Identify basic switching concepts and the operation of Cisco switches

Collision Domains The term collision domain defines the set of devices for which their frames could collide. All devices on a 10BASE2, 10BASE5, or 10BASE-T network using a hub risk collisions between the frames that they send, so all devices on one of these types of Ethernet networks are in the same collision domain.

Collision Domains LAN switches overcome the problem created by collisions and the CSMA/CD algorithm by removing the possibility of a collisions. Switches do not create a single shared bus, like a hub; they treat each individual physical port as a separate bus.

Collision Domains Switches use memory buffers to hold incoming frames as well, so when two attached devices send a frame at the same time, the switch can forward one frame while holding the other frame in a memory buffer, waiting to forward one frame until after the first one has been forwarded.

Collision Domains A Collision Domain es a set of network interface cards (NIC’s) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC, in the same collision domain.

Broadcast Domain A Broadcast Domain is a set of NICs for which a broadcast frame sent by one NIC will be received by all other NICs in the same broadcast domain.

Collision and Broadcast Domains Hubs do not actually segment an Ethernet into multiple segments. Bridges and switches do segment an Ethernet into different collision domains. Routers segment an Ethernet into different collision and broadcast domains.

Benefits when moving from one ethernet segment to multiple segments FeatureBridgingSwitchingRouting Greater cabling distance allowedYes Decrease in collisionsYes Decreased adverse impact of broadcast No Yes Decreased adverse impact of multicast No Yes Increase in bandwidthYes

Ways to switch Store and forward Cut through Fragment-free

Switch Internal Processing Switching Method Description Store-and-forwardThe switch fully receives al bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame. (The FCS is in the Ethernet trailer). Cut-throughThe switch performs the address table lookup as soon as the destination address field in the header is received. The first bits in the frame can be sent out the outbound port before the final bits in the incoming frame are received. This does not allow the switch to discard frames that fail the FCS check. Fragment-freeThis performs like cut-through switching, but the switch waits for 64 bytes to be received before forwarding the first bytes of the outgoing frame. According to Ethernet specifications, collision should be detected during the first 64 bytes of the frame, so frames in error because of a collision will not be forwarded.

CAM Table Content Addressable Memory table All Catalyst switch models use a Content Addressable Memory table for Layer 2 switching. As frames arrive on switch ports, the source MAC addresses are learned and recorded in the CAM table. The port of arrival and the VLAN are both recorded in the table along with a timestamp.

CAM Table Content Addressable Memory table If a MAC address learned on one switch port has moved to a different port, the MAC address and timestamp are recorded for the most recent arrival port. Then, the previous entry is deleted. If a MAC address is found already present in the table for the correct arrival port, only its timestamp is updated.

CAM Table Content Addressable Memory table To manage the CAM table space, stale entries (addresses that have not been heard from for a period of time) are aged out. By default, idle CAM table entries are kept for 300 seconds before they are deleted. The default timestamp setting can be changed using the following configuration command: ◦ Switch(config)#mac –address-table aging-time seconds