Trunk port comparison between cisco switches and IBM?

[asad ali]

Although very basic question, but i like clarification in termsn of different techonology offered by different vendors e.g IBM.

I'm dealing with IBM Flex System EN2092 1Gb in a pure flex chasis enviroment, I know in all other switches (e.g cisco) i have to enable one port as trunk to take tag traffic from one vlan to another. I want to know is by default if i don't explicitly specfiy a port as tunk any given port that i connect to layer 3 switch (uplink switch), how would it be treat that connected port(what would be its status)?. I know there is no ieee 802.1Q running on the port, in that case how the frames be handled?

I have a debate in my office, where my friends say that IBM switch quoted above, treats all ports as trunk, and i thought that doesn't fit with any technology, as what it should be that all ports are default switched, trunk is when it want to pass tag traffic. I will appreciate if someone clear the confusion.

Thanks.

[Peter Paluch]

Hello,

Regarding Cisco switches, the simple and somewhat imprecise answer is that the default port behavior is an access port in VLAN1. If you leave a port at its default settings then it behaves like an access port in the VLAN1, meaning:

• only untagged frames are being sent and received
• received untagged frames are processed in VLAN1
• only frames received in VLAN1 on other ports can be forwarded out this port, untagged
• received frames that are tagged are dropped

This would mean that you can not pass tagged frames - and thus traffic from various VLANs - through unconfigured switchports on Cisco switches.

A more precise but also a more complicated answer is that the default state of a port is the so-called dynamic state. This dynamic state allows the port to actually negotiate with its neighboring device whether it is going to be a trunk port (carrying frames from multiple VLANs and performing tagging) or an access port (belonging into a single VLAN and not using tagging). The goal of this dynamic negotiation is to make sure that both devices interconnected by a single link use this link in the same mode (either access or trunk, as mismatches in the operating mode can have disastrous results). This dynamic negotiation of the port's operating state is performed by a Cisco-proprietary Dynamic Trunk Protocol which is, to my best knowledge, supported only on Cisco switches. The port tries to negotiate its operating mode after it gets connected. If the neighboring device does not respond to negotiation attempts, the port will remain in the access mode for VLAN1.

So if you left the port unconfigured and connected a non-Cisco switch to that port, the port would be unable to successfully negotiate the operating mode, and would fall back to operation in VLAN1 access mode. If the connected device would be a Cisco switch then the outcome of this negotiation would also depend on the configuration of the neighboring switch.

If you want to pass tagged traffic through a Cisco switch, these requirements must be met:

1. The incoming and outgoing ports must be operating as trunks (either configured statically or negotiated with the neighboring device via DTP; static configuration is definitely preferred)
2. The VLANs whose traffic you want to forward must be created on the switch. A Cisco switch will not pass traffic for VLANs that are not created on the switch

Feel welcome to ask further!

Best regards,

Peter