Friday, March 21, 2008

Cisco CCNP / BSCI Exam Tutorial: EIGRP Stub Routing

By: Chris Bryant

Passing the BCSI exam and earning your CCNP certification requires you to know OSPF stub areas inside and out. Stub areas, total stub areas, a little study on not-so-stub stub areas ... and pretty soon your head is swimming. Then when you hear that EIGRP offers stub routing, your first reaction may be unprintable! But while EIGRP stub routing is effective in the right situation, it's not as complex as OSPF stub routing. Let's take a look at basic EIGRP stub routing.

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

For example, consider a network where R5 is the hub and R4, R6, and R7 are the spokes.

As long as R4, R6, and R7 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.

EIGRP stub routing doesn't give us all the options that OSPF stub routing does, but it is much simple to configure and can greatly reduce unnecessary Query packet transmission in a hub-and-spoke network.

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNP exam with Chris Bryant!

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