Smaller than a /24 for BGP?

Jon Lewis jlewis at
Wed Jan 25 23:39:31 UTC 2023

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:

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

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.

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
_________ for PGP public key_________

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