[c-nsp] An attribute with less priority than local preference for choosing a best path.

[Jakob Heitz (jheitz)]

Consider the cost community.
IOS-XR implements the pre-bestpath and the after IGP-nexthop-cost point of insertion.
https://datatracker.ietf.org/doc/html/draft-ietf-idr-custom-decision
https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r7-6/routing/command/reference/b-routing-cr-asr9000-76x/routing-policy-language-commands.html#wp3193927332
https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r7-6/routing/configuration/guide/b-routing-cg-asr9000-76x/implementing-bgp.html#con_1390979

Regards,
Jakob.

-----Original Message-----
From: Nick Hilliard <nick at foobar.org>
Drew Weaver wrote on 03/06/2022 16:48:
> Would it be better to adjust MED?

MED tends to be a better idea because it will accumulate the IGP metric 
with the BGP metric, which means you can fine-tune your routing in a 
dynamic basis in a way that you can't do with just localpref.

If you use MED, then you probably want:

router bgp xxxx
  bgp bestpath med always

Otherwise the best path selection algorithm will select the oldest route 
in favour of a MED.

The selection algorithm is documented here:

> https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

Worth reading.  Localpref overrides as-path length.  MEDs don't.

If you're a leaf network, you may also want to normalise the origin type 
to either IGP or EGP.  Some transit providers change theirs to IGP, 
which will cause traffic to unnaturally swing over to them, so higher 
sales for them.

Nick