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802.1Q between Catalyst 3750 en PowerConnect 6226

Configuring a 802.1Q connection isn’t that difficult, but you need to know the command line interface and the appropriate commands. Today I configured a 802.1Q connection between a Cisco Catalyst 3750G and a Dell PowerConnect 6226, while configuring I played a little with the trunking options on the PowerConnect and I noticed the following:

  • The Dell PowerConnect 6226 doesn’t support the configuration of a native vlan in switchport mode trunk

To solve this problem you should use the switchport mode general commands. I configured the 802.1Q connection on the Dell PowerConnect 6226 with the following commands:

interface ethernet 1/g24
description ‘802.1Q C3750G’
switchport mode general
switchport general pvid 10
no switchport general acceptable-frame-type tagged-only
switchport general allowed vlan remove 1
switchport general allowed vlan add 10 untagged
switchport general allowed vlan add 255 tagged
switchport general allowed vlan add 1128 tagged
switchport general allowed vlan add 1129 tagged

The command no switchport general acceptable-frame-type tagged-only prevents the switch from discarding untagged frames at ingress. I configured the Cisco Catalyst 3750 with the following commands:

interface GigabitEthernet1/0/4
description 802.1Q DELL POWERCONNECT 6226
switchport trunk encapsulation dot1q
switchport trunk native vlan 10
switchport trunk allowed vlan 10,255,1128,1129
switchport mode trunk
switchport nonegotiate
no ip address
no mdix auto

The above configuration is working perfectly. Packets from VLAN 10 are sent untagged over the link and all other VLAN’s are sent tagged over the link. I guess it would be easier if the switch supported the configuration of a native VLAN in trunking mode.

IEEE 802.3x FlowControl between Cat3750E and Cat2960

I have a network with two Catalyst 3750E switch stacks, which are connected with a 2 x 10Gbps Etherchannel. Every stack facilitates a ring topology of approximately 10 to 15 Catalyst 2960 switches. Two of the 2960 are connected with 1Gbps links to a switch stack to create the ring topology. So lets say that 7 24-ports Catalyst 2960 switch share a 1 Gbps link to the switch stack. With this customer, this won’t be any problem, because there are no heavy users and/or applications.

But let’s imagine that a link between a Catalyst 3750E and Catalyst 2960 switch or between two Catalyst 2960 switches is giving problems and the Catalyst 2960 cannot handle the receiving traffic. You need to find some way to slow done the traffic. I normally start thinking about the usage of IEEE 802.3x FlowControl.

Flow control enables connected Ethernet ports to control traffic rates during congestion by allowing congested nodes to pause link operation at the other end. If one port experiences congestion and cannot receive any more traffic, it notifies the other port by sending a pause frame to stop sending until the condition clears. Upon receipt of a pause frame, the sending device stops sending any data packets, which prevents any loss of data packets during the congestion period.

But after reading some documentation, FlowControl isn’t an option. When a link between both switches gets congested the Catalyst 2960 would have to send a pause frame to the Catalyst 3750E and that’s the problem.

Both, Catalyst 3750E and Catalyst 2960, can only receive, but not send, pause frames. So configuring FlowControl between Catalyst 3750E and Catalyst 2960 is useless, because no switch can inform the counterpart about the congested link.