Add a known issue in vCPE doc

Signed-off-by: Yang Xu <yang.xu@futurewei.com>
Issue-ID: INT-1127
Change-Id: I3354f5a468c79c83a40550fd8bcb20f9858b37a8
diff --git a/docs/docs_vCPE.rst b/docs/docs_vCPE.rst
index 0878045..939adde 100644
--- a/docs/docs_vCPE.rst
+++ b/docs/docs_vCPE.rst
@@ -191,4 +191,4 @@
   root>/opt/nat_service.sh
   root>vppctl restart
 
-
+2) During vCPE customer service instantiation, though vGW should come up successfully BRG vxlan tunnel configuration is likely to fail in SDNC cluster environment due to SDNC unreachable to BRG. See more detail in JIRA INT-1127. One workaround is to run vCPE use case with SDNC cluster disabled.