Switch interface e switch table. Chiarimento

Managed / Unmanaged Switch, Moduli e Connettori SFP
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jonnet
Messaggi: 39
Iscritto il: 06/03/2011, 18:19

Switch interface e switch table. Chiarimento

Messaggio da jonnet » 08/05/2019, 16:39

Ciao
Stavo leggendo un paragrafo di un libro in inglese sul networking.
Sono arrivato alla parte che tratta degli switch e la switch table, ma c'è un passaggio che non mi è del tutto chiaro.
Faccio copia e incolla qui del pezzo:


"To understand how switch filtering and forwarding work, suppose a frame with destination address DD-DD-DD-DD-DD-DD arrives at the switch on interface x. The switch indexes its table with the MAC address DD-DD-DD-DD-DD-DD. There are three possible cases:

• There is no entry in the table for DD-DD-DD-DD-DD-DD. In this case, the switch forwards copies of the frame to the output buffers preceding all interfaces except for interface x. In other words, if there is no entry for the destination address, the switch broadcasts the frame.

• There is an entry in the table, associating DD-DD-DD-DD-DD-DD with interface x. In this case, the frame is coming from a LAN segment that contains adapter DD-DD-DD-DD-DD-DD. There being no need to forward the frame to any of the other interfaces, the switch performs the filtering function by discarding the frame.

• There is an entry in the table, associating DD-DD-DD-DD-DD-DD with interface y=/x (non uguale). In this case, the frame needs to be forwarded to the LAN segment attached to interface y. The switch performs its forwarding function by putting the frame in an output buffer that precedes interface y.


Let’s walk through these rules for the uppermost switch in Figure 6.15 and its
switch table in Figure 6.22. Suppose that a frame with destination address 62-FE-F7-11-89-A3 arrives at the switch from interface 1. The switch examines its table and sees that the destination is on the LAN segment connected to interface 1 (that is, Electrical Engineering). This means that the frame has already been broadcast on the LAN segment that contains the destination. The switch therefore filters (that is, discards) the frame. Now suppose a frame with the same destination address arrives from interface 2. The switch again examines its table and sees that the destination is in the direction of interface 1; it therefore forwards the frame to the output buffer
preceding interface 1. It should be clear from this example that as long as the switch table is complete and accurate, the switch forwards frames toward destinations without any broadcasting.
In this sense, a switch is “smarter” than a hub.

Il punto che mi è poco chiaro è il due. Che poi mi sembra approfondito qui

"Let’s walk through these rules for the uppermost switch in Figure 6.15 and its switch table in Figure 6.22. Suppose that a frame with destination address 62-FE-F7-11-89-A3 arrives at the switch from interface 1. The switch examines its table and sees that the destination is on the LAN segment connected to interface 1 (that is, Electrical Engineering). This means that the frame has already been broadcast on the LAN segment that contains the destination. The switch therefore filters (that is, discards) the frame"


Arriva un frame con destinazione se stessa??

Aiutatemi a capire, c'è qualcosa che mi sfugge.
Grazie



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