EIGRP - Message TTL value of 2

EIGRP uses a TTL value of 2 in the IPv4 header of the messages it sends between EIGRP neighbors. Initially this may seem counterintuitive since by definition, EIGRP neighbors must be directly connected. Thus there will only ever be a single hop to reach the intended neighbor, therefore a TTL of 1 should suffice.

However, this TTL value of 2 is always used to specifically accommodate hub and spoke topologies such as Frame Relay or DMVPN. When you send a hello from spoke to spoke, it’s still on the same subnet, but due to the underlying hub and spoke technology, the packet must go through the hub router which will decrease the TTL by 1. Thus a TTL of 2 is needed.

EIGRP designers decided to let the TTL remain at a value of 2 rather than detect the topology type and have it changed accordingly. This simplifies the implementation of EIGRP.

Similarly, RIP also sets the TTL of all of its messages to 2 for the same reason.

https://forum.networklessons.com/t/ttl-2-of-eigrp-and-rip-packets-explained/1063/19?u=lagapidis

https://networklessons.com/rip/ttl-2-eigrp-rip-packets-explained