Extended communities ASN wildcard.

[Ondrej Zajicek]

On Mon, Mar 13, 2023 at 01:01:16PM +0000, Ochalski, Radoslaw via Bird-users wrote:
> Hi,
> 
> I was wondering if there is a way to match extended communities via all ASNs wildcard?
> 
> As per https://bird.network.cz/?get_doc&v=16&f=bird-5.html, it’s not possible:
> 
> EC sets use similar expressions like pair sets, e.g. (rt, 123, 10..20) or (ro, 123, *). Expressions requiring the translation (like (rt, *, 3)) are not allowed (as they usually have 4B range for ASNs).
> 
> This is kind a surprising, however maybe I am missing something?

Hi

Because our implementation of community sets is just a balanced tree with
intervals, expressions like '(*, 123)' for regular community sets are
implemented by expanding it to 64k nodes, while '(123, *)' is just one
node.

For ECs, expanding to 4G nodes would be unfeasible, and one could argue
that expressions like '(rt, *, 3)' do not really make sense, as it is the
global part - ASN, who is supposed to assign meaning to the local part,
so expecting consistent local parts is just misguided policy.

Note that for large communities, one can use for-cycle iteration over
bgp_large_community to implement pretty much any kind of policy:

  for lc c in bgp_large_community do
    if c.data1 = 123 then ...

But we do not have accessors for ext communities.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."