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Nexus arp issue


we are using N5K switches in our lan with 2 VDC: VDC2 for distribution layer to connect access switches+ VDC3 to connect our servers vlans.

a L3 linl  is between vdc2 and vdc3.

2 N5K are used for redendancy

we have vlan222 that is created in both vdc, but used for different things and each one has its separate subnets. normally it should work since we have different vdc. i read in cisco doc that this allowed.  but because of this i discovered that users who are in vlan 222 in vdc2 lost connection to other vlans from time to time.

using sniffer i found that when these users make arp request to find the mac address of their gateway that is in vdc2, they receive 3 arp reply: one from the gateway itself , one from vdc3 (node1) and one from vdc3(node2)

when the ones coming from vdc3 come first so user cannot communicate with other vlans.

when i changed vlan222 in vdc3 to another vlan id, the problem get resolved: i receive only one arp reply from the gateway in vdc2/vlan222.

is it a bug or an error in the config.? why vdc3 reply to an arp request for an IP that does not exist in vdc3?

vlan222 in vdc2: 172.222.x.x

vlan222 in vdc3: 10.222.x.x


Nexus arp issue


I could guess that you have coincident: VLAN 222 leaked between VDCs and proxy-arp is enabled.

Check STP status for VLAN 222 on VDCs - if they have common root bridge -> then that is your issue.