On Tue, 24 Jan 2023, Forrest Christian (List Account) wrote:
I have two thoughts in relation to this:
1) It's amazing how many threads end up ending in the (correct) summary that
making an even minor global change to the way the internet works and/or is
configured to
enable some potentially useful feature isn't likely to happen.
2) I'd really like to be able to tag a BGP announcement with "only use this
announcement as an absolute last resort" so I don't have to break my prefixes in
half in
those cases where I have a backup path that needs to only be used as a last
resort. (Today each prefix I have to do this with results in 3 prefixes in the
table where
one would do).
And yes. I know #2 is precluded from actually ever happening because of #1.
The irony is not lost on me.
With good upstreams (providing useful BGP communities support), you can do
this without cluttering the global table.
Say you have multiple upstreams, and you want provider A to treat a.b.c/23
as a "don't use this unless you have no other path" route. They should
support a community that allows you to cause them to set localpref lower
than their "peer default" (or transit if they admit to buying transit).
Then, when you advertise a.b.c/23 to both/all your providers, provider A
won't use the direct route unless they're not receiving it from any other
source. i.e. You don't have to advertise the /23 to A and a pair of /24s
to B, C, etc. to make sure A doesn't use the direct customer route.
I don't know that all "tier 1's" support it...but for example, Tata, GTT,
Cogent, and NTT do:
Tata:
Request for Local Preference Adjustment In AS6453 the default local
preference (LOCAL_PREF) value for customer-routes is 100, and for
peer-routes it is 90. Along the lines of RFC1998, a Tata Communications
customer may request other than thedefault local preference:
Adjust Local Preference
community action
6453:n, n={70, 80, 90, 110} assign local preferencen in AS6453
GTT:
3.2.2 Local Preference
Value Description
3257:1980 give routes localpref below normal customer route.
3257:1970 give routes localpref below normal peer route.
Cogent:
Local Preference
All customer routes announced to Cogent will have a local pref of 130.
The customer can control the local preference for their announcements by
using a community string that is passed to Cogent in the BGP session. The
following table lists the community strings and the corresponding local
preference that will be set when they are used.
Community String Local Pref Effect
174:10 10 Set customer route local preference to
10
(below everything-least preferred)
174:70 70 Set customer route local preference to
70
(below peers)
174:120 120 Set customer route local preference to
120
(below customer default)
174:125 125 Set customer route local preference to
125
(below customer default)
174:135 135 Set customer route local preference to
135
(above customer default)
174:140 140 Set customer route local preference to
140
(above customer default)
You get the idea. Everyone likely does it "their own way", so you need to
find the BGP community support info for the upstream with which you want
non-default behavior / localpref.
----------------------------------------------------------------------
Jon Lewis, MCP :) | I route
StackPath, Sr. Neteng | therefore you are
_________ http://www.lewis.org/~jlewis/pgp for PGP public key_________
