[arp] Ignore intermittent failure of "ip neigh flush all" (#820)

In ARP testing, the script needs to run "ip neigh flush all" couple of
times to clean up ARP table. Occasionally flushing ARP table may fail
with error "*** Flush not complete bailing out after 10 rounds" on ptf32
or ptf64 topology.

The reason is that BGP peers are configured on these topologies although
the PTF container does not have BGP running. From time to time, DUT will
try to contact the BGP peers. ARP requests are firstly sent out. This
will create some INCOMPLETE entries in ARP table for the BGP peers. If
"ip neigh flush all" is executed at the same moment, the command may
fail with "*** Flush not complete bailing out after 10 rounds".

The fix is to simply ignore the error of "ip neigh flush all". Purpose
of the "ip neigh flush all" command is to cleanup ARP entries generated
by previous testing. It doesn't matter when there are ARP entries
generated by other activities not flushed.

Meanwhile, I changed the command to "ip -stats neigh flush all" to
have more detailed output for debug in case of failure.

Signed-off-by: Xin Wang <[email protected]>

Author: wangxin

ansible/roles/test/tasks/arpall.yml

@@ -73,7 +73,8 @@
       delegate_to: "{{ ptf_host }}"
 
     - name: Clear DUT arp cache
-      command: ip nei flush all
+      command: ip -stats neigh flush all
+      ignore_errors: yes
       become: yes
 
     - name: Start PTF runner and Send correct unicast arp packets (10.10.1.3 to 10.10.1.2 with src_mac=00:06:07:08:09:00)
@@ -99,7 +100,8 @@
           - "{{ arptable['v4']['10.10.1.3']['interface'] == intf1 }}"
 
     - name: Clear DUT arp cache
-      command: ip nei flush all
+      command: ip -stats neigh flush all
+      ignore_errors: yes
       become: yes
 
     # Send correct ARP request from correct interface, expecting normal behavior
@@ -127,7 +129,8 @@
 
     ## check DUT won't reply ARP and install ARP entry when ARP request coming from other interfaces
     - name: Clear DUT arp cache
-      command: ip nei flush all
+      command: ip -stats neigh flush all
+      ignore_errors: yes
       become: yes
 
     - name: Send correct arp packets from other interface expect no reply(10.10.1.4 to 10.10.1.2 with src_mac=00:02:07:08:09:0a)
@@ -154,7 +157,8 @@
 
     ## check DUT won't reply ARP and install ARP entry when src address is not in interface subnet range
     - name: Clear DUT arp cache
-      command: ip nei flush all
+      command: ip -stats neigh flush all
+      ignore_errors: yes
       become: yes
 
     - name: Send Src IP out of interface subnet range arp packets, expect no reply and no arp table entry (10.10.1.22 to 10.10.1.2 with src_mac=00:03:07:08:09:0a)
@@ -181,7 +185,8 @@
 
     ## Test Gratuitous ARP behavior, no Gratuitous ARP installed when arp was not resolved before
     - name: Clear DUT arp cache
-      command: ip nei flush all
+      command: ip -stats neigh flush all
+      ignore_errors: yes
       become: yes
 
     - name: Send  garp packets (10.10.1.7 to 10.10.1.7)