BGP - weight

Stephen J. Wilcox steve at telecomplete.co.uk
Sat Feb 14 18:49:05 UTC 2004


On Sat, 14 Feb 2004, Sven Huster wrote:

> 
> On Sat, Feb 14, 2004 at 12:46:09PM -0500, Stephen J. Wilcox 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?
> > 
> > No it wont affect announcements, weight is local to the router you apply it.
> > 
> > > What I want to achieve is that traffic leaves through 
> > > the border router it arrived, rather than have it bounced around.
> > 
> > eBGP should be preferred over iBGP anyhow assuming all other things are equal, 
> > if theyre not equal then either make them equal or you probably want to choose a 
> > different path anyhow (eg shorter as path). 
> > 
> > if you dont want any traffic to go across your network why bother meshing the 
> > ibgp in the first place?
> 
> Just to make it a bit more clear:
> 
> Transit1  Peers       Transit2  Peers           Customers via BGP
>    |        |            |        |                 |
>    ----R1----            ----R2----                 R3
>        |                     |                      |
>        |                     |                      |
>        |                     |                      |
>        ---------------------Core---------------------
>                              |
>                              |
>                          Data Center
> 
> Full-mesh between R1,R2,R3 and Core
> 
> 
> We carry traffic from the DC as well as the customers in the core to transit and peers.
> We normally want to advertise full routes to customers, which are multi-homed.
> 
> > 
> > > 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. I know this doesn't solve the
> > > cause but the before digging for the initial reason I want a quick workaround.
> > 
> > hmm, i'd suggest emergency maintenance before doing some weird screwy stuff like 
> > that :)
> 
> The thing that happend was that the core believed that the best path out is via
> R1, which R1 thought it was via R2. So a little loop there.
> 
> We weren't able to reproduce the problem nor to find a source yet.

Is this all the same vendor hardware?

Check the bgp configs are identical eg deterministic-med, dampening, 
always-compare-med etc are all configured the same..

Steve

> So the plan right now was: if the core decides that traffic should go out via
> R1, R1 just just send it out via the best path it got from eBGP.
> So that we get some more time for debugging what's going on there.
> 
> Sven
> 
> 




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