Implements deterministic MED handling.

Thanks to Alexander V. Chernikov for many suggestions.

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 7f53f02..b8dabf4 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -1316,6 +1316,21 @@ for each neighbor using the following configuration parameters:
 	received from the same AS (which is the standard behavior).
 	Default: off.
 
+	<tag>deterministic med <m/switch/</tag> BGP route selection
+	algorithm is often viewed as a comparison between individual
+	routes (e.g. if a new route appears and is better than the
+	current best one, it is chosen as the new best one). But the
+	proper route selection, as specified by RFC 4271, cannot be
+	fully implemented in that way. The problem is mainly in
+	handling the MED attribute. BIRD, by default, uses an
+	simplification based on individual route comparison, which in
+	some cases may lead to temporally dependent behavior (i.e. the
+	selection is dependent on the order in which routes appeared).
+	This option enables a different (and slower) algorithm
+	implementing proper RFC 4271 route selection, which is
+	deterministic. Alternative way how to get deterministic
+	behavior is to use <cf/med metric/ option. Default: off.
+
 	<tag>igp metric <m/switch/</tag> Enable comparison of internal
  	distances to boundary routers during best route selection. Default: on.