bgp: Don't apply ImportExportPolicy to withdrawn nlri

Import/Export filters are meant to modify which advertised prefixes
are allowed. For Import, this is simply an allow-list that accepts a
subset of the advertised nlri in a received update. For Export, this
is an allow-list that accepts a subset of the locally originated nlri.
In neither case do you want to apply these filters to the list of
withdrawn nlri, as this can result in stale routes if a legitimate
withdrawal is not sent or received.

Fixes: #330

Signed-off-by: Trey Aspelund <[email protected]>

bgp/src/session.rs

@@ -1896,7 +1896,6 @@ impl<Cnx: BgpConnection + 'static> SessionRunner<Cnx> {
                 .collect::<BTreeSet<crate::messages::Prefix>>();
 
             update.nlri.retain(|x| message_policy.contains(x));
-            update.withdrawn.retain(|x| message_policy.contains(x));
         };
 
         let out = match self.shape_update(update, shaper_application)? {
@@ -1982,7 +1981,6 @@ impl<Cnx: BgpConnection + 'static> SessionRunner<Cnx> {
                 .collect::<BTreeSet<crate::messages::Prefix>>();
 
             update.nlri.retain(|x| message_policy.contains(x));
-            update.withdrawn.retain(|x| message_policy.contains(x));
         };
 
         self.update_rib(&update, id, peer_as);