Adds BGP option related to MED handling.

Adds option 'med metric' allows to compare MEDs
between routes received from different neighbors.

3 files changed, 14 insertions, 2 deletions

diff --git a/proto/bgp/attrs.c b/proto/bgp/attrs.c
index e1a3671..95eee9a 100644
--- a/proto/bgp/attrs.c
+++ b/proto/bgp/attrs.c
@@ -1118,7 +1118,17 @@ 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))
+  /* This is noncompliant. Proper RFC 4271 path selection cannot be
+   * interpreted as finding the best path in some ordering.
+   * Therefore, it cannot be implemented in BIRD without some ugly
+   * hacks. This is just an approximation, which in specific
+   * situations may lead to persistent routing loops, because it is
+   * nondeterministic - it depends on the order in which routes
+   * appeared. But it is also the same behavior as used by default in
+   * Cisco routers, so it is probably not a big issue.
+   */
+  if (new_bgp->cf->med_metric || old_bgp->cf->med_metric ||
+      (bgp_get_neighbor(new) == bgp_get_neighbor(old)))
     {
       x = ea_find(new->attrs->eattrs, EA_CODE(EAP_BGP, BA_MULTI_EXIT_DISC));
       y = ea_find(old->attrs->eattrs, EA_CODE(EAP_BGP, BA_MULTI_EXIT_DISC));
diff --git a/proto/bgp/bgp.h b/proto/bgp/bgp.h
index b06f20a..097faa6 100644
--- a/proto/bgp/bgp.h
+++ b/proto/bgp/bgp.h
@@ -25,6 +25,7 @@ struct bgp_config {
   int missing_lladdr;			/* What we will do when we don' know link-local addr, see MLL_* */
   int gw_mode;				/* How we compute route gateway from next_hop attr, see GW_* */
   int compare_path_lengths;		/* Use path lengths when selecting best route */
+  int med_metric;			/* Compare MULTI_EXIT_DISC even between routes from differen ASes */
   int igp_metric;			/* Use IGP metrics when selecting best route */
   int prefer_older;			/* Prefer older routes according to RFC 5004 */
   u32 default_local_pref;		/* Default value for LOCAL_PREF attribute */
diff --git a/proto/bgp/config.Y b/proto/bgp/config.Y
index e932a7f..93f832a 100644
--- a/proto/bgp/config.Y
+++ b/proto/bgp/config.Y
@@ -25,7 +25,7 @@ CF_KEYWORDS(BGP, LOCAL, NEIGHBOR, AS, HOLD, TIME, CONNECT, RETRY,
 	ADVERTISE, IPV4, CAPABILITIES, LIMIT, PASSIVE, PREFER, OLDER,
 	MISSING, LLADDR, DROP, IGNORE, ROUTE, REFRESH, INTERPRET,
 	COMMUNITIES, BGP_ORIGINATOR_ID, BGP_CLUSTER_LIST, IGP, TABLE,
-	GATEWAY, DIRECT, RECURSIVE)
+	GATEWAY, DIRECT, RECURSIVE, MED)
 
 CF_GRAMMAR
 
@@ -79,6 +79,7 @@ bgp_proto:
  | bgp_proto GATEWAY DIRECT ';' { BGP_CFG->gw_mode = GW_DIRECT; }
  | bgp_proto GATEWAY RECURSIVE ';' { BGP_CFG->gw_mode = GW_RECURSIVE; }
  | bgp_proto PATH METRIC bool ';' { BGP_CFG->compare_path_lengths = $4; }
+ | bgp_proto MED METRIC bool ';' { BGP_CFG->med_metric = $4; }
  | bgp_proto IGP METRIC bool ';' { BGP_CFG->igp_metric = $4; }
  | bgp_proto PREFER OLDER bool ';' { BGP_CFG->prefer_older = $4; }
  | bgp_proto DEFAULT BGP_MED expr ';' { BGP_CFG->default_med = $4; }