EIGRP Stub Routing Cisco CCNP / BSCI Exam Tutorial

EIGRP Stub Routing Cisco CCNP / BSCI Exam Tutorial
Passing the BCSI exam & earning your CCNP certification requires you to know OSPF stub areas inside & out. Stub areas, total stub areas, a little study on not-so-stub stub areas ... & 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 is 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 permit 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 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 one types of routes back to the hub - directly connected networks & summary routes. To alter 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

static Do advertise static routes

receive-only Set IP-EIGRP as receive only neighbor

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

summary Do advertise summary routes

As long as R4, R6, & R7 have a neighbor relationship only with the hub, they can be configured as stub routers. they will then advertise their directly connected networks & summary routes back to the hub & will receive only a default route back from the hub. If R5 loses a successor & has no feasible successor, it won't 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 & can greatly reduce unnecessary Query packet transmission in a hub-and-spoke network.