EIGRP Query Process

Although EIGRP is an advanced distance vector routing protocol and convergence time is low, an EIGRP router still relies on its neighbor to advertise routing information. To achieve fast convergence, EIGRP can't rely on a flush timer like IGRP. EIGRP needs to actively search for the lost routes for fast convergence. This process is called the query process, and it was briefly discussed in the previous few sections. In the query process, queries are sent when the primary route is lost and no feasible successors are available. At this stage, the route is said to be in the Active state.

Queries are sent out to all the neighbors and on all interfaces except for the interface to the successor. If the neighboring routers do not have the lost route information, more queries are sent to the neighboring routers' neighbors until the query boundary is reached. Query boundary consists of either the end of the network, the distribute list boundary, or the summarization boundary. The distribute list and summarization boundaries are defined by the router that has the distribute list or summarization configured. When the queries are sent, the router must wait for all the replies from the neighbors before the router calculates the successor information. If any neighbor fails to reply in three minutes, the route is said to be stuck in active (SIA), and the neighbor relationship of the router that didn't reply to the query is reset. Chapter 7, "Troubleshooting EIGRP," addresses the SIA problem and tells how to troubleshoot it in greater detail.