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Right now I have a 7204 load sharing four T1's. Two from MCI and two
from Global Crossing. We run BGP and announce a class C range that we
own, over both ISP's. We get full routing tables from both.

It has given us pretty even load-sharing considering both MCI and GBX
are Tier 1 providers. This method has been fine for years but it is
about to change.

The Global Crossing connection is going up to a partial DS3 (18Mb
woohoo!), while the MCI will remain at 3Mb total for now. It may go
up to 3 or 4 T1's later, but right now the ratio is 3Mb to 18Mb.

All Internet routes need to be available to both ISP's, so failover
and redundancy are important. I don't want to limit the BGP updates I
get from MCI to balance the links.

What is the best way to load-share disproportionate links? If at all
possible, I would like most of the traffic to go over the Global
Crossing T3 with only the closest ASN's getting time on MCI. And if
the Global Crossing DS3 took a dive, I'd want everything to route over
MCI until it was restored.

What is the best practice being used today to solve this question?

Or would it be better just to treat the MCI circuits as a pure backup
since in 95% of the instances, the Global circuit would be faster?


-Robert

Rob <> wrote:

>What is the best way to load-share disproportionate links? If at all
>possible, I would like most of the traffic to go over the Global
>Crossing T3 with only the closest ASN's getting time on MCI. And if
>the Global Crossing DS3 took a dive, I'd want everything to route over
>MCI until it was restored.

I'd keep prepending your AS onto your MCI advertisements until you've
reached an acceptable traffic split. If Gblx takes a dive, then the
MCI line will take the strain until it comes back. Use a route map and
set the prepend in there.

If you're going to utilise your 18MB cct fully, then failing over to a
3MB cct will bring along problems of its own. You'll flood the MCI cct
badly and latency will rise horribly. I'd sit down and think about the
traffic levels and types of traffic you need to pass over the MCI cct
during a Gblx outage very carefully.

Tim

Thanks Tim.
But doesn't prepending the AS affect traffic in only one direction?

Yes, I know that sending 18Mb through a 3Mb pipe would be bad, but
it's better than nothing. We're also contemplating bumping that up to
a MLPPP bundle of four T1's.



On Sat, 21 Feb 2004 13:39:43 GMT,
(Tim Thorne) wrote:

>Rob <> wrote:
>
>>What is the best way to load-share disproportionate links? If at all
>>possible, I would like most of the traffic to go over the Global
>>Crossing T3 with only the closest ASN's getting time on MCI. And if
>>the Global Crossing DS3 took a dive, I'd want everything to route over
>>MCI until it was restored.
>
>I'd keep prepending your AS onto your MCI advertisements until you've
>reached an acceptable traffic split. If Gblx takes a dive, then the
>MCI line will take the strain until it comes back. Use a route map and
>set the prepend in there.
>
>If you're going to utilise your 18MB cct fully, then failing over to a
>3MB cct will bring along problems of its own. You'll flood the MCI cct
>badly and latency will rise horribly. I'd sit down and think about the
>traffic levels and types of traffic you need to pass over the MCI cct
>during a Gblx outage very carefully.
>
>Tim

Rob <> wrote:

>Thanks Tim.
>But doesn't prepending the AS affect traffic in only one direction?

You can tag inbound routes with AS path prepends to engineer your
outbound traffic. I dislike leaving links idle. If something happens
to your announcements over that link you'll never see the problem
until you need to utilise the link. Your quickest and easiest solution
would be the one you've mentioned, use the MCI link purely as backup.

