Inter-switch link was developed by Cisco Systems as means of supporting multiple VLANs over a single link. ISL is an encapsulation protocol that allows Ethernet frames to get encapsulated with the proper VLAN information. ISL adds a 26 byte header and a new4 byte CRC frame at the end of the packet. With an ISL trunk port, all received packets are expected to be encapsulated with an ISL header, and all transmitted packets are sent with an ISL header. Native (non-tagged) frames received from an ISL trunk port are dropped. Catalyst 3550 Series switches, also participate in DTP (Dynamic Trunking Protocol) that enables two trunk capable switches to negotiate a trunk. The modes available are desirable and auto, which is default. When in desirable mode the switch will negotiate a trunk with another capable device that is in either auto or desirable mode. If two switches are in auto mode, they will not negotiate a trunk.
Dot1q is an IEEE standard for relaying multiple VLANs across the same link. 802.1q only adds 4 new bytes of information to the Ethernet frame, and replaces the old CRC with a new value.
The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree. Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default settings in the 802.1D spanning tree), which is critical for networks carrying delay-sensitive traffic such as voice and video.
Root port - provides the best path (lowest cost) when the switch forwards packets to the
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