[add-topo] Increase server IPv6 route cache size from default 4096 to…

… 16384 (sonic-net#11680)

On testbed server, the IPv6 route cache may be full. When the issue is hit,
we can observe messages like below in dmesg:

[2615625.047483] Route cache is full: consider increasing sysctl net.ipv[4|6].route.max_size.
[2615625.967008] Route cache is full: consider increasing sysctl net.ipv[4|6].route.max_size.

The impact is that static IPv6 configured on testbed server interface
may stop working. Just ping the IP would get "Network is unreachable".

Reason:
The current SONiC testbed design needs to create many bridges
on test server. Each bridge may consume some IPv6 route cache.
When multiple testbeds are deployed on single server, the default
route cache size 4096 may be hit.

The fix is to increase IPv6 route cache size on test server from
default 4096 to 16384 while deploying a testbed.

Signed-off-by: Xin Wang <xiwang5@microsoft.com>

ansible/roles/vm_set/tasks/main.yml

@@ -168,6 +168,13 @@
     sysctl_set: yes
   become: yes
 
+- name: Increase IPv6 route cache size
+  sysctl:
+    name: "net.ipv6.route.max_size"
+    value: "16384"
+    sysctl_set: yes
+  become: yes
+
 - name: Setup external front port
   include_tasks: external_port.yml
   when: external_port is defined