Tim

In article <>,
Tim Thorne <> wrote:
>Rob <> wrote:
>
>>Thanks Tim.
>>But doesn't prepending the AS affect traffic in only one direction?
>
>You can tag inbound routes with AS path prepends to engineer your
>outbound traffic. I dislike leaving links idle. If something happens
>to your announcements over that link you'll never see the problem
>until you need to utilise the link. Your quickest and easiest solution
>would be the one you've mentioned, use the MCI link purely as backup.
>
>Tim

Regardless of the approach you take, if higher availability is desired,
it is essential that you monitor the state of both links so repairs can
be initiated before the second link fails. Unless you routinely monitor
your BGP neighbor tables (does anyone?) or otherwise have a dedicated
monitor (such as WhatsUp Gold against the IP of the other end of the
link), the easiest way to detect a failure is to keep an eye on your
MRTG or other traffic monitor, use load sharing, and jump into action
any time you see one of the links drop to zero traffic.

Good luck and have fun!
--
Vincent C Jones, Consultant Expert advice and a helping hand
Networking Unlimited, Inc. for those who want to manage and
Tenafly, NJ Phone: 201 568-7810 control their networking destiny
http://www.networkingunlimited.com

On Sat, 21 Feb 2004 08:11:23 -0500, Rob <> wrote:

>Right now I have a 7204 load sharing four T1's. Two from MCI and two
>from Global Crossing. We run BGP and announce a class C range that we
>own, over both ISP's. We get full routing tables from both.
>
>It has given us pretty even load-sharing considering both MCI and GBX
>are Tier 1 providers. This method has been fine for years but it is
>about to change.
>
>The Global Crossing connection is going up to a partial DS3 (18Mb
>woohoo!), while the MCI will remain at 3Mb total for now. It may go
>up to 3 or 4 T1's later, but right now the ratio is 3Mb to 18Mb.
>
>All Internet routes need to be available to both ISP's, so failover
>and redundancy are important. I don't want to limit the BGP updates I
>get from MCI to balance the links.
>
>What is the best way to load-share disproportionate links? If at all
>possible, I would like most of the traffic to go over the Global
>Crossing T3 with only the closest ASN's getting time on MCI. And if
>the Global Crossing DS3 took a dive, I'd want everything to route over
>MCI until it was restored.
>
>What is the best practice being used today to solve this question?
>
>Or would it be better just to treat the MCI circuits as a pure backup
>since in 95% of the instances, the Global circuit would be faster?

I'd either treat the MCI circuit purely as a backup, or accept only
partial routes and a default from MCI (i.e., MCI & their direct
customers only), which will cause everything else to be sent via GBX.
If you're really opposed to getting customer-only routes from MCI, get
them all and use a route-map to raise the local preference of routes
that only have AS701 in the AS-PATH, or ask MCI if they can send you
all routes but tag customer routes with a community which you can use
to differentiate them.

-Terry

On Sat, 21 Feb 2004 12:52:01 -0600, Terry Baranski
<0VE> wrote:


>If you're really opposed to getting customer-only routes from MCI, get
>them all and use a route-map to raise the local preference of routes
>that only have AS701 in the AS-PATH, or ask MCI if they can send you
>all routes but tag customer routes with a community which you can use
>to differentiate them.
>
>-Terry

That sounds interesting. Is there a writeup on CCO?

-Robert

On Sat, 21 Feb 2004 14:07:18 -0500, Rob <> wrote:

>On Sat, 21 Feb 2004 12:52:01 -0600, Terry Baranski
><0VE> wrote:
>
>
>>If you're really opposed to getting customer-only routes from MCI, get
>>them all and use a route-map to raise the local preference of routes
>>that only have AS701 in the AS-PATH, or ask MCI if they can send you
>>all routes but tag customer routes with a community which you can use
>>to differentiate them.
>>
>>-Terry
>
>That sounds interesting. Is there a writeup on CCO?

http://www.cisco.com/en/US/tech/tk36...8009456d.shtml
may help, though the book "Internet Routing Architectures" should be
considered required reading for those who work with BGP.

I forgot to mention that you'll also want to manipulate your outbound
advertisement to MCI in some way so that only MCI and it's direct
customers use your MCI link to get traffic to you. Best to talk to
MCI and see what they recommend, as they may have a specific community
you can tag your advertisement with to accomplish this based on GBX
(probably) being a peer of theirs. If not, prepending is probably the
way to go.

-Terry