Why does the show arp returns a different IP than the PC's gateway on the Switch

Question

I have a PC connected to a switch connected to a router : why does the show arp returns a different IP than the PC's gateway on the Switch ? :

PC $ arp -a | grep -w gateway
gateway (172.16.15.1) at 00:00:5e:00:01:01 [ether] on bond0
PC $

Switch#show mac-address-table address 00:00:5e:00:01:01

Codes: *N - VLT Peer Synced MAC
*I - Internal MAC Address used for Inter Process Communication
VlanId     Mac Address           Type          Interface        State
 4      00:00:5e:00:01:01       Dynamic         Te 0/0          Active
 6      00:00:5e:00:01:01       Dynamic         Te 0/0          Active
 8      00:00:5e:00:01:01       Dynamic         Te 0/0          Active
 9      00:00:5e:00:01:01       Dynamic         Te 0/0          Active
 166    00:00:5e:00:01:01       Dynamic         Te 0/0          Active
Switch#show arp macaddress 00:00:5e:00:01:01

Protocol    Address         Age(min)  Hardware Address    Interface      VLAN             CPU
---------------------------------------------------------------------------------------------
Internet    172.17.10.1         192   00:00:5e:00:01:01   Te 0/0         Vl 6             CP

Answer 1

As it seems, the PC is connected to a different subnet (the gateway 172.16.15.1 might indicate 172.16.15.0/24) than what the switch shows for VLAN 6 (172.17.10.1 might indicate 172.17.10.0/24).

Since a router is connected to multiple subnets, that one seems to be connected to both subnets, with different IP addresses (necessarily) but using the same MAC address on both (entirely legal).

Note that there must be a unique mapping from an IP address to a MAC address, but a single MAC address may be mapped by multiple IP addresses, even on different subnets as in your case.

Also note that the switch's ARP cache is only populated and relevant when it's routing (for a layer-3 switch) or used by management. A plain layer-2 switch uses neither IP addresses nor ARP (except for management).