isisd: fix metric calculation of classic lfa backup prefixes

The isis-lfa-topo1 topotest shows backup routes with lower metrics than
the primary ones.

> rt1# show isis route
> Area 1:
> IS-IS L1 IPv6 routing table:
>
>  Prefix                Metric  Interface  Nexthop                    Label(s)
>  ------------------------------------------------------------------------------
>  2001:db8:1000::1/128  0       -          -                          -
>  2001:db8:1000::2/128  20      eth-rt2    fe80::d091:eeff:fe09:31cd  -
>  2001:db8:1000::3/128  20      eth-rt3    fe80::946b:d4ff:fe5b:414b  -
>  2001:db8:1000::4/128  20      eth-rt4    fe80::5ced:29ff:feed:59c4  -
> (...)
>
> rt1# show isis route backup
> Area 1:
> IS-IS L1 IPv6 routing table:
>
>  Prefix                Metric  Interface  Nexthop                    Label(s)
>  ------------------------------------------------------------------------------
>  2001:db8:1000::2/128  15      eth-rt3    fe80::946b:d4ff:fe5b:414b  -
>                                eth-rt5    fe80::847d:feff:fe74:bdde  -
>                                eth-rt6    fe80::ac8c:dff:feac:8a8d   -
>  2001:db8:1000::3/128  15      eth-rt2    fe80::d091:eeff:fe09:31cd  -
>                                eth-rt5    fe80::847d:feff:fe74:bdde  -
>  2001:db8:1000::4/128  45      eth-rt5    fe80::847d:feff:fe74:bdde  -
> (...)

Backup routes metrics are incorrect because they only take into account
the path metric but not the prefix metric.

Add the prefix metric to the path metric on backup routes.

After the patch with a prefix metric of 10:

> rt1# show isis route backup
> Area 1:
> IS-IS L1 IPv6 routing table:
>
> Area 1:
> IS-IS L1 IPv6 routing table:
>
>  Prefix                Metric  Interface  Nexthop                    Label(s)
>  ------------------------------------------------------------------------------
>  2001:db8:1000::2/128  25      eth-rt3    fe80::8c6f:8aff:fe10:ad0d  -
>                                eth-rt5    fe80::b08e:5cff:fe90:62dd  -
>                                eth-rt6    fe80::4810:47ff:fe81:2b9a  -
>  2001:db8:1000::3/128  25      eth-rt2    fe80::e855:12ff:fe31:5765  -
>                                eth-rt5    fe80::b08e:5cff:fe90:62dd  -
>  2001:db8:1000::4/128  55      eth-rt5    fe80::b08e:5cff:fe90:62dd  -

Fixes: e886416f81 ("isisd: add support for classic LFA (RFC 5286)")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>

1 files changed, 11 insertions, 7 deletions

diff --git a/isisd/isis_lfa.c b/isisd/isis_lfa.c
index 800cac852..b348c876d 100644
--- a/isisd/isis_lfa.c
+++ b/isisd/isis_lfa.c
@@ -1836,7 +1836,7 @@ static bool clfa_loop_free_check(struct isis_spftree *spftree,
 				 struct isis_vertex *vertex_S_D,
 				 struct isis_spf_adj *sadj_primary,
 				 struct isis_spf_adj *sadj_N,
-				 uint32_t *lfa_metric)
+				 uint32_t *path_metric)
 {
 	struct isis_spf_node *node_N;
 	uint32_t dist_N_D;
@@ -1882,7 +1882,7 @@ static bool clfa_loop_free_check(struct isis_spftree *spftree,
 			   dist_N_S, dist_S_D);
 
 	if (dist_N_D < (dist_N_S + dist_S_D)) {
-		*lfa_metric = sadj_N->metric + dist_N_D;
+		*path_metric = sadj_N->metric + dist_N_D;
 		return true;
 	}
 
@@ -2082,7 +2082,7 @@ void isis_lfa_compute(struct isis_area *area, struct isis_circuit *circuit,
 		      struct isis_spftree *spftree,
 		      struct lfa_protected_resource *resource)
 {
-	struct isis_vertex *vertex;
+	struct isis_vertex *vertex, *parent_vertex;
 	struct listnode *vnode, *snode;
 	int level = spftree->level;
 
@@ -2099,7 +2099,7 @@ void isis_lfa_compute(struct isis_area *area, struct isis_circuit *circuit,
 		struct isis_vertex_adj *vadj_primary;
 		struct isis_spf_adj *sadj_primary;
 		bool allow_ecmp;
-		uint32_t best_metric = UINT32_MAX;
+		uint32_t prefix_metric, best_metric = UINT32_MAX;
 		char buf[VID2STR_BUFFER];
 
 		if (!VTYPE_IP(vertex->type))
@@ -2133,6 +2133,9 @@ void isis_lfa_compute(struct isis_area *area, struct isis_circuit *circuit,
 		vadj_primary = listnode_head(vertex->Adj_N);
 		sadj_primary = vadj_primary->sadj;
 
+		parent_vertex = listnode_head(vertex->parents);
+		prefix_metric = vertex->d_N - parent_vertex->d_N;
+
 		/*
 		 * Loop over list of SPF adjacencies and compute a list of
 		 * preliminary LFAs.
@@ -2140,7 +2143,7 @@ void isis_lfa_compute(struct isis_area *area, struct isis_circuit *circuit,
 		lfa_list = list_new();
 		lfa_list->del = isis_vertex_adj_free;
 		for (ALL_LIST_ELEMENTS_RO(spftree->sadj_list, snode, sadj_N)) {
-			uint32_t lfa_metric;
+			uint32_t lfa_metric, path_metric;
 			struct isis_vertex_adj *lfa;
 			struct isis_prefix_sid *psid = NULL;
 			bool last_hop = false;
@@ -2190,7 +2193,7 @@ void isis_lfa_compute(struct isis_area *area, struct isis_circuit *circuit,
 
 			/* Check loop-free criterion. */
 			if (!clfa_loop_free_check(spftree, vertex, sadj_primary,
-						  sadj_N, &lfa_metric)) {
+						  sadj_N, &path_metric)) {
 				if (IS_DEBUG_LFA)
 					zlog_debug(
 						"ISIS-LFA: LFA condition not met for %s",
@@ -2198,6 +2201,7 @@ void isis_lfa_compute(struct isis_area *area, struct isis_circuit *circuit,
 				continue;
 			}
 
+			lfa_metric = path_metric + prefix_metric;
 			if (lfa_metric < best_metric)
 				best_metric = lfa_metric;
 
@@ -2208,7 +2212,7 @@ void isis_lfa_compute(struct isis_area *area, struct isis_circuit *circuit,
 
 			if (vertex->N.ip.sr.present) {
 				psid = &vertex->N.ip.sr.sid;
-				if (lfa_metric == sadj_N->metric)
+				if (path_metric == sadj_N->metric)
 					last_hop = true;
 			}
 			lfa = isis_vertex_adj_add(spftree, vertex, lfa_list,