Every network starts with a switch, that connects a couple of devices and eventually a device that is being used as a default gateway that would enable external reachability.
One should keep in mind that Switches make hosts communicate using their MAC Addresses, not their IP Addresses. To make that possible switches use a protocol called ARP (Address Resolution Protocol) that matches IP addresses to MAC addresses. They’re usually stored in a tabled called the ARP Table.
To view the ARP table we use the command:
SAMPLE-SW#show ip arp Protocol Address Age (min) Hardware Addr Type Interface Internet 10.10.10.12 0 0009.0f09.0012 ARPA Vlan30 Internet 10.10.10.222 10 0050.56ae.8dab ARPA Vlan30 Internet 10.10.10.232 - 4ce1.755d.ebe5 ARPA Vlan30 Internet 10.10.10.234 4 308b.b28d.179e ARPA Vlan30 Internet 10.10.10.235 131 6c41.0ec5.cb24 ARPA Vlan30
As noticed, the ARP table contains the IP address, the MAC address and the interface it was learned on.
ARP works when a switch receives a packet with an IP address that is not being matched to a known MAC address so it Floods that packet to all of the available ports with the exception of the original port
Once an ARP request is fulfilled, the learned MAC address and the interface that it was learned on are added to the MAC address table with an entry type of dynamic.
Once an ARP request is fulfilled, the learned MAC address and the interface that it was learned on are added to the MAC address table with an entry type of dynamic.
the command to view the MAC Address table is
CORE_SWITCH#sh mac address-table dynamic vlan 30 Mac Address Table ------------------------------------------- Vlan Mac Address Type Ports ---- ----------- -------- ----- 30 000c.29cf.7491 DYNAMIC Gi1/0/2 30 000c.820d.08ea DYNAMIC Gi1/0/48 30 0020.85df.956e DYNAMIC Gi1/0/16 30 0020.85df.9580 DYNAMIC Gi1/0/17 30 0045.1d6b.754b DYNAMIC Gi1/0/45 30 0050.565b.2608 DYNAMIC Gi1/0/12 30 0050.565d.acda DYNAMIC Gi1/0/3 30 0050.565e.5eee DYNAMIC Gi1/0/2 30 0050.565f.8174 DYNAMIC Gi1/0/11 30 0050.56a0.022f DYNAMIC Gi1/0/2 30 0050.56a0.034c DYNAMIC Gi1/0/2
Whenever more than 1 MAC address is mapped to a single port, it means that another switch (or a wireless access point) is connected to the other end.
