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Cisco CCNA / CCNP Certification: Introduction To BGP Attributes |
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BGP is one of the most complex topics you'll study when pursuing
your CCNP, if not the most complex. I know from personal experience
that when I was earning my CCNP, BGP is the topic that gave
me the most trouble at first. One thing I keep reminding today's
CCNP candidates about, though, is that no Cisco technology
is impossible to understand if you just break it down and
understand the basics before you start trying to understand
the more complex configurations.
BGP attributes are one such topic. You've got well-known
mandatory, well-known discretionary, transitive, and non-transitive.
Then you've got each individual BGP attribute to remember,
and the order in which BGP considers attributes, and what
attributes even are... and a lot more! As with any other Cisco
topic, we have to walk before we can run. Let's take a look
at what attributes are and what they do in BGP.
BGP attributes are much like what metrics are to OSPF, RIP,
IGRP, and EIGRP. You won't see them listed in a routing table,
but attributes are what BGP considers when choosing the best
path to a destination when multiple valid (loop-free) paths
exist.
When BGP has to decide between such paths, there is an order
in which BGP considers the path attributes. For success on
the CCNP exams, you need to know this order. BGP looks at
path attributes in this order:
Highest weight (Cisco-proprietary BGP value)
Highest local preference (LOCAL_PREF)
Prefer locally originated route.
Shortest AS_PATH is preferred.
Choose route with lowest origin code. Internal paths are
preferred over external paths, and external paths are preferred
over paths with an origin of "incomplete". Lowest
multi-exit discriminator (MED)
External BGP routes preferred over Internal BGP routes.
If no external route, select path with lowest IGP cost to
the next-hop router for iBGP.
Choose most recent route.
Choose lowest BGP RID (Router ID).
If you don't know what these values are, or how they're configured,
don't panic! The next several parts of this BGP tutorial will
explain it all. So spend some time studying this order, and
in part II of this free BGP tutorial, we'll look at each of
these values in detail. Keep studying!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNP and CCNA 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”, just visit the website!
You can also get FREE CCNA and CCNP exam questions every day!
Pass the CCNP exam with The Bryant Advantage!
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