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EIGRP Dual Queries, SIA, and Stub Routers
EIGRP is a major subject of the CCNA exam, and Cisco goes into even more detail with EIGRP on your CCNP exams. Part of that detail is the purpose and configuration of EIGRP stub routers.
A problem with EIGRP comes in when a successor is lost and there is no feasible successor. DUAL doesn't give up that easily, though. DUAL will mark the route as Active, indicating that the route is being calculated and cannot be used to route data, and will send out a Query message.
A DUAL Query is basically one neighbor asking another, "Hey, do you know how to get to this network I just lost my route to?" If that neighbor has a route, the query will be answered with that route; if the neighbor doesn't have such a route, that neighbor will ask its neighbors. The process continues until a downstream router replies with the desired route, or the EIGRP downstream routers run out of neighbors to ask.
It's a good idea to limit the scope of your DUAL queries, otherwise routes may go into Stuck In Active state during this reconfiguration. Route summarization helps to limit queries, as does configuration of EIGRP stub routers.
While EIGRP does not have the stub area options that OSPF does, EIGRP does allow a router to be configured as stub. This is commonly done with a hub-and-spoke configuration where the spoke routers do not have the resources to keep a full routing table. Since the spoke's next hop will always be the hub, all the spoke really needs is a default route. For this reason, the only neighbor an EIGRP stub router can have is the hub router. (Obviously, the hub would never be configured as stub.)
Configuring EIGRP stub routers also combats the SIA problem. EIGRP stub routers are not queried for routes when the hub does not have a feasible successor for a successor route that has gone down.
By default, EIGRP stub routers advertise information about two types of routes back to the hub - directly connected networks and summary routes. To change this default, use the eigrp stub command followed by the types of routes you want the stub to advertise back to the hub. (The eigrp stub command run by itself configures the router as stub.)
R1(config)#router eigrp 100
R1(config-router)#eigrp stub ?
connected Do advertise connected routes
receive-only Set IP-EIGRP as receive only neighbor
static Do advertise static routes
summary Do advertise summary routes
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Assume a network where R5 is the hub and R4, R6, and R7 are spokes. As long as the spokes have a neighbor relationship only with the hub, they can be configured as stub routers. They will then advertise their directly connected networks and summary routes back to the hub and will receive only a default route back from the hub. If R5 loses a successor and has no feasible successor, it will not send a query packet to any of the stub routers.
About the Author
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com), home of free CCNP and CCNA tutorials! For my FREE "How To Pass The CCNA" or "CCNP" ebook, visit the website and download your copies. Pass your CCNP exam with The Bryant Advantage!
