Implements RFC 5004 - prefer older external routes.

4 files changed, 16 insertions, 3 deletions

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 0c2b8fbe..40793ed3 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -981,6 +981,11 @@ for each neighbor using the following configuration parameters:
 	<tag>path metric <m/switch/</tag> Enable comparison of path lengths
 	when deciding which BGP route is the best one. Default: on.
 
+	<tag>prefer older <m/switch/</tag> Standard route selection algorithm
+	breaks ties by comparing router IDs. This changes the behavior
+	to prefer older routes (when both are external and from different
+	peer). For details, see RFC 5004. Default: off.
+
 	<tag>default bgp_med <m/number/</tag> Value of the Multiple Exit
 	Discriminator to be used during route selection when the MED attribute
 	is missing. Default: 0.
diff --git a/proto/bgp/attrs.c b/proto/bgp/attrs.c
index d839ed09..9c7bc30a 100644
--- a/proto/bgp/attrs.c
+++ b/proto/bgp/attrs.c
@@ -1044,7 +1044,6 @@ bgp_rte_better(rte *new, rte *old)
     return 0;
 
   /* RFC 4271 9.1.2.2. c) Compare MED's */
-
   if (bgp_get_neighbor(new) == bgp_get_neighbor(old))
     {
       x = ea_find(new->attrs->eattrs, EA_CODE(EAP_BGP, BA_MULTI_EXIT_DISC));
@@ -1082,12 +1081,19 @@ bgp_rte_better(rte *new, rte *old)
   y = ea_find(old->attrs->eattrs, EA_CODE(EAP_BGP, BA_ORIGINATOR_ID));
   n = x ? x->u.data : new_bgp->remote_id;
   o = y ? y->u.data : old_bgp->remote_id;
+
+  /* RFC 5004 - prefer older routes */
+  /* (if both are external and from different peer) */
+  if ((new_bgp->cf->prefer_older || old_bgp->cf->prefer_older) &&
+      !new_bgp->is_internal && n != o)
+    return 0;
+
+  /* rest of RFC 4271 9.1.2.2. f) */
   if (n < o)
     return 1;
   if (n > o)
     return 0;
 
-
   /* RFC 4271 9.1.2.2. g) Compare peer IP adresses */
   return (ipa_compare(new_bgp->cf->remote_ip, old_bgp->cf->remote_ip) < 0);
 }
diff --git a/proto/bgp/bgp.h b/proto/bgp/bgp.h
index 0a82be2b..c487aafb 100644
--- a/proto/bgp/bgp.h
+++ b/proto/bgp/bgp.h
@@ -24,6 +24,7 @@ struct bgp_config {
   ip_addr source_addr;			/* Source address to use */
   int next_hop_self;			/* Always set next hop to local IP address */
   int compare_path_lengths;		/* Use path lengths when selecting best route */
+  int prefer_older;			/* Prefer older routes according to RFC 5004 */
   u32 default_local_pref;		/* Default value for LOCAL_PREF attribute */
   u32 default_med;			/* Default value for MULTI_EXIT_DISC attribute */
   int capabilities;			/* Enable capability handshake [RFC3392] */
diff --git a/proto/bgp/config.Y b/proto/bgp/config.Y
index 7360820f..94011750 100644
--- a/proto/bgp/config.Y
+++ b/proto/bgp/config.Y
@@ -22,7 +22,7 @@ CF_KEYWORDS(BGP, LOCAL, NEIGHBOR, AS, HOLD, TIME, CONNECT, RETRY, KEEPALIVE,
 	BGP_PATH, BGP_LOCAL_PREF, BGP_MED, BGP_ORIGIN, BGP_NEXT_HOP,
 	BGP_ATOMIC_AGGR, BGP_AGGREGATOR, BGP_COMMUNITY, SOURCE, ADDRESS,
 	PASSWORD, RR, RS, CLIENT, CLUSTER, ID, AS4, ADVERTISE, IPV4,
-	CAPABILITIES, LIMIT, PASSIVE)
+	CAPABILITIES, LIMIT, PASSIVE, PREFER, OLDER)
 
 CF_GRAMMAR
 
@@ -65,6 +65,7 @@ bgp_proto:
  | bgp_proto MULTIHOP expr VIA ipa ';' { BGP_CFG->multihop = $3; BGP_CFG->multihop_via = $5; }
  | bgp_proto NEXT HOP SELF ';' { BGP_CFG->next_hop_self = 1; }
  | bgp_proto PATH METRIC bool ';' { BGP_CFG->compare_path_lengths = $4; }
+ | bgp_proto PREFER OLDER bool ';' { BGP_CFG->prefer_older = $4; }
  | bgp_proto DEFAULT BGP_MED expr ';' { BGP_CFG->default_med = $4; }
  | bgp_proto DEFAULT BGP_LOCAL_PREF expr ';' { BGP_CFG->default_local_pref = $4; }
  | bgp_proto SOURCE ADDRESS ipa ';' { BGP_CFG->source_addr = $4; }