highly recommended for all IP routing environments. There should never be more than seven hops between any two routing devices on an internetwork. The propagation delay and query process across multiple hops when changes occur may slow down convergence of the network.
  • Number of alternate paths through the network: A network should provide alternate paths to avoid single points of failure. However, too many alternate paths can also create EIGRP convergence problems, because the EIGRP routing process needs to use queries to explore all possible paths for lost routes. This complexity creates an ideal condition for a router to become stuck in active (SIA) as it awaits a response to queries that are being propagated through these many alternate paths.

    • Content 2.6 Using EIGRP in the Enterprise 2.6.2 EIGRP Queries As an advanced distance vector protocol, EIGRP relies on neighboring routers to provide routing information. When a router loses a route and does not have a feasible successor in its topology table, it looks for an alternate path to the destination. This is referred to as going active on a route. A route is considered passive when a router is not performing recompilation on that route. The router sends query packets to all neighbors on interfaces other than the one used to reach the previous successor (split horizon), inquiring whether they have a route to the given destination. If a router has an alternate route, it answers the query and does not propagate it further. If a neighbor does not have an alternate route, it queries each of its own neighbors for an alternate path. The queries then propagate through the network, creating an expanding tree of queries. When a router answers a query, it stops the spread of the query through that branch of the network; however, the query can still spread through other branches of the network as other routers attempt to find alternate paths, which might not exist.
    Content 2.6 Using EIGRP in the Enterprise 2.6.3 SIA Connections EIGRP uses a reliable multicast approach to search for an alternate to a lost route. Therefore, it is imperative that EIGRP receive a reply for each query it generates in the network.Once a route goes active and the query sequence is initiated, the route can only come out of the active state and move to passive state when it receives a reply for every generated query. If the router does not receive a reply to all the outstanding queries within 3 minutes (the default time), the route goes to the SIA state. SIA routes can be some of the most challenging problems to resolve in an EIGRP network. When a route goes to SIA state, the querying router resets the neighbor relationship to the neighbor that fails to reply. This setting causes the router to go active on all routes known through the lost neighbor and to readvertise all the routes that it knows about to the lost neighbor. The most common reasons for SIA routes are as follows:
    Content 2.6 Using EIGRP in the Enterprise 2.6.4 EIGRP Stubs The stability of large-scale EIGRP networks is often dependent on the range of queries through the network. Hub-and-spoke network topologies commonly use stub routing. In a hub-and-spoke topology, the remote router forwards all traffic that is not local to a hub router; the remote router does not need to retain a complete routing table. Generally, the hub router needs to send only a default route to the remote routers. In a hub-and-spoke topology, having a full routing table on the remote router serves no functional purpose, because the path to the corporate network and the Internet is always through the hub router. Additionally, having a full routing table at the spoke router increases the amount of memory required. Route summarization and route filtering can be used to conserve bandwidth and memory requirements on the spoke routers. Traffic from a hub router should not use a remote router as a transit path. A typical connection from a hub router to a remote router has significantly less bandwidth than a connection at the network core. Attempting to use the connection to a remote router as a transit path typically results in excessive congestion, as illustrated in Figure . The EIGRP stub routing feature can prevent this problem by restricting the remote router from advertising the hub router routes back to other hub routers. Figure illustrates how routes recognized by the remote router E from hub router A are not advertised to hub router B. Because the remote router does not advertise the hub routes back to the hub routers, the hub routers do not use the remote routers as a transit path.
    Content 2.6 Using EIGRP in the Enterprise 2.6.5 EIGRP Stubs To improve network stability, reduce resource utilization, and simplify stub router configuration, you can configure the EIGRP stub routing feature. This feature was first introduced in Cisco IOS Software Release 12.0(7)T, and it can be configured on remote routers only. A stub router sends a special peer information packet to all neighboring routers to report its status as a stub router. Any neighbor that receives a packet informing it of the stub status does not query the stub router for any routes. Therefore, a router that has a stub peer does not query that peer; instead, hub routers connected to the stub router answer the query on behalf of the stub router. The stub routing feature does not prevent routes from being advertised to the remote router. The EIGRP stub routing feature also simplifies the configuration and maintenance of hub-and-spoke networks. When stub routing is enabled in dual-homed remote configurations, you do not have to configure filtering on remote routers to prevent them from appearing as transit paths to the hub routers. Caution
    EIGRP stub routing should be used on stub routers only. A stub router is defined as a router connected to the network core or hub layer through which core transit traffic should not flow. A stub router should only have hub routers for EIGRP neighbors. Ignoring this restriction may cause undesirable behavior.
    Content 2.6 Using EIGRP in the Enterprise 2.6.6 Configuring EIGRP Stubs To configure a router as an EIGRP stub, use the eigrp stub command, as illustrated in Figure . A router configured as a stub with this command shares information about connected and summary routes with all neighboring routers. Figure describes the parameters of the eigrp stub command. Note
    In OSPF, the stub command must be configured on the hub router and stub router. In EIGRP, the stub command is only configured on the actual stub router. The parameters of this command can be used in any combination, with the exception of the receive-only keyword. If one of these keywords, except receive-only, is used individually, the connected and summary routes are not sent automatically. The EIGRP stub routing feature does not automatically enable route summarization on the hub router. In most cases, route summarization should be configured on the hub routers. If a true stub network is required, the hub router can be configured to send a default route to the spoke routers. This approach is