----- Original Message -----
From: "Zhang, Ou (David)" <OuDavid.Zhang@gs.com>
> All,
Hi David:
>
> Can someone name the pros and cons of using local preference and MED ?
> For instance, one problem of using local preference is that it overwrites
> as-path. The problem of using MED is that you have to rely on other
> organizations. Thanks.
I tend to use local preference for broad classification of prefixes by
source where you want to influence path decision before AS_PATH is
evaluated, in order to implement things like:
- prefer customer routes over peering or transit (set localpref high)
- prefer peering routes over transit (set localpref middle)
- take transit path as last option (set localpref low)
which is a good policy for financial routing: your customers pay you so
prefer their prefixes across their links regardless of how much they pad the
AS_PATH, your peers are no-cost or low-cost so prefer prefixes advertised to
you from them over any higher-cost transit regardless of AS_PATH, then
default to transit as a last-choice path.
I've used MED on backup circuits with peers, ensuring that no traffic takes
the backup unless the primary is not working, and it works quite well with
peers if they're accepting your MEDs and sending you reasonable ones -- many
peers like the ability to do their own traffic engineering in your network,
and some may insist on it.
I haven't otherwise found MEDs very useful as a means to statically
influence internal path decisions, since I generally want traffic to follow
the IGP if localpref is equal, AS_PATH is equal, and I don't have a *very*
good reason (read: internal congestion) to want to preempt normal path
selection via IGP. The additional route-map complexity of statically
defining MEDs at the border and taking this into account for traffic
engineering is often not worth it -- and you can find yourself in a
situation where a link fails and the MEDs are making your congestion worse
by preempting IGP.
cheers.
-travis
>
> -David
>
>
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