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OECG – Chapter 2 Definitions


VLAN – a virtual LAN. Ports defined to be a common broadcast domain. Can span multiple switches through the use of trunks.

broadcast domain – devices that are on the same ethernet area. When a broadcast is sent to this area, everyone in the area gets it. Typically, one subnet per broadcast domain.

DTP – dynamic trunking protocol. What a Cisco switch uses to dynamically decide to trunk (or not) and what trunking protocol to use.

VTP pruning – VTP will “prune” VLAN’s from carrying broadcast traffic (and unknown unicast traffic) for a particular trunk if there’s no ports assigned to that VLAN on the switch on the far side of the trunk.

802.1Q – a VLAN trunking protocol that uses 4 byte tags. The first 2 bytes of the tag are 0x8100, and the last 2 are the VLAN number. The tag gets popped into the header right after the source address, and the 0x8100 tells the ethernet device that those bytes are 802.1q VLAN related.

ISL – interswitch link, a proprietary Cisco protocol used to as an alternative to 802.1q trunking. ISL performs encapsulation instead of tagging, placing a 26-byte header and new trailer to accommodate the new FCS value. If 2 Cisco devices are using DTP, ISL will be the winning protocol if both support it.

native VLAN – Frames sourced from this VLAN will not be tagged as they traverse a trunk. 802.1q only – not supported in ISL.

encapsulation – Implying that a frame gets a header and a trailer put around it. ISL encapsulates, 802.1q tags.

private VLAN – Used primarily to conserve IP address while still providing security.

promiscuous port – In a private VLAN, promiscuous ports can talk to all other ports, in the primary private VLAN.

community VLAN – In a private VLAN, community ports can only talk to promiscuous and ports in the same community. Community ports are members of a secondary VLAN.

isolated VLAN – In a private VLAN, isolated ports can only talk to promiscuous ports. Isolated ports are members of a secondary VLAN.

802.1Q-in-Q – Nested tags allow 802.1Q traffic to tunnel across a service provider network, while allowing the customer to retain his 802.1Q tags.

Layer 2 protocol tunneling – another name for 802.1Q-in-Q

By Ethan Banks
Ethan Banks On productivity.

You probably know Ethan Banks because he writes & podcasts about IT. For example, he co-authored "Computer Networks Problems & Solutions" with Russ White.

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