Cisco Nexus 3k Route Selection\Packet Forwarding Debugging

Mon Apr 3 12:48:17 UTC 2023

What started this investigation was a client complained of traffic coming from another upstream instead of our direct connection. 

----- 
Mike Hammett 
Intelligent Computing Solutions 
http://www.ics-il.com 

Midwest-IX 
http://www.midwest-ix.com 

----- Original Message -----

From: "Mike Hammett" <nanog at ics-il.net> 
To: "NANOG" <nanog at nanog.org> 
Sent: Monday, April 3, 2023 12:21:29 AM 
Subject: Cisco Nexus 3k Route Selection\Packet Forwarding Debugging 

We have a Nexus 3064 that is setup with partial BGP tables and is routing based on that. 

I've done a show ip bgp for an IP of interest and it has an expected next hop IP. I show ip arp on that next hop IP and it has the expected interface. 

However, sFlows show the packets leaving on a different interface, the one that would carry the default route for routes not otherwise known. 

If the next hop IP is expected and the ARP of that next hop IP is expected, why are packets leaving out an unexpected interface? 

----- 
Mike Hammett 
Intelligent Computing Solutions 
http://www.ics-il.com 

Midwest-IX 
http://www.midwest-ix.com 

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