Active and Stuck-in-Active Routes

160 In the example, there are two paths from Router D1A to 10.25.3.0 : one through D1C and the other through D1D. If both of these links have the same metric, then EIGRP uses equal-cost multipath routing. In most cases, the router simply alternates the traffic flows between the different paths. The packets belonging to a particular TCP session are called a flow. So equal cost multipath routing keeps all of these packets on the same path. However, as each new session is established, the router attempts to balance the paths since they have the same cost. If the packets from several different flows are coming sufficiently quickly, the router sends the second packet out the second path before the first packet has finished departing along the first path. This allows a simple form of load sharing between the paths. However, this form of load sharing is not terribly bandwidth efficient, so you will get considerably less than twice the bandwidth of one path in practice.

6.4.2 Active and Stuck-in-Active Routes

EIGRP uses an interesting technique for keeping its routing tables up-to-date. Even though it only uses the best route, the EIGRP topology table keeps a list of every path to every subnet. This way, if the best path goes away it can select a feasible successor. But if there are no feasible successors when a route disappears, the router puts this route into an ACTIVE state and queries its neighbors to find a new path to the destination. If one or more of the neighbors knows a path to this destination network or a summary route that contains this one, they respond. But if they do not have a route, they in turn query their neighbors. Sometimes the destination is simply nowhere to be found. This can happen because a failure somewhere in the network has isolated some subnets. Sometimes the process of trying to find a new path can fail to converge. In the ever-expanding chain of queries from one router to the next, each device is waiting for a response. If the network is too large or if it contains too many high-latency sections, it may become difficult for this process to converge. If the queries for an ACTIVE route are not satisfied within the timeout period of a few minutes, the router gives the dreaded Stuck In Active message. It then clears the neighbor relationship with the router that failed to respond. This can happen either because the route has disappeared or because a communication problem has broken the chain of queries somewhere in the network. Either way, Stuck In Active represents a serious problem, particularly if it happens repeatedly. When the routers in an EIGRP network issue large numbers of Stuck In Active messages, it is important to determine where thing are getting Stuck. This is by far more serious than the ACTIVE problem, which just means that a route is missing. When these messages appear, the network engineer should find out which neighbor relationships are being reset. This could be happening anywhere in the network, not necessarily on or adjacent to the router that reports the Stuck In Active. The easiest way to find these problems is to ensure that EIGRP is configured to log neighbor status changes. Then, when the Stuck In Active messages appear, attempt to track where the neighbor relationships are flapping. In some cases the neighbors that are changing are randomly dispersed throughout the network. This may indicate that the EIGRP AS has simply become too large to be stable. This can happen particularly when the automatic route summarization features of EIGRP are not used effectively.

6.4.3 Interconnecting Autonomous Systems