bgpd: Blackhole nexthops are not reachable

When bgp registers for a nexthop that is not reachable due
to the nexthop pointing to a blackhole, bgp is never going
to be able to reach it when attempting to open a connection.

Broken behavior:

<show bgp nexthop>
 192.168.161.204 valid [IGP metric 0], #paths 0, peer 192.168.161.204
  blackhole
  Last update: Thu Feb 11 09:46:10 2021

eva# show bgp ipv4 uni summ fail
BGP router identifier 10.10.3.11, local AS number 3235 vrf-id 0
BGP table version 40
RIB entries 78, using 14 KiB of memory
Peers 2, using 54 KiB of memory

Neighbor             EstdCnt DropCnt ResetTime Reason
192.168.161.204            0       0     never Waiting for peer OPEN

The log file fills up with this type of message:
2021-02-09T18:53:11.653433+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:21.654005+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:31.654381+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:41.654729+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:51.655147+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument

As that the connect to a blackhole is correctly rejected by the kernel

Fixed behavior:

eva# show bgp ipv4 uni summ
BGP router identifier 10.10.3.11, local AS number 3235 vrf-id 0
BGP table version 40
RIB entries 78, using 14 KiB of memory
Peers 2, using 54 KiB of memory
Neighbor             V         AS   MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt Desc
annie(192.168.161.2) 4      64539    126264        39        0    0    0 00:01:36           38       40 N/A
192.168.161.178      4          0         0         0        0    0    0    never       Active        0 N/A
Total number of neighbors 2
eva# show bgp ipv4 uni summ fail
BGP router identifier 10.10.3.11, local AS number 3235 vrf-id 0
BGP table version 40
RIB entries 78, using 14 KiB of memory
Peers 2, using 54 KiB of memory
Neighbor             EstdCnt DropCnt ResetTime Reason
192.168.161.178            0       0     never Waiting for NHT
Total number of neighbors 2
eva# show bgp nexthop
Current BGP nexthop cache:
 192.168.161.2 valid [IGP metric 0], #paths 38, peer 192.168.161.2
  if enp39s0
  Last update: Thu Feb 11 09:52:05 2021
 192.168.161.131 valid [IGP metric 0], #paths 0, peer 192.168.161.131
  if enp39s0
  Last update: Thu Feb 11 09:52:05 2021
 192.168.161.178 invalid, #paths 0, peer 192.168.161.178
  Must be Connected
  Last update: Thu Feb 11 09:53:37 2021
eva#

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

diff --git a/bgpd/bgp_nht.c b/bgpd/bgp_nht.c
index bc5da0ee21d769d97731563ae0ffce9407bc9c09..5e31c3d86fa9dfe5f544f018191426919408ae42 100644 (file)
--- a/bgpd/bgp_nht.c
+++ b/bgpd/bgp_nht.c
@@ -843,9 +843,17 @@ static void evaluate_paths(struct bgp_nexthop_cache *bnc)
        if (peer) {
                int valid_nexthops = bgp_isvalid_nexthop(bnc);
 
-               if (valid_nexthops)
-                       peer->last_reset = PEER_DOWN_WAITING_OPEN;
-               else
+               if (valid_nexthops) {
+                       /*
+                        * Peering cannot occur across a blackhole nexthop
+                        */
+                       if (bnc->nexthop_num == 1
+                           && bnc->nexthop->type == NEXTHOP_TYPE_BLACKHOLE) {
+                               peer->last_reset = PEER_DOWN_WAITING_NHT;
+                               valid_nexthops = 0;
+                       } else
+                               peer->last_reset = PEER_DOWN_WAITING_OPEN;
+               } else
                        peer->last_reset = PEER_DOWN_WAITING_NHT;
 
                if (!CHECK_FLAG(bnc->flags, BGP_NEXTHOP_PEER_NOTIFIED)) {