North American Network Operators Group

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Re: BGP - weight

  • From: Danny McPherson
  • Date: Sat Feb 14 11:26:40 2004

On Feb 14, 2004, at 5:23 AM, Sven Huster wrote:

Dumb question:
If I apply a equal weight to all our transit/peers, will
that affect route announcements to iBGP or eBGP peers anyhow?
Yes, given that it's a local parameter (i.e., not BGP,
per se, though it does impact what's installed in the BGP
RIB and what's subsequently advertised to your peers),
you'll likely begin preferring more routes via eBGP
learned peers per subsequent attributes in the best path
selection algorithm (e.g., MED, AS_PATH, even LOCAL_PREF)
won't be considered.

We got a very simple setup:
- 2 routers on the border to transit/peers (that's were i want
  to set the weight)
- 1 edge router to customers
- core router running BGP as well

What I want to achieve is that traffic leaves through
the border router it arrived, rather than have it bounced
Perhaps you should first look at other reasons why this may
be occurring (e.g., accepting MEDs from one peer and not the
other, accepting MEDs from both but different policies are
employed to derive their values, AS_PATH "suggests" a better
path, etc..) -- then comes preference for eBGP over iBGP.

We had some recent issues were it looks like the core got
"out of sync" with the border (looks more like a sw issue than
just convergence delay) and packets bounced back and forth
between them.
This could be any of a number of things..  Without more
information I'd be hesitant to start tweaking knobs.

I know this doesn't solve the cause but the before digging
for the initial reason I want a quick workaround.
"Weight" is a very influential parameter.  I'm not a big fan
of configuring routing policies that are entirely local to a
system, for obvious reasons.  But I do suspect it would
accomplish what you're trying to achieve.