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satish_txt_2254's avatar
satish_txt_2254
Icon for Cirrus rankCirrus
Apr 26, 2017

STP issue with Nexus 5548

We have above network and here is the cisco trunk configuration.

Nexus 5548

 interface Ethernet1/9
  description F5-10G-Link
  switchport mode trunk
  switchport trunk allowed vlan 201

Cisco 3850 (non-root switch)

interface Port-channel20
 description F5-2G-Etherchannel trunk
 switchport trunk allowed vlan 201
 switchport mode trunk
end

interface GigabitEthernet1/0/45
 description F5-1G-a
 switchport trunk allowed vlan 201
 switchport mode trunk
 channel-protocol lacp
 channel-group 20 mode active
end

!
interface GigabitEthernet1/0/46
 description F5-1G-b
 switchport trunk allowed vlan 201
 switchport mode trunk
 channel-protocol lacp
 channel-group 20 mode active
end

links are up but if i check in STP configuration on cisco my 10G link is in 

blocking
mode and 2G link is 
forwarding
mode, as per STP cost calculation 10G link should be in forwarding because it has total cost to reach root is 
4
and 2G link total cost to root is 
5
right?

Even i tried to increase spanning-tree cost of 2G link but it didn't work still that link is in 

FWD
mode. 

SW2sh spanning-tree int po20

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0201            Desg FWD 3         128.2046 P2p      


Nexussh spanning-tree int e1/9

Vlan             Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
VLAN0201         Altn BLK 2         128.137  P2p

If i am trying to shutdown 2G trunk then on Nexus switch i am seeing following spanning-tree status which is weird

Nexus sh spanning-tree int e1/9

Vlan             Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
VLAN0201         Desg BKN*2         128.137  P2p *LOOP_Inc

and this interface never come into 

forwarding
state.

what i am doing wrong here? is my cisco config is wrong on trunk?

application delivery
BIG-IP
  • satish_txt_2254's avatar
    satish_txt_2254
    Icon for Cirrus rankCirrus
    Apr 27, 2017

    One more thing, This is new F5 and they are using default STP setting which is "Passthrough" But in our Cisco network we are using RSTP (PV-STP) spanning-tree. Do you think i need to change any setting at F5 side? or should i use default?

     

  • crengifo_232216's avatar
    crengifo_232216
    Icon for Nimbostratus rankNimbostratus
    Apr 27, 2017

    Assuming that you haven't configured the priority on the root switch for VLAN 201, you can try this (in config mode):

     

    • non-root switch: spanning-tree vlan 201 priority 40960
    • Nexus switch: spanning-tree vlan 201 priority 36864
    • satish_txt_2254's avatar
      satish_txt_2254
      Icon for Cirrus rankCirrus
      Apr 27, 2017

      I have priority set on c3850 switch that's is why it's root bridge and second c3850 is sencandary root bridge.

       

      This is not an issue about root bridge, it's about STP failover.. even I sent very high cost on 2G link my STP doesn't getting switch to 10G.. you can see in output.

       

    • satish_txt_2254's avatar
      satish_txt_2254
      Icon for Cirrus rankCirrus
      Apr 27, 2017

      Should I run F5 is passthrough mode? We are using PVSTP+ ?

       

      F5 has very poor documents related it.