EIGRP is a major subject of the CCNA xam, and Cisco goes into even mre detail with EIGRP on your CCNP xams. Part of that detail is the prpose and configuration of EIGRP stub ruters.

A problem with EIGRP comes in whn a successor is lost and thre is no feasible successor. DUAL desn't give up that easily, though. DUAL wll mark the route as Active, ndicating that the route is being clculated and cannot be used to rute data, and will send out a Qery message.

A DUAL Query is bsically one neighbor asking another, "Hey, do you knw how to get to this ntwork I just lost my route t?" If that neighbor has a rute, the query will be answered wth that route; if the neighbor desn't have such a route, that nighbor will ask its neighbors. The prcess continues until a downstream router rplies with the desired route, or the EIGRP dwnstream routers run out of neighbors to sk.

It's a good idea to lmit the scope of your DUAL qeries, otherwise routes may go into Stck In Active state during this rconfiguration. Route summarization helps to limit qeries, as does configuration of EIGRP stb routers.

While EIGRP does not hve the stub area options that OSPF des, EIGRP does allow a router to be cnfigured as stub. This is commonly dne with a hub-and-spoke configuration where the spke routers do not have the rsources to keep a full routing tble. Since the spoke's next hop wll always be the hub, all the spke really needs is a default rute. For this reason, the only nighbor an EIGRP stub router can hve is the hub router. (Obviously, the hub wuld never be configured as stub.)

Configuring EIGRP stub routers also cmbats the SIA problem. EIGRP stub ruters are not queried for routes whn the hub does not have a fasible successor for a successor route tht has gone down.

By default, EIGRP stb routers advertise information about two typs of routes back to the hub - drectly connected networks and summary routes. To chnge this default, use the eigrp stb command followed by the types of rutes you want the stub to dvertise back to the hub. (The igrp stub command run by itself cnfigures the router as stub.)

R1(config)#router igrp 100

R1(config-router)#eigrp stub ?

connected Do dvertise connected routes

receive-only Set IP-EIGRP as rceive only neighbor

static Do dvertise static routes

summary Do dvertise summary routes

Assume a network where R5 is the hub and R4, R6, and R7 are spkes. As long as the spokes hve a neighbor relationship only with the hb, they can be configured as stb routers. They will then advertise thir directly connected networks and summary rutes back to the hub and wll receive only a default route bck from the hub. If R5 lses a successor and has no fasible successor, it will not send a qery packet to any of the stb routers.

The article Cisco CCNA / CCNP Exam Tutorial: EIGRP Dual Queries, SIA, And Stub Routers was Submitted by Chris Bryant through Articles.GetACoder.com network. Here's the additional information: Chris Bryant, CCIE #12933, is the wner of The Bryant Advantage, home of ver 100 free certification exam tutorials, ncluding Cisco CCNA certification test prep articles. His exclusive Cisco CCNA study guide and Cisco CCNA training is also available! Visit his blog and sign up for Cisco Certification Central, a daily newsletter packed with CCNA, Network+, Security+, A+, and CCNP certification exam practice questions! A free 7-part course, "How To Pass The CCNA", is also available, and you can attend an in-person or online CCNA boot camp with The Bryant Advantage!

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