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

[Mike Hammett <nanog () ics-il net>]

I don't see any route-maps applied to interfaces, so there must not be any PBR going on. I only see ACLs, setting 
communities, setting local pref, etc. in the route maps that are applied to neighbors. 



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

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


From: "Mike Hammett" <nanog () ics-il net> 
To: "Matthew Huff" <mhuff () ox com> 
Cc: "NANOG" <nanog () nanog org> 
Sent: Monday, April 3, 2023 8:26:30 AM 
Subject: Re: Cisco Nexus 3k Route Selection\Packet Forwarding Debugging 

Only two VRFs, default and manangement. IIRC, everything I saw before mentioned the default VRF. 

I do see a ton of route-maps. It's mostly Greek to me, so I'll have to dig through this a bit to see what's going on. 



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

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


From: "Matthew Huff" <mhuff () ox com> 
To: "Mike Hammett" <nanog () ics-il net> 
Cc: "NANOG" <nanog () nanog org> 
Sent: Monday, April 3, 2023 8:06:51 AM 
Subject: RE: Cisco Nexus 3k Route Selection\Packet Forwarding Debugging 

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 () ics-il net> 
Sent: Monday, April 3, 2023 9:00 AM 
To: Matthew Huff <mhuff () ox com> 
Cc: NANOG <nanog () 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 () ox com> 
To: "Mike Hammett" <mailto:nanog () ics-il net> 
Cc: "NANOG" <mailto:nanog () 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 () ics-il net> 
Sent: Monday, April 3, 2023 8:47 AM 
To: Matthew Huff <mailto:mhuff () ox com> 
Cc: NANOG <mailto: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" <mailto:mhuff () ox com> 
To: "Mike Hammett" <mailto:nanog () ics-il net>, "NANOG" <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 <mailto:nanog-bounces+mhuff=ox.com () nanog org> On Behalf Of Mike Hammett 
Sent: Monday, April 3, 2023 1:21 AM 
To: NANOG <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