[copp]: Use non-zero trap priority for default trap group (sonic-net#3502)

* Use non-zero trap priority for default trap group
This priority is used internally in some vendor SAI implementations and causes
undesirable packet trapping behavior.

How I verified it

By running copp tests which include rate-limiting tests for TTL 1 packets on on Cisco/Mellanox/Arista platforms.
By manual tests with TTL 1 packets generated by scapy

Author: prabhataravind

orchagent/copporch.cpp

@@ -188,13 +188,17 @@ void CoppOrch::initDefaultTrapIds()
     attr.value.oid = m_trap_group_map[default_trap_group];
     trap_id_attrs.push_back(attr);
 
-    /* Mellanox platform doesn't support trap priority setting */
-    /* Marvell platform doesn't support trap priority. */
+    /*
+     * Use a default trap priority > 0 to avoid undesirable packet trapping
+     * behavior on some platforms that use 0 as default SAI-internal priority.
+     * Note: Mellanox and Marvell platforms don't support trap priority setting.
+     */
+
     char *platform = getenv("platform");
     if (!platform || (!strstr(platform, MLNX_PLATFORM_SUBSTRING) && (!strstr(platform, MRVL_PRST_PLATFORM_SUBSTRING))))
     {
         attr.id = SAI_HOSTIF_TRAP_ATTR_TRAP_PRIORITY;
-        attr.value.u32 = 0;
+        attr.value.u32 = 1;
         trap_id_attrs.push_back(attr);
     }
 

tests/test_copp.py

@@ -232,6 +232,8 @@ def validate_trap_group(self, trap_oid, trap_group):
         queue = ""
         trap_action = ""
         trap_priority = ""
+        default_trap_queue = "0"
+        default_trap_prio = "1"
 
         for fv in trap_fvs:
             if fv[0] == "SAI_HOSTIF_TRAP_ATTR_PACKET_ACTION":
@@ -262,6 +264,11 @@ def validate_trap_group(self, trap_oid, trap_group):
                 assert trap_group_oid != "oid:0x0"
                 if keys == "queue":
                     assert queue == trap_group[keys]
+                    # default trap in copp config doesn't specify a trap priority
+                    # this is instead set internally in swss
+                    # confirm that default trap uses a priority 1
+                    if queue == default_trap_queue:
+                        assert trap_priority == default_trap_prio
                 else:
                     assert 0