From 67d412fe1af9acfb134372f34c18d16257e3e63d Mon Sep 17 00:00:00 2001
From: Trisha Biswas <tbiswas@fastly.com>
Date: Fri, 14 Apr 2023 04:28:37 +0200
Subject: [PATCH] BGP: Add 'allow bgp_med' option for EBGP sessions

This option allows to treat bgp_med as regular transitive attribute
on EBGP sessions (without hacks in filters).

Minor changes from committer.
---
 doc/bird.sgml      | 15 +++++++++++----
 proto/bgp/attrs.c  |  2 +-
 proto/bgp/bgp.h    |  1 +
 proto/bgp/config.Y |  1 +
 4 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/doc/bird.sgml b/doc/bird.sgml
index f75fa9fe..557140b8 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -2625,12 +2625,19 @@ using the following configuration parameters:
 	keeps MED attribute). Default: disabled.
 
 	<tag><label id="bgp-allow-local-pref">allow bgp_local_pref <m/switch/</tag>
-	A standard BGP implementation do not send the Local Preference attribute
-	to eBGP neighbors and ignore this attribute if received from eBGP
+	Standard BGP implementations do not send the Local Preference attribute
+	to EBGP neighbors and ignore this attribute if received from EBGP
 	neighbors, as per <rfc id="4271">.  When this option is enabled on an
-	eBGP session, this attribute will be sent to and accepted from the peer,
+	EBGP session, this attribute will be sent to and accepted from the peer,
 	which is useful for example if you have a setup like in <rfc id="7938">.
-	The option does not affect iBGP sessions. Default: off.
+	The option does not affect IBGP sessions. Default: off.
+
+	<tag><label id="bgp-allow-med">allow bgp_med <m/switch/</tag>
+	Standard BGP implementations do not propagate the MULTI_EXIT_DESC
+	attribute unless it is configured locally. When this option is enabled
+	on an EBGP session, this attribute will be sent to the peer regardless,
+	which is useful for example if you have a setup like in <rfc id="7938">.
+	The option does not affect IBGP sessions. Default: off.
 
 	<tag><label id="bgp-allow-local-as">allow local as [<m/number/]</tag>
 	BGP prevents routing loops by rejecting received routes with the local
diff --git a/proto/bgp/attrs.c b/proto/bgp/attrs.c
index 1e234b16..204151c3 100644
--- a/proto/bgp/attrs.c
+++ b/proto/bgp/attrs.c
@@ -1834,7 +1834,7 @@ bgp_update_attrs(struct bgp_proto *p, struct bgp_channel *c, rte *e, ea_list *at
 
     /* MULTI_EXIT_DESC attribute - accept only if set in export filter */
     a = bgp_find_attr(attrs0, BA_MULTI_EXIT_DISC);
-    if (a && !(a->fresh))
+    if (a && !a->fresh && !p->cf->allow_med)
       bgp_unset_attr(&attrs, pool, BA_MULTI_EXIT_DISC);
   }
 
diff --git a/proto/bgp/bgp.h b/proto/bgp/bgp.h
index 0461ea28..302f58e7 100644
--- a/proto/bgp/bgp.h
+++ b/proto/bgp/bgp.h
@@ -108,6 +108,7 @@ struct bgp_config {
   int interpret_communities;		/* Hardwired handling of well-known communities */
   int allow_local_as;			/* Allow that number of local ASNs in incoming AS_PATHs */
   int allow_local_pref;			/* Allow LOCAL_PREF in EBGP sessions */
+  int allow_med;			/* Allow BGP_MED in EBGP sessions */
   int allow_as_sets;			/* Allow AS_SETs in incoming AS_PATHs */
   int enforce_first_as;			/* Enable check for neighbor AS as first AS in AS_PATH */
   int gr_mode;				/* Graceful restart mode (BGP_GR_*) */
diff --git a/proto/bgp/config.Y b/proto/bgp/config.Y
index a2dfa747..013d14af 100644
--- a/proto/bgp/config.Y
+++ b/proto/bgp/config.Y
@@ -199,6 +199,7 @@ bgp_proto:
  | bgp_proto ALLOW LOCAL AS ';' { BGP_CFG->allow_local_as = -1; }
  | bgp_proto ALLOW LOCAL AS expr ';' { BGP_CFG->allow_local_as = $5; }
  | bgp_proto ALLOW BGP_LOCAL_PREF bool ';' { BGP_CFG->allow_local_pref = $4; }
+ | bgp_proto ALLOW BGP_MED bool ';' { BGP_CFG->allow_med = $4; }
  | bgp_proto ALLOW AS SETS bool ';' { BGP_CFG->allow_as_sets = $5; }
  | bgp_proto GRACEFUL RESTART bool ';' { BGP_CFG->gr_mode = $4; }
  | bgp_proto GRACEFUL RESTART AWARE ';' { BGP_CFG->gr_mode = BGP_GR_AWARE; }