that after a failed NSX-T Edge transport node is back online, it does not take over the gateway services thus causing a short service outage.


Enable standby relocation of the Tier-1 gateway.

Ensures that if an edge failure occurs, a standby Tier-1 gateway will be created on another edge node.