CCNP ROUTE 642-902 Exam Training:

EIGRP Stub Routing

By Chris Bryant, CCIE #12933

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Trying to keep the different OSPF stub area options straight was tough enough during your CCNA studies, but now that you're working toward your CCNP ROUTE exam pass, you've got to know EIGRP stub routing as well!

I know, I know.... you're not very excited to hear this.

I also know that keeping the stub, total stub, not-so-stubby stub, and Negative Zone stub areas in OSPF is a little tough at first - but you'll find EIGRP stubs to be easier to work with.

(Okay, I made that last stub area up. I was just catching up on my old FF comics last night. But you have to admit there's a certain ring to it.)

So what's the big difference between OSPF and EIGRP stub areas?

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 wasted bandwidth and router resources that occur as a result of a Stuck-In-Active (SIA) route.  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. 

Note the <cr> at the bottom of the options shown, which means that eigrp stub by itself is a legal command. The eigrp stub command run by itself simply 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
  <cr>

As long as R4, R6, and R7 have a neighbor relationship only with the hub, R5, they can be configured as EIGRP 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 DUAL query packet to any of the stub routers, since it knows they are indeed stub routers and cannot possibly have or acquire a replacement for the lost successor route.

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To your success,

Chris Bryant

CCIE #12933

"The Computer Certification Bulldog"

chris@thebryantadvantage.com