a) Why, despite the carrier sense functionality, collisions may still occur in t

Question

a) Why, despite the carrier sense functionality, collisions may still occur in the CSMA/CD
Medium Access Control? Answer (give a cause of collisions) in one short sentence. [2]
b) How does an Ethernet switch know where to forward a frame with a given destination MAC
address? Name and briefly (in 1-2 sentences) describe the applicable protocol. [2]
c) If you disconnect a host from one port on the switch and connect it to another port, how does
the switch know that the host is no longer connected to the old port? Name and briefly describe
the mechanism responsible for “dropping” MAC addresses from port forwarding tables. [2]

Explanation / Answer

a. Because of propagation delays. When 2 or more stations may sense that the channel is free, they start transmitting at time instants that are close enough for a collision to occur. b. The switch learns MAC addresses by examining the source MAC address of each frame the bridge receives to make good forwarding choices. The switch then decides when to forward a frame or when to filter (not forward) it based on the destination MAC address. The switch then looks at the previously learned MAC addresses in an address table to decide where to forward the frames. c. Because by default, all ports on a switch are VLAN 1 and that’s how they all can communicate. As soon as you change the VLAN assignment for a switch port to another VLAN, that switch port won’t be able to communicate with the rest of the devices on other ports, therefore knowing host no longer connected. Unicast Reverse Path Forwarding (URPF) discards packets that lack a consistent source IP address. It uses tables to verify that the source addresses and the interfaces on which packets were received are consistent with the forwarding tables on the supervisor/main engine. If the packet was received from reverse path routes, the packet is forwarded. If there is no reverse path route on the interface on which the packet was received, the packet fails the URPF check and is discarded.

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