Switching Tables (Part-2)
MAC Address Table
In the Part-1, the switch made a forwarding decision based on the destination host’s hardware address. The switch essentially forwarded the frame from one port’s ingress queue to another port’s egress queue.
To perform this forwarding decision, a switch consults its hardware address table. For Ethernet switches, this is referred to as the MAC address table, or the Layer-2 forwarding table. When a switch is first powered on, the MAC address table will be empty.
The switch will build the table through a dynamic learning process, by observing the source MAC address of frames:
1. Initially, the switch will have no knowledge of the MAC addresses of HostA and HostB.
2. When HostA sends a frame to HostB, the switch will add HostA’s MAC address to its table, associating it with port ethernet1.
3. The switch will not learn HostB’s MAC address until HostB sends a frame back to HostA, or to any other host connected to the switch.
4. HostB’s MAC address will then be associated with port ethernet2. Remember: a switch will only add MAC address table entries based on the source MAC address in a frame.
The MAC address table is stored in fast volatile memory, allowing lookups to be performed very quickly. However, this also results in dynamically learned MAC addresses being lost if the switch is rebooted or powered off. Stale (or idle) entries in the table will be aged out.
By default on Cisco switches, idle entries will be purged after 300 seconds. Most switches support statically configuring MAC addresses into the table, which will survive a reboot or power failure, and never be purged. Statically configuring entries in the table is only required in limited circumstances.
Ref:
https://www.routeralley.com/guides.html
In the Part-1, the switch made a forwarding decision based on the destination host’s hardware address. The switch essentially forwarded the frame from one port’s ingress queue to another port’s egress queue.
To perform this forwarding decision, a switch consults its hardware address table. For Ethernet switches, this is referred to as the MAC address table, or the Layer-2 forwarding table. When a switch is first powered on, the MAC address table will be empty.
The switch will build the table through a dynamic learning process, by observing the source MAC address of frames:
1. Initially, the switch will have no knowledge of the MAC addresses of HostA and HostB.
2. When HostA sends a frame to HostB, the switch will add HostA’s MAC address to its table, associating it with port ethernet1.
3. The switch will not learn HostB’s MAC address until HostB sends a frame back to HostA, or to any other host connected to the switch.
4. HostB’s MAC address will then be associated with port ethernet2. Remember: a switch will only add MAC address table entries based on the source MAC address in a frame.
The MAC address table is stored in fast volatile memory, allowing lookups to be performed very quickly. However, this also results in dynamically learned MAC addresses being lost if the switch is rebooted or powered off. Stale (or idle) entries in the table will be aged out.
By default on Cisco switches, idle entries will be purged after 300 seconds. Most switches support statically configuring MAC addresses into the table, which will survive a reboot or power failure, and never be purged. Statically configuring entries in the table is only required in limited circumstances.
Ref:
https://www.routeralley.com/guides.html
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