Cisco Nexus 3k Route Selection\Packet Forwarding Debugging

Mon Apr 3 10:38:23 UTC 2023

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