bgp: implement the "replace-peer-as" configuration option

Signed-off-by: Renato Westphal <[email protected]>

holo-bgp/src/events.rs

@@ -166,6 +166,7 @@ fn process_nbr_update(
                 rib,
                 reach.prefixes.clone(),
                 attrs,
+                instance.config.asn,
                 instance.shared,
                 &instance.state.policy_apply_tasks,
             );
@@ -192,6 +193,7 @@ fn process_nbr_update(
                         rib,
                         prefixes,
                         attrs,
+                        instance.config.asn,
                         instance.shared,
                         &instance.state.policy_apply_tasks,
                     );
@@ -208,6 +210,7 @@ fn process_nbr_update(
                         rib,
                         prefixes,
                         attrs,
+                        instance.config.asn,
                         instance.shared,
                         &instance.state.policy_apply_tasks,
                     );
@@ -257,7 +260,8 @@ fn process_nbr_reach_prefixes<A>(
     nbr: &Neighbor,
     rib: &mut Rib,
     nlri_prefixes: Vec<A::IpNetwork>,
-    attrs: Attrs,
+    mut attrs: Attrs,
+    local_asn: u32,
     shared: &InstanceShared,
     policy_apply_tasks: &PolicyApplyTasks,
 ) where
@@ -278,6 +282,12 @@ fn process_nbr_reach_prefixes<A>(
         PeerType::External => RouteType::External,
     };
 
+    if nbr.config.as_path_options.replace_peer_as {
+        // Replace occurrences of the peer's AS in the AS_PATH with the local
+        // autonomous system number.
+        attrs.base.as_path.replace(nbr.config.peer_as, local_asn);
+    }
+
     // Update pre-policy Adj-RIB-In routes.
     let table = A::table(&mut rib.tables);
     let route_attrs = rib.attr_sets.get_route_attr_sets(attrs.clone());

holo-bgp/src/packet/attribute.rs

@@ -705,6 +705,16 @@ impl AsPath {
         }
     }
 
+    pub(crate) fn replace(&mut self, from: u32, to: u32) {
+        for segment in self.segments.iter_mut() {
+            for member in segment.members.iter_mut() {
+                if *member == from {
+                    *member = to;
+                }
+            }
+        }
+    }
+
     pub(crate) fn contains(&self, asn: u32) -> bool {
         self.segments.iter().any(|segment| segment.contains(asn))
     }