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-rw-r--r--doc/bird.sgml16
1 files changed, 11 insertions, 5 deletions
diff --git a/doc/bird.sgml b/doc/bird.sgml
index 5ee9562..8fa55f8 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -784,7 +784,7 @@ for each neighbor using the following configuration parameters:
<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: infinite.
+ is missing. Default: 0.
<tag>default bgp_local_pref <m/number/</tag> Value of the Local Preference
to be used during route selection when the Local Preference attribute
@@ -806,10 +806,16 @@ with `<tt/O/') are optional.
selection among multiple BGP routes (see the selection rules above). It's
used as an additional metric which is propagated through the whole local AS.
- <tag>int <cf/bgp_med/ [IO]</tag> The Multiple Exit Discriminator of the route
- is an optional attribute which is often used within the local AS to
- reflect interior distances to various boundary routers. See the route selection
- rules above for exact semantics.
+ <tag>int <cf/bgp_med/ [O]</tag> The Multiple Exit Discriminator of the route
+ is an optional attribute which is used on on external (inter-AS) links to
+ convey to an adjacent AS the optimal entry point into the local AS.
+ The received attribute may be also propagated over internal BGP links
+ (and this is default behavior). The attribute value is zeroed when a route
+ is exported from a routing table to a BGP instance to ensure that the attribute
+ received from a neighboring AS is not propagated to other neighboring ASes.
+ A new value might be set in the export filter of a BGP instance.
+ See RFC 4451<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt">
+ for further discussion of BGP MED attribute.
<tag>enum <cf/bgp_origin/</tag> Origin of the route: either <cf/ORIGIN_IGP/
if the route has originated in an interior routing protocol or