|
List Info Thread: Multihomed to 2 ISPs - Load Balance?
[1-4]
|
||||||||||||||||||||||||||||||||||||
|
about | contact Other archives ( Real Estate discussion Medical topics ) |
||||||||||||||||||||||||||||||||||||
Multihomed to 2 ISPs - Load Balance? |
|
List Info Thread: Multihomed to 2 ISPs - Load Balance?
[1-4]
|
||||||||||||||||||||||||||||||||||||
|
about | contact Other archives ( Real Estate discussion Medical topics ) |
||||||||||||||||||||||||||||||||||||
(why dangerous? something breaks in your leak mechanism and
you drop 'full
internet routes' on ospf/eigrp/isis... network go boom! it
is fun to
watch though.)
> >> install EBGP ECMP routes, how do we advertise
this information to our
> >> downstream peers? As far as my working
knowledge of BGP4 goes, it wouldnt
> >> let me do this.
correct... BGP selects 'best path' and sends that along to
it's neighbors.
There is a flag on one vendor I believe to force it to send
'all paths',
but this is also dangerous, or could be if misused. Perhaps
someone who's
used that feature could speak up?
> >> I wish to understand how other network
operators do this?
>
> > You don't, not usually anyway. You advertise the
best path to your
> > downstreams. If you want to 'load balance' per
packet or otherwise to one
> > or more upstreams that's an internal/your AS
decision only. There's
> > nothing to tell the downstreams about from BGP's
point of view.
> >
> I think there is a need to tell my downstream peers
about ASes the
> traffic is gonna go through.
There isn't a facility in bgp to tell a neighbor more than
one possible
aspath... or not one that most network folk use currently.
I suppose for a subset of routes you might hack up some
community based
solution, but it'd be a horrible hack, and it'd cause you
to keep churning
your router configs on a very regular basis as things up
stream changed.
If the downstream has a connection only to you does it
matter where they
send packets? everything has to go through your AS to get
anywhere...
right? If they have a multihomed solution (you and another
isp) they are
going to have to decide on some other internal metric
(interal to them
based perhaps on non-routing-table information, like 'john
has a oc-12 to
provider-Y, Jim only has a T1.... send to John!') whete to
send traffic.
>
> I'm thinking wildly, and it may not make a lot of
sense but heres the
> scenario i have in mind: You load balance (per stream
which is usually
per flow... is the normal terminology I think, but sure.
> what most of the vendors do) and you distribute your
traffic through
> ASes 10 and 20. Now you are advertising only one BGP
path, say the one
> through AS 10. Isnt this a problem? Isnt
"Advertise what you yourself
> use" one of the basic shibboleths of BGP or
routing for that matter?
>
BGP will only pick 'one best path', So, unless you did
some local static
or IGP based thing (see the leak suggestion above) you'll
only really be
using one path to AS10 or AS20, and only be sending
internally (then
externally on the other side of the network) one path.
If you were sending to AS10 initially and that link failed
or otherwise
became 'worst path' you churn on your edge then ship an
update with new
path info along to your ebgp peers... They have to then
churn and decide
which path is 'best' and move forward. What benefit is
there in sending
them 2 paths? They still must remove a path and re-converge,
eh? (if you
could even send them 2 paths of course)
Oh, and to throw in another monkey wrench... if you really
wanted to do
this for some reason you COULD provide ebgp-multihop peers
to your border
routers to all customers (ebgp neighbors) that wanted this
'service'...
again, this is messy and ugly, but it'd get them multiple
copies of the
same route, they could then decide on 'best path' based on
this
information. (this also is not recommended, just a thought)
(glad someone atleast replied offline 
cluenet.de -- dr