bgpd: support for DF election in EVPN-MH

DF (Designated forwarder) election is used for picking a single
BUM-traffic forwarded per-ES. RFC7432 specifies a mechanism called
service carving for DF election. However that mechanism has many
disadvantages -
1. LBs poorly.
2. Doesn't allow for a controlled failover needed in upgrade
scenarios.
3. Not easy to hw accelerate.

To fix the poor performance of service carving alternate DF mechanisms
have been proposed via the following drafts -
draft-ietf-bess-evpn-df-election-framework
draft-ietf-bess-evpn-pref-df

This commit adds support for the pref-df election mechanism which
is used as the default. Other mechanisms including service-carving
may be added later.

In this mechanism one switch on an ES is elected as DF based on the
preference value; higher preference wins with IP address acting
as the tie-breaker (lower-IP wins if pref value is the same).

Sample output
=============
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
torm-11# sh bgp l2vpn evpn es 03:00:00:00:00:01:11:00:00:01
ESI: 03:00:00:00:00:01:11:00:00:01
 Type: LR
 RD: 27.0.0.15:6
 Originator-IP: 27.0.0.15
 Local ES DF preference: 100
 VNI Count: 10
 Remote VNI Count: 10
 Inconsistent VNI VTEP Count: 0
 Inconsistencies: -
 VTEPs:
  27.0.0.16 flags: EA df_alg: preference df_pref: 32767
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
torm-11# sh bgp l2vpn evpn route esi 03:00:00:00:00:01:11:00:00:01
*> [4]:[03:00:00:00:00:01:11:00:00:01]:[32]:[27.0.0.15]
                    27.0.0.15                          32768 i
                    ET:8 ES-Import-Rt:00:00:00:00:01:11 DF: (alg: 2, pref: 100)
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/bgpd/bgp_attr_evpn.c b/bgpd/bgp_attr_evpn.c
index aa0c59f3a7..7cc9ecd79e 100644
--- a/bgpd/bgp_attr_evpn.c
+++ b/bgpd/bgp_attr_evpn.c
@@ -26,6 +26,7 @@
 #include "log.h"
 #include "memory.h"
 #include "stream.h"
+#include "vxlan.h"
 
 #include "bgpd/bgpd.h"
 #include "bgpd/bgp_attr.h"
@@ -146,6 +147,43 @@ uint8_t bgp_attr_default_gw(struct attr *attr)
 }
 
 /*
+ * Fetch and return the DF preference and algorithm from
+ * DF election extended community, if present, else 0.
+ */
+uint16_t bgp_attr_df_pref_from_ec(struct attr *attr, uint8_t *alg)
+{
+	struct ecommunity *ecom;
+	int i;
+	uint16_t df_pref = 0;
+
+	*alg = EVPN_MH_DF_ALG_SERVICE_CARVING;
+	ecom = attr->ecommunity;
+	if (!ecom || !ecom->size)
+		return 0;
+
+	for (i = 0; i < ecom->size; i++) {
+		uint8_t *pnt;
+		uint8_t type, sub_type;
+
+		pnt = (ecom->val + (i * ECOMMUNITY_SIZE));
+		type = *pnt++;
+		sub_type = *pnt++;
+		if (!(type == ECOMMUNITY_ENCODE_EVPN
+		      && sub_type == ECOMMUNITY_EVPN_SUBTYPE_DF_ELECTION))
+			continue;
+
+		*alg = (*pnt++) & ECOMMUNITY_EVPN_SUBTYPE_DF_ALG_BITS;
+
+		pnt += 3;
+		pnt = ptr_get_be16(pnt, &df_pref);
+		(void)pnt; /* consume value */
+		break;
+	}
+
+	return df_pref;
+}
+
+/*
  * Fetch and return the sequence number from MAC Mobility extended
  * community, if present, else 0.
  */