Please help our simple bgp

Jimmy Hess mysidia at
Sat Feb 4 16:51:25 UTC 2012

On Mon, Jan 30, 2012 at 8:27 PM, Ann Kwok <annkwok80 at> wrote:

> We discover the routes is going to ISP A only even the bandwidth 100M is
> full
> Can we set the weight to change to ISP B to use ISP B as preference routes?
> Can the following configuration work?

Tuning weights and  localpreference values can influence traffic that your
equipment sends _to_ ISP A and ISP B.   These options do not control what
incoming traffic  gets forwarded into your network _from_  ISP A or ISP
B;   you need a separate strategy for incoming bits.

The config you listed should do just what the vendor documentation says it
does, so I can't say it doesn't "work";  it just  does nothing to help
remedy the situation.

That is, if you have two ISP links each 100M full duplex;  and one of them
is at 100% outbound usage,   increasing the weight of all the other
neighbor's paths  assuming the set of prefixes received over BGP are the
same,   will mean that ISP B is the preferred next-hop for each path;
which means ISP A  outgoing utilization should drop to near 0,  and  then
ISP B  should be just as fully utilized as ISP A  is currently.

You could instead identify some specific paths that are heavy users or
would carry a high percentage of the outgoing traffic,   and  use
filters/route maps  to  adjust local preference  of specific paths,  to
take outgoing load off ISP A.

or  increase the weight for   on routes received from ISP B,
and allow your outgoing traffic  to rest of the address space  to utilize
ISP A, for example.

But the preferred fix for this problem would be to upgrade ISP A and ISP B
links to at least double their current capacity.

Weight is a vendor-specific parameter,  local to your router.  I would
consider increasing the default local preference for ISP A first, by the

But as long as you only have one router on which ISP A and ISP B sessions
when you have an identical prefix from ISP A and B,  the outgoing path
ISP B  is to be preferred by your local router,  if the path has a higher

"  What suggest to this weight no. too? "

With weights the magnitude of the numbers and the numerical difference
between weights is not significant;  it just matters if one path has a
higher weight, or both paths have equal weight.

I would suggest weights that are uniformly spaced apart and easy to
remember, e.g. 100 200 300 400.    When you want to add  ISP C  later,  you
will also have flexibility  without re-assigning your existing weights.

If this works, how is ISP B upstream connection is down?
> Can it still be failover to ISP A automatically?
> If it won't work, Do you have any suggestion?
> Thank you for your help


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