Cisco Nexus 3k Route Selection\Packet Forwarding Debugging

Mon Apr 3 12:59:53 UTC 2023

It could be an sFlow bug, but I come at this from a reported problem and gathering data on that problem as opposed to looking at data for problems. 

The snmp if index reported by the Nexus matches the if index in ElastiFlow. 

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

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

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

From: "Matthew Huff" <mhuff at ox.com> 
To: "Mike Hammett" <nanog at ics-il.net> 
Cc: "NANOG" <nanog at nanog.org> 
Sent: Monday, April 3, 2023 7:50:08 AM 
Subject: RE: Cisco Nexus 3k Route Selection\Packet Forwarding Debugging 

SFlow misconfiguration or bug on either the nexus or the sflow monitor? On the monitor, can you verify that the snmp interfaces are mapped to the correct ones on the nexus? 

From: Mike Hammett <nanog at ics-il.net> 
Sent: Monday, April 3, 2023 8:47 AM 
To: Matthew Huff <mhuff at ox.com> 
Cc: NANOG <nanog at nanog.org> 
Subject: Re: Cisco Nexus 3k Route Selection\Packet Forwarding Debugging 

It shows the desired result. 

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

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

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

From: "Matthew Huff" < mhuff at ox.com > 
To: "Mike Hammett" < nanog at ics-il.net >, "NANOG" < nanog at nanog.org > 
Sent: Monday, April 3, 2023 5:38:23 AM 
Subject: RE: Cisco Nexus 3k Route Selection\Packet Forwarding Debugging 

switch-core1# sh forwarding route x.x.x.x 

slot 1 
======= 

IPv4 routes for table default/base 

------------------+-----------------------------------------+----------------------+-----------------+----------------- 
Prefix | Next-hop | Interface | Labels | Partial Install 
------------------+-----------------------------------------+----------------------+-----------------+----------------- 
x.x.x.x/24 x.x.x.250 Ethernet1/29 

switch-core1# show routing hash x.x.x.x y.y.y.y 
Load-share parameters used for software forwarding: 
load-share mode: address source-destination port source-destination 
Hash for VRF "default" 
Hashing to path *y.y.y.y Eth1/29 
For route: 
y.y.y.0/24, ubest/mbest: 1/0 
*via z.z.z.z, Eth1/29, [90/3072], 1w2d, eigrp-100, internal 

From: NANOG < nanog-bounces+mhuff=ox.com at nanog.org > On Behalf Of Mike Hammett 
Sent: Monday, April 3, 2023 1:21 AM 
To: NANOG < nanog at nanog.org > 
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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