Communication among Same IP subnets in different EPGs in one BD

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Mar 15th, 2017
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Hi All,

Since we can configure same IP subnet in different EPGs in same BD. What is the path if the end points of those EPGs communicate where the default Gw is defined in external firewall?

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dpita Mon, 03/20/2017 - 04:54
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  • Cisco Employee,

If the gateway is outside, there is no need for a subnet under the BD, much less under the EPG

from a forwarding perspective, same BD with two different EPGs using the same subnet should be ok so long as they are different VLANs to the ACI side. at this point, traffic from subnet-A EPG-A will need pass through a contract to subnet-A EPG-B. if you need to communicate to the gw on an external device, that gets complicated. 

would probably be best to use three VLANs, one for the external bridge network, the real vlan your FW is using, a VLAN for EPG-A and a VLAN for EPG-B and contracts between all of them. of course, all of them tied to the same BD

hope that helps

Thushan Pramod Tue, 03/21/2017 - 01:20
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Hi Dpita,

Thanks for the reply. Do we really need to worry about the vlan concept in ACI? for the forwarding does ACI check and worry about the vlan configuartion?

Here you are proposing three vlans as below right. If iam wrong pls comment.

Vlan10 - EPG-A (IP Subnet 1)

Vlan20 - EPG-B (IP Subnet 1)

here does external bridge network means external EPG which connects FW?


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