cisco Nexus 6001 ARP-3-REQ_IP if Null0 routes

[YuriiIvanov92199]

Hi,
Got problem in logs on cisco Nexus 6001:

%ARP-3-REQ_IP: arp [3857] Sending ARP request for invalid IP address 0.0.0.0 on Null0, request from pid: 3981

To reproduce this error I simply should make route to 1.104.196.17 unavailable

# show ip route 1.150.62.0/23
1.150.62.0/23, ubest/mbest: 1/0
*via 1.104.196.17, [1/0], 18:35:22, static
via Null0, [254/0], 3y23w, static

To make it unavailable need to take down or simply remove 1.104.196.17 static route.

# show ip interface vlan 951
IP Interface Status for VRF "default"(1)
Vlan951, Interface status: protocol-up/link-up/admin-up, iod: 292,
IP address: 1.104.196.18, IP subnet: 1.104.196.16/30 route-preference: 0, tag: 0

Btw, 1.104.196.18 also my router-id.

After Null0 route becomes best %ARP-3-REQ_IP is generated.

How can I find what is sending arp requests?

[ashok_boin]

Hi,

This is the default behaviour of Cisco router when it got a static route pointing to a next-hop IP address pointing through an Ethernet interface. 

The static route is pointing to x.x.x.17 for which Router is not getting ARP reply. 

Please look into the below detailed document to understand your issue as well as solutions.

Static Routes With Next Hop As An Exit Interface Or An IP Address - Cisco Community

[YuriiIvanov92199]

Thanks, let me clarify.

This is valid for Null0 routes like

1.150.62.0/23, ubest/mbest: 1/0
*via Null0, [254/0], 3y23w, static

What address arp tries to determine in case of Null0 interface?

P.S. In my case after deleting both static routes resolved issue

no ip route 1.150.62.0/23 1.104.196.17
no ip route 1.150.62.0/23 Null0 254

But I don't understand why arp try to work on Null0 interface.

[ashok_boin]

ARP requests originating w.r.t your static route having 1.104.196.17, not
Null0.

The static route point to Null 0 will be considered when you don't have any
other specific route in the routing table.

[YuriiIvanov92199]

This ok, but how to describe such behavior after deleting static route 1.104.196.17:

# show ip route 91.250.62.0/23
1.150.62.0/23, ubest/mbest: 1/0
    *via Null0, [254/0], 00:00:05, static

With only Null route nexus generates  %ARP-3-REQ_IP: arp [3857] Sending ARP request for invalid IP address 0.0.0.0 on Null0, request from pid: 3981 message.

Deleting Null route stops generating arp log message.

Moreover another Null0 static routes doesn't generate such arp messages.

Maybe some internal process try to generate arp for certain routes?

And maybe I can somehow watch what arp enables this error message?

I find process by pid but don't find more information.

# show processes | include 3981
 3981      S  415c4642            1     - rpm

Thanks in advance.

[ashok_boin]

You might be hitting a bug of Nexus...

Cisco Bug: CSCut29361 - ARP request on invalid IP error seen during
configuring agg-address