From: Voss, David (dvoss@heidrick.com)
Date: Fri Mar 14 2003 - 00:19:24 GMT-3
I have dual circuits at work. I think PREPEND is a very weak method in to
try to dictate traffic mainly because it will not work in every scenario.
What if you prepend 5 5 5 5 5 to make one circuit less preffered but your
"customer" is:
CUSTOMER 100 200 300 5 5 5 5 5
YOUR AS
CUSTOMER 100 200 300 400 500 600 700 800 9000 1000 1100 1200 1300 1400
YOUR AS
The customer will still prefer the circuit that you prepend in this case.
I have chosed to advertise one network out a T1 and one network out our DS3.
I then use advertise-map / non-exist-map to advertise the network out the
other circuit should the router or circuit fail.
The other obvious method is to play with MED's, but I had the luxery of 2
Class C network to play with.
Regardless, AS-PATH prepend, I believe, is not a 100% reliable way to dicate
traffic.
-----Original Message-----
From: Anthony Pace [mailto:anthonypace@fastmail.fm]
Sent: Thursday, March 13, 2003 12:43 PM
To: ccielab@groupstudy.com
Subject: BGP load balancing (via best path)
All of the implementations I have seen, (as well as the examples in
Halabi's book) use a Primary/Failover method for both INGRESS and EGRESS
traffic when BGP is used. Outbound advertisements with AS-PREPEND'S to
the backup to cause the "world" to come in via the Primary; and perhaps
better local preference for routes learned on the PRimary to cause our
outbound traffic to use the primary.
There are many flavors of this,but the principle remains consistant. For
any given route a primary and a failover.
Here is my question:
What is the detriment of taking in the all BGP routes from 2 providers
and choosing the best path for outbound traffic; What is the detriment of
advertising my own address space to 2 providers with no PREPENDS. Just
let the traffic come in and go out based on the "end systems" proximity
to either of my 2 providers?
Without AS-PREPENDS to the BACKUP provider, I understand that if the
PRIMARY provider gave me a block of their address space, I would wind up
seeing the traffic all come in via the BACKUP link (as the backup
provider would advertise a more specific, route and the PRIMARY would
summarize my space into an aggrigate)
Does that sound correct?
If the address space belonged to neither provider, then couldn't I just
advertise it to both and let the traffic find me through both?
Am I overlooking something very obvious?
Anthony Pace CCIE#10349
-- Anthony Pace anthonypace@fastmail.fm-- http://www.fastmail.fm - One of many happy users: http://www.fastmail.fm/docs/quotes.html
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