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NaOH
Asked: 2020-10-01 12:00:05 +0800 CST

Traffic not traversing VLAN over multiple hops?

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I am configuring a new Cisco edge switch stack, and temporarily I am trunking this stack through another edge switch back to my core switch (no spare fiber pairs to use to trunk directly). This is how they're configured (lots of extraneous stuff removed):

**2960X Edge Stack**
  vtp mode transparent

  vlan 10 (old data subnet, 10.1.10.0/24)
  vlan 1000 (management subnet, 10.255.10.0/24)
  vlan 1080 (new data subnet, 10.1.80.0/24)

  int gi1/0/48 (trunk to 3560 edge switch)
    switchport mode trunk
    switchport trunk native vlan 1000
    switchport trunk allowed vlan 10,1000,1080
    spanning-tree portfast network
  
  int gi3/0/30 (edge port)
    switchport mode access
    switchport access vlan 1080
    spanning-tree portfast edge

  ip routing
  ip route 0.0.0.0 0.0.0.0 10.255.10.254

**3560 Edge Switch**
  vtp mode transparent

  vlan 10
  vlan 1000
  vlan 1080

  int gi0/3 (edge port)
    switchport mode access
    switchport access vlan 1080
    spanning-tree portfast

  int gi0/20 (trunk to 2960 edge stack)
    switchport mode trunk
    switchport trunk encapsulation dot1q
    switchport trunk native vlan 1000
    switchport trunk allowed vlan 10,1000,1080
    spanning-tree portfast

  int gi0/49 (trunk to 9300 core switch)
    switchport mode trunk
    switchport trunk encapsulation dot1q
    switchport trunk native vlan 1000
    switchport trunk allowed vlan 10,1000,1080
    spanning-tree portfast

  ip routing
  ip route 0.0.0.0 0.0.0.0 10.255.10.254

**9300 Core Switch**
  vtp mode transparent

  vlan 10
  vlan 100 (server subnet, 10.1.0.0/24)
  vlan 1000
  vlan 1080

  int te1/1/2 (trunk to 3560 edge switch)
    switchport mode trunk
    switchport trunk native vlan 1000
    switchport trunk allowed vlan 10,1000,1080
  
  int Vlan10
    ip address 10.1.10.254 255.255.255.0
    ip helper-address 10.1.0.1

  int Vlan100
    ip address 10.1.0.254 255.255.255.0

  int Vlan1000
    ip address 10.255.10.254 255.255.255.0

  int Vlan1080
    ip address 10.1.80.254 255.255.255.0
    ip helper-address 10.1.0.1

  ip routing

A device connected to an edge port on the 2960X in VLAN1080 (gi3/0/30 in this case) cannot get a DHCP address, and even with a statically-assigned address can't ping the gateway (10.1.80.254). If I put that port in VLAN10 instead, it will get a DHCP address and can ping everything as expected.

A device connected to an edge port on the 3560 in VLAN1080 will get a DHCP address in the correct scope (10.1.80.0/24) and can ping the gateway (10.1.80.254).

What seems to be happening is that the 3560 is maybe stripping the 802.1Q tags when traffic is traversing from the 2960X to the 9300, but I don't see any reason it should be doing that. Can anyone suggest what's happening here and how to fix it?

vlan cisco switch

1 Answers

  1. Issue was actually with spanning-tree... port Gi1/0/48 on the 2960X as shown above was in BKN state. Disabling "spanning-tree portfast network" corrected this issue.

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