From: Steven Hodgson (steven.hodgson@inxi.com)
Date: Tue Feb 12 2008 - 18:24:47 ARST
A combination of a & c.
MED is easy, but you need to understand it. It only matters if you have
multiple routes to the same network. A provider may give metrics to
certain routes to try to influence the BGP best path selection of your
AS. Obviously if they only advertise an aggregate route they lose some
granularity to their ability to control those metrics (only 1 route
versus many). So a provider could "leak" more specific routes to you if
they want to affect the metric of just those routes.
You will get it. Halabi gives a complicated explanation. Try labbing
it up and it will make more sense.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jersey Guy
Sent: Monday, February 11, 2008 8:24 PM
To: Cisco certification
Subject: can't understand BGP Theory
From Halabi's book, Internet Routing Architectures, 2nd edition, page
167:
*MEDs are somewhat handicapped by aggregation scenarios in which
providers
announce a given CIDR block from multiple locations in their network and
suppress the smaller routes from the block. Utilizing MEDs in this
scenario
could potentially result in suboptimal routing because the more-specific
routes of the CIDR block could be scattered throughout the AS and MEDs
associated with more-granular routes are no longer available.
When using MEDs to perform what's commonly referred to as best-exit
routing,
some providers leak the more-specifics of their CIDR blocks to select
peers
to remove the offshoots introduced by aggregation. The problem with this
is
that controlling the more-specific announcements is sometimes complex,
and
failure to do so can result in some very suboptimal routing situations.
*
I read the above two paragraphs five times but didn't understand it.
Which
of the following is true:
a) I have no choice but to understand this stuff, to pass the lab. I
need to
understand *everything* in Halabi's book, period.
b) The lab is tough but not THAT tough. I can skip certain convoluted
sections of every topic and still manage to get by.
c) Forget it. I am not going to make it. MED is a piece of cake; what's
so
hard to understand??
d) I need to read "How to grow gray matter and raise IQ" book before
Halabi's.
Thing is....how thoroughly do I need to pound away at theory/reading
before
hitting the equipment, lab scenarios and excercises?
thanks, JG
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