Nexus 2000 FEX and LAN Switch connection

[jamesserna]

Hello,

I have a big doubt. Can I connect a LAN switch (i.e. 2960, 3560 or 3750) to a couple of Nexus 2000 FEX (i.e 2232P or 2248TP) and have a Port-Channel or vPC with STP active and several VLANs through this Port-Channel? Or Can I only connect servers to these N2K FEX?

Thank you so much!

[Reza Sharifi]

Hi,

No, the FEX ports are for host connectivity and not switch.'

From the FEX guide:

The                  Fabric Extender                 provides end-host connectivity into the network fabric. As a result,  BPDU Guard is enabled on all its host interfaces. If you                 connect a bridge or switch to a host interface, that interface is  placed in an                 error-disabled state when a BPDU is received.

You cannot disable BPDU Guard on the host interfaces of the                  Fabric Extender.

more info:

http://www.cisco.com/en/US/partner/docs/switches/datacenter/nexus2000/sw/configuration/guide/rel_521/Configuring_the_Cisco_Nexus_2000_Series_Fabric_Extender_rel_5_2_chapter1.html#con_1197054

HTH

[Gregory Snipes]

This is a tricky one. In theory you cannot connect a switch to a FEX due to the permanent BPDU guard on FEX ports. Not allowing spanning tree to be run on FEX ports is one of the ways Cisco was able to get the port counts so high on the Nexus equipment. If you tired to run spanning tree on 2000 ports the switch would melt.

That said; there are work arounds to this problem. If you do not run spanning tree on the switches you connect to the FEX, YOU CAN DO IT. In order to make this happen you need to disable the transmission of BPDUs from the switch you connect by using BPDU filter. You can then use some type of non spanning tree technology to give you redundancy, these include port-channeling (VPC to traditional port channel) or the use of Flexlinks.

From the Cisco Nexus 2000 Series NX-OS Fabric Extender Software Configuration Guide:

Any edge switch that leverages a link redundancy mechanism not dependent on spanning tree such as Cisco FlexLink or vPC (with the BPDU Filter enabled) can be connected to a Fabric Extender host interface. Because spanning tree is not used to eliminate loops, you should ensure a loop-free topology below the Fabric Extender host interfaces.

Best Regards,

Greg

[Leo Laohoo]

I agree with Gregory.  Disable STP on your switch and you can connect the switch to your FEX.

BPDU Guard is permanently enabled on the FEX.

[Trilok Kataria]

Hi, 

I have a situation where I am looking to connect a router C3925 to FEX, is that something supported, will work, anyone done that?

r-d Cisco 3925 connected to r-agg Cisco 6500

on r-d:

G0/1.200

  Vlan 200

  Ip address x.x.x.x

  TAP PROD

G0/1.60

  Vlan 60

  Vrf admin

  Ip address x.x.x.x

  ADMIN

…

iBGP

Neighbor … IP of r-agg …

On r-agg

Int G0/0/0

Switch port mode trunk

Allowed vlan 60, 200, etc …

Int vlan 200

Ip address x.x.x.xx.

iBGP

Neighbor … IP of r-d …

Want to move r-d Cisco 3925 connected to r-agg Cisco Nexus 2k (copper ports) – connected to Nexu 5k parent switch to avoid buying SFP on the N5k

I have read about some buffer overflow on FEX and over utilisation of the FEX uplinks to the parent switch other than it's NOT recommended Cisco design.

Any thoughts/inputs will be highly appreciated.

Thanks

[want2asku]

yes you can.. but you need to spare 1 port on the downlink switch (2960)

in 2960 do this (assume i configure it to access vlan 1)

!

interface GigabitEthernet1/0/1
 description ### For NEXUS  ###
 switchport mode access
 switchport backup interface Gi1/0/2
!
interface GigabitEthernet1/0/2
 description ### backup port - DO NOT USE - ###
 switchport mode access

!

where gi1/0/1 is the port that you connect to n2k & leave gi1/0/2 empty, do not connect any cable on that port.