Let’s consult the diagram below to learn how addresses are stored and retrieved.
Let’s pretend we just flipped the light on. Their MAC table is empty. According to the layout, we plug in the hosts’ cables to the switch’s inputs. When Host A makes first contact with Host D, the subsequent events take place:
The frame is sent from Host A (fe0/0) to Host D. (MAC address:0000.43c5.334c).
- The switch looks at the frame’s Source Address and stores the MAC address of Host A and the corresponding Interface number in its database.
- The frame’s Destination Address is checked by the switch. Given that it does not have the MAC address for Hosts D in its table, it creates a broadcast frame and sends it out over all interfaces except the one it came in on.
- Host D confirms its status as the intended receiver and sends a reply to Host A. As soon as the switch receives the reply frame on interface fe0/11, it stores the SA and the originating interface number in its table.
Any further data exchanged between the two hosts will be redirected to the appropriate interfaces according to the MAC table entries.
This happens whenever a new host is connected to the switch and begins sending and receiving data. The switch removes hosts from its MAC table if they haven’t sent any traffic in a while and adds them back in when they start sending packets again in an effort to keep the table as up-to-date as possible.
Processing in the Spanning Tree Protocol (STP)
The Spanning Tree Protocol (STP) is in charge of finding redundant links and turning them off to prevent network loops. The network’s switches negotiate the root bridge by sending and receiving BPDU messages. After the root bridge has been selected, all of the other switches must decide which of their ports will talk to the root port.
If multiple links are connected to the root bridge, only one will be allowed through (the “Designated Port”), while the others will be closed off. Here’s a practical application of STP to help illustrate how it works. To better grasp how STP functions, we will use the topology depicted below.
It’s crucial to vote for the root bridge. The root bridge can be located using its MAC address and Priority values, which are two separate fields. Without any sort of configuration, all switches are treated equally, leaving the selection of the root bridge up to the MAC address. The router with the fewest MAC addresses becomes the primary connection point. The preceding illustration has chosen Switch C as its root bridge.
Each switch must then choose a single root port, the one that is geographically closest to the route bridge, after the root bridge has been elected. The status of this port will always be “forwarding.” All of the route bridge’s ports will automatically be set to the forwarding mode. In addition, only one port per segment (the designated port) may be in the forwarding state at any given time.
We’ll use a scenario where two switches A and B each have four ports, but only two of those ports are part of the same logical network segment. Thus, in order to prevent infinite loops, it is necessary to block two of them. Ports on switch B must be disabled due to its higher MAC address value (and consequently lower priority).
As a consequence, there is always exactly one route from any given switch to every other switch. Result achieved!
Considerations Regarding STP
- Supporting redundant links while avoiding switching loops in the network are two primary goals of the Spanning Tree Protocol, a link management protocol. Because of its usefulness, it should be enabled on all switch interfaces.
- Since STP requires up to a minute to converge and provide redundancy, it has a high convergence time. The Rapid Spanning Tree Protocol is a recent enhancement to the Spanning Tree Protocol (RSTP). The latter keeps all the benefits of STP while drastically cutting down on convergence time.