Calculating EIGRP Retransmission Time-Out (RTO)

[rod.sizemore]

How does the IOS determine the "Retransmission Time-Out" (RTO) value displayed for EIGRP neighbors with the "show ip eigrp neighbor" command?

Is there a default value or does the router use some formula to calculate this value?

[thisisshanky]

The IOS waits for the RTO time period, before which it retransmits a packet from the retransmission queue for the neighbor.

EIGRP sends hellos as multicast packets to neighbors, on 224.0.0.10.

When reply to the multicast fails, from the neighbor (say neighbor is dead), it starts unicasting the hello packet that particular neighbor. RTO is the time that the software waits for receipt of the acknowledgement for the Unicast hello sent. That means RTO is the time interval between each unicast hello message sent. I think EIGRP software tries this for 10 successive times.

I think the default value of RTO is 200 ms...... and it is increased by the EIGRP software, each time it sends a unicast (and neighbor fails to respond each time). If you wanna test this, first create neighbor relationship between 2 routers using EIGRp, clear the neihbor using clear ip eigrp nei command, and check on the other router, the increase in RTO.