From: Gary Duncanson (gary.duncanson@googlemail.com)
Date: Tue Feb 12 2008 - 17:02:49 ARST
Multiexit discriminator. A way of telling the downstream AS about a preferred
path when there is more than one way into the AS announcing routes with with
different MEDs. Aggregate route announcements typically embrace IP space way
beyond what you want to influence so playing with MED may not be a good idea
in that case. You may find things heading the right way in some cases, perhaps
for the range you are interested in but in the case of the rest of the space
covered by the aggregate perhaps not at all...
Using MEDs with smaller announcements from the block can give you
configuration headaches and administration problems i.e scale.
Agree with Greg. MED is just one of many things you need to read in depth at
least as deep as it takes to understand what it's for and how to use it. If
it's any consolation when I started reading about BGP in 2001 I worked through
the papers by Pete Welcher who proceeded to advise one to read numerous URLs
and RFCs 'even the stuff you don't understand'. Persevere!
You need to clear the written before scheduling the lab, if you cover the
reading list thoroughly and patiently your practice will make a lot more
sense!
HTH
Gary
----- Original Message -----
From: "Jersey Guy" <guy.jersey@gmail.com>
To: "Cisco certification" <ccielab@groupstudy.com>
Sent: Tuesday, February 12, 2008 2:23 AM
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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