BGP: Improve custom BGP attributes

 - Implement EA_GET for custom BGP attributes
 - Forbid EA_SET on existing opaque attributes
 - Forbid redefining existing attributes
 - Document possible compatibility problems

1 files changed, 8 insertions, 4 deletions

diff --git a/doc/bird.sgml b/doc/bird.sgml
index a023473f..5e795450 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -3561,10 +3561,14 @@ some of them (marked with `<tt/O/') are optional.
 	name="local role"> is configured it set automatically.
 </descrip>
 
-<p>For attributes unknown by BIRD, the user can assign a name (on top level)
-to an attribute by its number. This defined name can be used then to both set
-(by a bytestring literal, transitive) or unset the given attribute even though
-BIRD knows nothing about it:
+<p>For attributes unknown by BIRD, the user can assign a name (on top level) to
+an attribute by its number. This defined name can be used then to get, set (as a
+bytestring, transitive) or unset the given attribute even though BIRD knows
+nothing about it.
+
+<p>Note that it is not possible to define an attribute with the same number
+as one known by BIRD, therefore use of this statement carries a risk of
+incompatibility with future BIRD versions.
 
 <tt><label id="bgp-attribute-custom">attribute bgp <m/number/ bytestring <m/name/;</tt>