Arista EOS Tunneling Mechanism (1) - Tunnel vs NHG/Decap

In Arista EOS, there are 2 types of tunnel mechanisms,

1. Regular tunnel interface - starting with "interface tunnel 10"
2. Next-hop-group + decap-group; 

The first one is easy to understand and other vendors have similar features. The second one is very unique. Based on the EOS manual, section 35.2 and 35.3 

• The decap group is a data structure that receives encapsulated packets and extracts the payload.
• A nexthop group is a data structure that defines a list of nexthop addresses and a tunnel type for packets routed to the specified address.

Comparison between Tunnel and next-hop-group/decap-group:

• The tunnel is 2-way vs NHG/Decap is a 1-way
• The tunnel is for both control plane and data plane, while NHG/Decap is ONLY for data plane, so no routing protocol over it
• Before 4.21.1F, the tunnel is software forwarding which is capped by the CoPP policy. NHG/Decap is hardware forwarding from day-1. 

Or you can think about it in this way:

• The tunnel interface is a traditional tunnel with a routing protocol, which is responsible for programing the forwarding information. 
• The nhg/decap is purely a data plane tunnel, and network admin can use EAPI to program the router's forwarding. 

In the following blog posts, will cover the details of configuration and troubleshooting for some different flavor of setup. Here is a (probably uncompleted) list of EOS NHG/DecapG and tunnel features