Can someone help me to find the cisco official definition of hub, bridge, switch and router. I mean i know how these devices works, i just want to read how cisco defined them.
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Not Cisco official, but a hub is very generally a center of multiple spokes, term often used for a L2 multi-port Ethernet repeater.
A bridge is generally a L2 device that applies some intelligence to traffic forwarding. It controls replication of ingress traffic to egress and/or it might do media conversion.
A switch generally is a multi-port L2 bridge without media conversion.
A router is basically the L3 version of a bridge although there's always a L3 forwarding decision (unless device is configured for L2 bridging).
In the Cisco line-up, term usage can be confusing. For instance, a 7600 and a 6500 if exactly the same hardware (except "chassis") and if running exactly the same IOS (before they were split into different IOS trains), labels the 7600 a router and the 6500 a switch. (NB: to be fair, the 7600 and 6500 each supports/supported hardware and software features unique to their router/switch roles, although probably this was a marketing decision, not a technical decision.)
And don't forget MLS or MPLS which "blur" the distinction between L2 bridges and L3 routers.
Please refer to the CCNA Certification Guide for the closest thing to an "official Cisco definition". It states:
hub. A LAN device that provides a centralized connection point for LAN cabling, repeating any received electrical signal out all other ports, thereby creating a logical bus.
bridge. Not separately defined but described as a device that segments and Ethernet by creating separate collision domains.
switch. A network device that filters, forwards and flood Ethernet frames based on the destination address of each frame.
router. Not separately defined but described in that they collectively provide the main feature of the network layer—the capability to forward packets end-to-end through a network. Routers forward packets by connecting to various physical network links, like Ethernet, serial links, and Frame Relay, and then using Layer 3 routing logic to choose where to forward each packet.