Docs: added information about difference in export table usage on v2/3.

diff --git a/doc/migration-bird3.md b/doc/migration-bird3.md
index 1a088d59f4b810ebb97a7c39c53ceb153b047a9b..6098c10fbb40d8a1337802ac2b174115cd4aec64 100644 (file)
--- a/doc/migration-bird3.md
+++ b/doc/migration-bird3.md
@@ -93,3 +93,7 @@ internal attribute, and when it is too high, the route is discarded.
 The export table now shows the state immediately before sending on the wire,
 thus without custom attributes and after all updates done by BGP. This should
 help with performance a little, while also being more accurate.
+
+When changing the export filter the export table is used in BIRD 3, but ignored in BIRD 2.
+This means that BIRD 2 sends all routes again, regardless of having export table enabled. 
+BIRD 3 is smarter and uses the export table to filter out routes when export filter has changed.