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Thread Index Re:[isp-bgp] isp-bgp digest: December 07, 2004
Hello All
We must be careful to distinguish
between load-sharing and load-balancing. Even I
blurred the distinction of what I was talking about in
my last post. Load-balancing (with mulipath) is not be
possible when multihomed to two AS'es but load sharing
is - for outbound traffic.
But here's a possible solution:
When multi-homed to two AS'es,
1. Arrange to receive full routing updates from both.
2. Then weed out ip prefixes that belong to the
specific AS'es only. You'll need a pencil and paper
and the internet for this.
3. Take the balance list of prefixes that is left, and
divide it into two sets that are roughly equal. For
prefixes common to both i.e advertised by both, use a
route-map for each BGP peer in the neighbor statements
and assign a weight (assuming cisco implementation) to
one set of incoming routes with corresponding
access-lists - so that matching routes prefer the path
thru the corresponding BGP neighbor, and another
route-map matching the other set of incoming routes
(access-list deny {1st set} permit {2nd set}),
assingning a weight to matching routes preferring the
second ISP. You will have to update the access-lists
manually and regularly ....
Your comments?
Referring to Ejay's post, AS prepending is only
possible on the advertising router, let me clarify.
And getting your ISP to modify routes for you will
cost you in the best case - in which case you need not
assign weights on your side. Worst case is that most
likely your request will be be ignored or refused...
Rgrds
Rahul Sawarkar
--- ISP-BGP Discussion List digest
<isp-bgp@...> wrote:
> ISP-BGP Digest for Tuesday, December 07, 2004.
>
> 1. Re: load balancing with EBGP
>
>
----------------------------------------------------------------------
>
> Subject: Re: load balancing with EBGP
> From: "Ejay Hire" <ejayhire@...>
> Date: Tue, 07 Dec 2004 05:31:30 -0600
> X-Message-Number: 1
>
> Hello.
>
> Outbound load balancing to two different providers
> is absolutely possible!
>
> The simplest way is to accept only a default route
> from each provider, and
> leave ip route-cache turned on on the interfaces
> connected to the ISP. In
> this scenario, the trafffic will not be "truly" load
> balanced, because
> traffic to one destination will always take only one
> link because of he
> route-cache. This is called "per-destination load
> balancing" and it works
> really well.
>
> The second option, which is slightly more
> complicated to configure, allows
> you to still use the best path abilities of BGP and
> still quasi-load
> balance. To do this, you accept a full routing
> table from both providers,
> and then use route-maps to modify the routes
> recieved from each provider to
> make the routes recieved roughly equal in as-path
> length. This sounds
> really complicated, but in practice it's not too
> bad.
>
> Example 1.
>
> ISP1 MCI --- EXAMPLENETWORK -- ISP2 ATT
> In this scenario, you have two isps of roughly the
> same size. Simply
> accepting a full routing table from both providers
> would probably result in
> decent load balancing. It will never be exactly
> 50/50, but should be ok.
>
> Example 2.
> ISP1 MCI --- EXAMPLENETWORK -- ISP2 Mom&PopIsp
> For this situation, you have a big isp, and a small
> ISP. To load balance
> here will require you to as-prepend the routes from
> MCI as you recieve them,
> to make the smaller ISP's routes look better to your
> router.
>
> All of this has been related to outbound (from you
> to the internet) traffic.
> It is also possible to roughly load-balance your
> traffic inbound from the
> internet. The quick version of this is you use
> as-prepends, and in some
> cases provider defined communities to make the
> internet see both of the
> routes you advertise as roughly equal.
>
> Other thoughts.
> Sockeye, which has been bought by Internap, and they
> are selling it under a
> different name makes a box that will dynamically
> adjust bgp and do load
> balancing outbound. We demo'd one, and it works.
> Personally, I felt it was
> too expensive, but internap may have dropped the
> pricing.
>
> Two default routes should not tax the memory of even
> the lowest cisco
> router. A full routing table from two providers
> will barely squeeze into
> 128mb, and you should look for a router with 256mb
> if you don't want to
> upgrade in a year.
>
> <plug>
> I offer BGP and Network consulting services. If you
> are interested in
> outsourcing this to someone with years of ISP and
> enterprise experience,
> please keep me in mind.
> </plug>
>
> Ejay Hire
> Network Engineer
> CCNP/CCDP
>
> >On Sun, 21 Nov 2004 21:15:05 -0800 (PST), suraj
> <suraj_rt@...> wrote:
> > > Hi,
> > >=20
> > > Please provide me the solution to the following
> > > problem;
> > >=20
> > > Protocol running :BGP
> > > Scenario: We have a single router where two
> different
> > > links of that router are terminated with two
> differnt
> > > service providers.These links are used for
> internet to
> > > ours users sitting in our Lan.Is it possible to
> make
> > > utilize these two links simultaniously( Is it
> posible
> > > to load balance these two links using the BGP,I
> > > believe Load balancing is not possible with BGP
> as BGP
> > > takes the best path).PLease let me know whether
> is
> > > there any other way to acheive load sharing/load
> > > balancing with BGP?
> > >=20
> > > Suraj
> > > Consultant
> > > ACC technologies
>
>
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> ---
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> END OF DIGEST
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