Figuring out the Exit Interface of an OMP Route in vEdges by Routing Tables

Let's start from show ip routes command in CLI.

(Click on the image to see in actual size)

You can see there are 2 VPNs (0 and 1) and several routes learned via static, connected, OSPF and OMP. All other routes except OMP are straight forward just like in normal Cisco IOS; either there is a next hop or else directly connected. 

In OMP (Overlay Management Protocol), there is this concept called TLOC (Transport Locator) to identify the hop and transport which the traffic should be forwarded to.

TLOC contains of 3 components;

1. TLOC IP (System IP of the Hop)

2. Color (the transport eg:- biz-internet, MPLS, etc)
3. Encapsulation Method

OMP routes are learned via a Service VPN only which in this case the VPN 1. Let's analyze a specific route, 192.168.22.0/24

So there are 2 TLOCs which the traffic should be forwarded to, which has the same TLOC IP but different transports. Which means this subnet is learned from these 2 transports from same WAN edge.

Now let's see how to reach this TLOC IP. 

The thing is that the TLOC IP is not need to be routed from other WAN edges, it's the System IP of the WAN edge just like a OSPF router id. So let's find the public IPs of the TLOC transports by the following command..

show omp tlocs | b 10.1.1.112

Now it's the time to look for the next hops in VPN 0 routes for the above 2 public IPs.

So the traffic is load balanced through the 2 interfaces.

All the above was done to resolve next hop IP and exit interface by examining routing tables. Following command will give you the same information at once and hope now you know how it is derived and that is nothing but the forwarding table.

show ip fib

Red box shows the next hop addresses of the route in the Service VPN (Overlay VPN) and Purple lines show the real next hop addresses in VPN 0 (underlay VPN) if any and the exit physical interfaces.

Note that since AAR (App Aware Routing) in SD-WAN is defining the traffic forwarding in case it is configured. You can simulate to visualize the actual traffic flow in vManage GUI interface.

If AAR is not configured, it will show the same result as we found from above CLI analysis.

Go to vManage > Monitor > Devices > Select the Device

Troubleshooting (left side)  > Simulate Flows under Traffic and enter the values.