[Fanout] Gateway information for Fanout switches generated from is not correct.

[lvphuc133]

Description
Gateway information for Fanout switches generated from ansible/library/conn_graph_facts.py is not correct.

Steps to reproduce the issue:

1. Source file ansible/library/conn_graph_facts.py
2. In function parse_graph()
   ...
   management_ip = l3info.find('ManagementIPInterface').attrib['Prefix']
   deviceinfo[hostname]['ManagementIp'] = management_ip
   mgmtip = ipaddress.IPNetwork(management_ip)
   deviceinfo[hostname]['mgmtip'] = str(mgmtip.ip)
   management_gw = str(mgmtip.network+1)
   deviceinfo[hostname]['ManagementGw'] = management_gw

Describe the results you received:
The management_gw shoud is calculated as network + 1 which is not correct for all Lab. environment. In my case the gateway IP is network + 254.

Describe the results you expected:
The management_gw shoud be retrieved from the XML file rather than network + 1.

[okanchou9]

@lvphuc133

I have the same issue as yours.
And you will notice that the gateway of management interface of DUT is hard coding to "+1" also.
See the following link that I open in sonic-buildimage if need:
sonic-net/sonic-buildimage#857

For now I'm using self-modified script to create a tag called "mgmt_gw" and assign its value to management_gw to workaround this issue.