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The role of Preference and Weight in Cisco SDWAN Viptela Solution

Today we are going to discuss about the parameters like Preference and Weight which influence the path of the traffic via WAN on the SDWAN. 

Most of you already knew about the use of the Preference and Weight in the BGP environment and some of you know how to use that in the Cisco Viptela SDWAN environment specially when you are using multiple paths on the same vEdge devices.

Let's talk about both of these parameters one by one, Let's start with the preference first. 

Preference as a parameter
Preference as name suggest is tell us about the direction of the traffic to the tunnel. A higher value is preferred. When a vEdge router has multiple tunnels like Multiple TLOCs e.g. MPLS, VPLS, Internet or so on.

Fig 1.1- Preference between two TLOC's

TLOCs with the highest preference are chosen using inbound path selection. However, traffic is influenced in both the directions which means inbound as well as outbound. If all TLOCs have the same preference and no policy is applied that affects traffic flow, traffic flows are evenly distributed among the tunnels, using ECMP. 

For example, when a preference of 100 on one TLOC just say MPLS and a preference of 90 on the other TLOC just say VPLS path is set, the preference chosen is the TLOC with a preference of 100 which is a MPLS path. 


Other case if we have two path with the same preference value let's say 100 on both MPLS and VPLS, then Load balancing using ECMP across the paths. 


Note: The criteria set in preferences work correctly when there are no other configurations that may alter the traffic flow. For example, if preferences are used with color restrict (color color restrict ), there is a possibility of the reverse traffic going through a different tunnel than what is expected based on the configured preferences.

Hope it clarifies the concept of the preference in SDWAN environment. 

Now let's talk about the other parameter named as "Weight". 

Weight as a parameter
Weight to use to balance traffic across multiple tunnels or multiple TLOCs. A higher value sends more traffic to the tunnel. You typically set the weight based on the bandwidth of the TLOC. 

When a vEdge router has multiple TLOCs, all with the highest preference, traffic distribution is weighted according to the configured weight value. 


For example, if TLOC A has weight 10, and TLOC B has weight 1, and both TLOCs have the same preference value, then roughly 10 flows are sent out TLOC A for every 1 flow sent out TLOC B.

So in short, you can say that when ever you need to use as Primary secondary path, use preference and when you want to put more traffic on one specific path or TLOC and less on other path or TLOC, then use weight.

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