[isp-bgp] RE: BGP advantages

[Dean Anderson <dean@...>]

> From: "Arie Vayner" <ariev@....il>
> 
> You would be loosing the ease of load balancing. The advantage of the
> Link Proof that it load balances automatically, where with BGP you would
> have to "tweak" it manually all the time.

People do tweak BGP to adjust load on links, but this is usually dubious.  
I've heard that an MIT group studying BGP has found that 20% of all
network failures are preceeded by BGP alterations by about 15 minutes.  
While not actually conclusive, the obvious suggestion is that admins cause
20% of the outages, and their activity is indicated 15 minutes prior to
failure.  Such tweaking can only directly affect routes taken by packets
going out, though communities do allow some adjustment of how advertised
routes are used.

> Also real load balancing would be effective only if you have many users
> and they are all going out on the internet with different ip addresses
> (and not via a single hide-nat address)
> BGP works well only on a highly statistical network (with many users and
> destinations)

I wouldn't go that far. BGP does not prevent fine grained load balancing.  
But the large router vendor uses a flow cache that sends all traffic in
that flow out the same interface. Thus, only new flows get to choose new
routes. This is good because the router doesn't have to search 150K routes 
for each packet, but the usually much smaller flow cache. The flow cache 
has also been acl checked, so that is quicker as well.  If a flow is 
already cached, the routing tables and acls won't be consulted.   Of 
course, a new hardware generation could keeps sets of outgoing interfaces, 
or simply expire the cache more quickly.

> From: Jon Lewis <jlewis@...>

> On Mon, 6 Sep 2004, Dean Anderson wrote:
> 
> > You need to have portable address space before BGP is going to useful.
> 
> That's not entirely true.  Multihoming to 2 or more providers using
> BGP to announce PA address space from one of your providers to all of your
> providers works just fine generally.  The main disadvantage is you're
> still tied to that provider for the IPs and will have to renumber if you
> want to dump that provider.

The more specific prefix will get advertised and used if both prefixes are
in table, but this isn't a /good thing/:  It creates untenable route table
growth.  You can imagine what might happen if another 100,000 companies
wanted to advertise /28's.  Also some ISPs will reject small prefixes, or 
won't accept non/16 prefixes from legacy class B space (verio).


		--Dean








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