RTP and DUAL

Reliable Transport Protocol

The Reliable Transport Protocol (RTP) manages the delivery and reception of EIGRP packets. Reliable delivery means that delivery is guaranteed and that packets will be delivered in order. Guaranteed delivery is accomplished by means of a Cisco – proprietary algorithm known as reliable multicast, using the reserved class D address 224.0.0.10. Each neighbor that is receiving a reliable multicast packet, will unicast an acknowledgment.

Ordered delivery is ensured by including two sequence number in the packet. Each packet includes a sequence number assigned by the sending router. This sequence number is incremented by one each time the router sends a new packet. In addition, the sending router places in the packet the sequence number of the last packet received from the destination route.

In some cases, RTP may be unreliable delivery. No acknowledgement is required, and no sequence number will be included for unreliably delivery EIGRP packets.

DUAL finite state machine

DUAL, the Diffusing Update Algorithm, is the algorithm used by Cisco’s EIGRP routing protocol to ensure that a given route is recalculated globally whenever it might cause a routing loop. According to Cisco, the full name of the algorithm is DUAL finite-state machine (DUAL FSM). EIGRP is responsible for the routing within an autonomous system and DUAL responds to changes in the routing topology and dynamically adjusts the routing tables of the router automatically.

EIGRP uses a feasibility condition to ensure that only loop-free routes are ever selected. The feasibility condition is all routes to a destination although some are loop-free.

When no feasible route to a destination is available, the DUAL algorithm invokes a Diffusing Computation to ensure that all traces of the problematic route are eliminated from the network.