From: Joerg Kohlenz (j.kohlenz@itknetwork.com)
Date: Fri Jan 03 2003 - 07:01:36 GMT-3
Hi,
you are right. If imagine there were the same router id's than it would
choose the oldest on, so entry 2. But in the decision process the rid is
coming before he looks at the age of the entry.
Hope it helps !
Bye,
joerg
-----Urspr|ngliche Nachricht-----
Von: nobody@groupstudy.com [mailto:nobody@groupstudy.com] Im Auftrag von
cebuano
Gesendet: Freitag, 3. Januar 2003 06:46
An: ccielab@groupstudy.com
Betreff: Clarification needed on BGP and MEDs
Hi group.
I need to clarify my reading of this CCO page.
The following examples demonstrate how the bgp deterministic med and bgp
always-compare-med commands can influence MED-based path selection.
Note: Cisco Systems recommends enabling the bgp deterministic med
command in all new network rollouts. For existing networks, the command
must either be deployed on all routers at the same time, or
incrementally, with care to avoid possible internal BGP (iBGP) routing
loops.
For example, consider the following routes for network 10.0.0.0/8:
entry1: AS(PATH) 500, med 150, external, rid 172.16.13.1
entry2: AS(PATH) 100, med 200, external, rid 1.1.1.1
entry3: AS(PATH) 500, med 100, internal, rid 172.16.8.4
The order in which the BGP routes were received is entry3, entry2, and
entry1 (entry3 is the oldest entry in the BGP table and entry1 is the
newest one).
Note: When BGP receives multiple routes to a particular destination, it
lists them in the reverse order they were received, from the newest to
the oldest. BGP then compares the routes in pairs starting with the
newest entry and moving toward the oldest entry (starting at top of the
list and moving down). For example, entry1 and entry2 are compared. The
best of these two is then compared to entry3, and so on.
Example 1: Both Commands Disabled
Entry1 and entry2 are compared first. Entry2 is chosen as the best of
these two because it has a lower router ID. The MED is not checked since
the paths are from a different neighbor AS.
Shouldn't example 1 instead say, "Entry2 is chosen as the best of these
two because it has an OLDER received path"?
Step 10 of the BGP Bestpath Selection states.
"When both paths are EXTERNAL, prefer the OLDEST path."
Isn't Entry2 "older" than Entry1?
Or is something wrong with my understanding of this English?
TIA.
Elmer
BTW - do you guys/gals have a recommendation on very good lab scenarios
to bring the true IOS behavior of these different BGP "knobs" to life?
There are so many changes to the 12.2 release that it makes me not want
to dwell too much on Halabi's book. I wish Doyle had THREE chapters on
this instead of two (pity he spent three chapters on Multicast instead).
.
.
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