northd: Use lower priority for all src routes.

The current implementation prefers dst routes over src routes only if
they have the same prefix length. This is not very useful for real world
use cases, because it doesn't make much sense to compare the prefix
length between different fields of the IP header (dst IP v.s. src IP).
The prefix length should make sense only when comparing for the same
field, either dst or src.

This patch changes the behavior by always prefering dst routes over src
routes, regardless of the prefix length, and comparing prefix length
only within the same type of routes.

E.g., there are two routes:

10.0.0.0/8      nexthop A dst-ip
192.168.11.0/24 nexthop B src-ip

For a packet 192.168.11.123 -> 10.1.2.3:

Before: nexthop will be B

After: nexthop will be A

Reported-by: Girish Moodalbail <gmoodalbail@gmail.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2020-May/050049.html
Signed-off-by: Han Zhou <hzhou@ovn.org>
Acked-by: Dumitru Ceara <dceara@redhat.com>

Author: hzhou8

NEWS

@@ -36,6 +36,8 @@ Post v25.03.0
    - Added a new ACL option "--all" to "acl-list" command. When set,
      "acl-list" command will also list port groups ACLs associated with
      each port of the target logical switch.
+   - Adjusted priorities of src-ip based static routes to be lower than other
+     types of routes regardless of prefix length.
 
 OVN v25.03.0 - 07 Mar 2025
 --------------------------

northd/northd.c

@@ -339,11 +339,12 @@ static const char *reg_ct_state[] = {
  *  1. (highest priority) connected routes
  *  2. static routes
  *  3. routes learned from the outside via ovn-controller (e.g. bgp)
- *  4. (lowest priority) src-ip routes */
-#define ROUTE_PRIO_OFFSET_MULTIPLIER 8
-#define ROUTE_PRIO_OFFSET_LEARNED 2
-#define ROUTE_PRIO_OFFSET_STATIC 4
-#define ROUTE_PRIO_OFFSET_CONNECTED 6
+ * (src-ip routes have lower priority than all other routes regardless of
+ * prefix length, so not included here.) */
+#define ROUTE_PRIO_OFFSET_MULTIPLIER 6
+#define ROUTE_PRIO_OFFSET_LEARNED 0
+#define ROUTE_PRIO_OFFSET_STATIC 2
+#define ROUTE_PRIO_OFFSET_CONNECTED 4
 
 /* Returns the type of the datapath to which a flow with the given 'stage' may
  * be added. */
@@ -11832,6 +11833,7 @@ build_route_match(const struct ovn_port *op_inport, uint32_t rtb_id,
 {
     const char *dir;
     int ofs = route_source_to_offset(source);
+    int base = 0;
 
     /* The priority here is calculated to implement longest-prefix-match
      * routing. */
@@ -11840,6 +11842,9 @@ build_route_match(const struct ovn_port *op_inport, uint32_t rtb_id,
         ofs = 0;
     } else {
         dir = "dst";
+        /* dst routes have higher priority than all src routes regardless of
+         * prefix length. */
+        base = (128 + 1) * ROUTE_PRIO_OFFSET_MULTIPLIER;
     }
 
     if (op_inport) {
@@ -11853,7 +11858,7 @@ build_route_match(const struct ovn_port *op_inport, uint32_t rtb_id,
     if (has_protocol_match) {
         ofs += 1;
     }
-    *priority = (plen * ROUTE_PRIO_OFFSET_MULTIPLIER) + ofs;
+    *priority = base + (plen * ROUTE_PRIO_OFFSET_MULTIPLIER) + ofs;
 
     ds_put_format(match, "ip%s.%s == %s/%d", is_ipv4 ? "4" : "6", dir,
                   network_s, plen);

ovn-nb.xml

@@ -4325,9 +4325,9 @@ or
     </p>
 
     <p>
-      When multiple routes match a packet, the longest-prefix match is chosen.
-      For a given prefix length, a <code>dst-ip</code> route is preferred over
-      a <code>src-ip</code> route.
+      When multiple routes match a packet, a <code>dst-ip</code> route is
+      preferred over a <code>src-ip</code> route. Among the same type of
+      routes, the longest-prefix match is chosen.
     </p>
 
     <p>