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CCNP / BSCI Exam Tutorial: EIGRP Stub Routing |
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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.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages.
For a FREE copy of his latest e-books, “How To Pass
The CCNA” and “How To Pass The CCNP”, visit
the website and download your free copies. You can also get
FREE CCNA and CCNP exam questions every day! Pass the CCNA
exam with The Bryant Advantage!
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