RE: Cisco Nexus 3k Route Selection\Packet Forwarding Debugging

[Matthew Huff <mhuff () ox com>]

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 () ics-il net>
Sent: Monday, April 3, 2023 8:47 AM
To: Matthew Huff <mhuff () ox com>
Cc: NANOG <nanog () 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

________________________________
From: "Matthew Huff" <mhuff () ox com<mailto:mhuff () ox com>>
To: "Mike Hammett" <nanog () ics-il net<mailto:nanog () ics-il net>>, "NANOG" <nanog () nanog org<mailto:nanog () 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 () nanog org<mailto:nanog-bounces+mhuff=ox.com () nanog org>> On Behalf Of Mike 
Hammett
Sent: Monday, April 3, 2023 1:21 AM
To: NANOG <nanog () nanog org<mailto:nanog () 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