Cisco Nexus 3k Route Selection\Packet Forwarding Debugging

Mon Apr 3 13:06:51 UTC 2023

What about VRFs and/or policy based routing?  

switch-core1# show vrf
VRF-Name                           VRF-ID State   Reason                        
default                                 1 Up      --                            
management                              2 Up      --                            

switch-core1# show route-map 
route-map rmap_bgp_to_eigrp_b2b, permit, sequence 10 
  Match clauses:
    interface: Ethernet1/33 
    route-type: internal 
  Set clauses:
    metric 40000000 10 255 1 1500 
route-map rmap_bgp_to_eigrp_b2b, permit, sequence 20 
  Match clauses:
    interface: Ethernet1/34 
    route-type: internal 
  Set clauses:
    metric 40000000 30 255 1 1500 
route-map rmap_static_to_eigrp, permit, sequence 10 
  Match clauses:
    ip address prefix-lists: prefix_static_to_eigrp 
  Set clauses:
route-map rmap_static_to_eigrp_v6, permit, sequence 10 
  Match clauses:
    ipv6 address prefix-lists: prefix_ipv6_static_to_eigrp 
  Set clauses:

From: Mike Hammett <nanog at ics-il.net> 
Sent: Monday, April 3, 2023 9:00 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 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

________________________________________
From: "Matthew Huff" <mailto:mhuff at ox.com>
To: "Mike Hammett" <mailto:nanog at ics-il.net>
Cc: "NANOG" <mailto: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 <mailto:nanog at ics-il.net> 
Sent: Monday, April 3, 2023 8:47 AM
To: Matthew Huff <mailto:mhuff at ox.com>
Cc: NANOG <mailto: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

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