IP address listed in CDP is not be that of the direct interface.

[conf-t]

Hello,

I have a L3 routed connection between NX-OS switch and ACI Leaf, When I run "show cdp neighbors eth1/1 detail" on the NX-OS switch, it shows me a different ip address than that of the direct interface! it seems to be a L3out interface address eth1/5 of an SVI used for another L3out to another router!! I want just to understand this behavoir! Thank you 

Note: ports eth1/1 are up and working. 

[AshSe]

Hey @conf-t 

The behavior you're observing with the show cdp neighbors eth1/1 detail command on your NX-OS switch showing a different IP address than expected can be explained by understanding how Cisco Discovery Protocol (CDP) works and how IP addresses are advertised.

Understanding CDP Behavior

CDP is a Layer 2 protocol that Cisco devices use to share information about directly connected devices. It can provide details such as device identifiers, IP addresses, port identifiers, capabilities, and platform information.

Possible Reasons for the Observed Behavior

1. CDP Advertises Management IP:

   • CDP typically advertises the management IP address of the neighboring device. If the ACI leaf switch has a management IP address configured, it might be advertising this IP address instead of the IP address of the directly connected interface.

2. SVI IP Address Advertisement:

   • In some cases, CDP might advertise the IP address of an SVI (Switched Virtual Interface) if it is configured on the ACI leaf switch. This could explain why you are seeing the IP address of an SVI used for another L3out.

3. Multiple IP Addresses:

   • If the ACI leaf switch has multiple IP addresses configured (e.g., for different L3outs or SVIs), CDP might be advertising one of these IP addresses. This can happen if the management IP or another SVI IP is prioritized for CDP advertisements.

4. CDP Configuration:

   • Check the CDP configuration on both the NX-OS switch and the ACI leaf switch. There might be specific settings that influence which IP address is advertised.

Steps to Verify and Understand the Behavior

1. Check CDP Configuration:

   • On the NX-OS switch, run the following command to check the CDP configuration:

     show cdp interface eth1/1

   • On the ACI leaf switch, check the CDP settings to see if there are any specific configurations that might influence the advertised IP address.

2. Verify Management IP Address:

   • Check the management IP address configured on the ACI leaf switch. This can be done through the APIC GUI or CLI.

3. Inspect SVI Configuration:

   • Review the SVI configuration on the ACI leaf switch to see if the IP address you are seeing is associated with an SVI. This can be done through the APIC GUI or CLI.

4. Check CDP Advertisements:

   • On the NX-OS switch, run the following command to see detailed CDP information:

     show cdp neighbors detail

   • This will provide more information about the neighboring devices and the IP addresses they are advertising.

Example Commands

On NX-OS Switch:

show cdp neighbors eth1/1 detail

show cdp interface eth1/1

show cdp neighbors detail

On ACI Leaf Switch (via APIC CLI):

show cdp interface

show cdp neighbors detail

Conclusion

The behavior you are observing is likely due to CDP advertising an IP address that is not directly associated with the physical interface but rather a management IP or an SVI IP. By checking the CDP configuration and the IP addresses configured on the ACI leaf switch, you should be able to understand why this specific IP address is being advertised.

If you need further assistance or have additional questions, feel free to ask!

Happy Troubleshooting!!!

AshSe

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