Switch Showing mac address table but arp is empty

[GurpreetSidhu60753]

hey all,

I am a beginner and am using cisco packet tracer, i have a network with a 2960 switch connected 2 4 pcs, after pinging pc to pc the arp table is not populating in the switch but the mac address table is, I'm not sure why this occurs and how would i be able to get the arp table to populate in the switch.

Thanks in advance Gurpreet

[Giuseppe Larosa]

Hello @GurpreetSidhu60753 ,

if the C2960 acts as a L2 only switch what you see is normal:

the PC MAC addresses table are learned on the L2 switch and placed in the CAM table.

For having an ARP table that is not empty the C2960 needs to have a L3 interface on the same Vlan / IP subnet where the PCs connect to and an IP address in that subnet.

Example vlan 10

subnet 10.10.10.0/24    subnet mask 255.255.255.0

interface vlan 10

desc SVI  for L2 domain vlan 10

ip addres 10.10.10.1 255.255.255.0

no shut

int fas0/1

switchport

switchport access vlan 10

desc PC1

int fas0/2

switchport

switchport access vlan 10

desc PC2

int fas0/3

switchport

switchport access vlan 10

desc PC3

int fas0/4

switchport

switchport access vlan 10

desc PC4

in this case the ARP table should list the MAC addresses of PC1-PC4 and their IP addresses in subnet 10.10.10.0/24

Hope to help

Giuseppe

[GurpreetSidhu60753]

hey 

so i would configure the vlan like in this pdf https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/25ew/configuration/guide/conf/l3_int.pdf

[Giuseppe Larosa]

Hello @GurpreetSidhu60753 ,

this document explains multi layer switching and the use of L3 logical interfaces called SVI (interface Vlan) like in my previous example in this same thread

Hope to help

Giuseppe

[Richard Burts]

I would take a slightly different approach in explaining why the arp table might be empty. The first thing is to recognize that this switch is operating as a layer 2 switch. The layer 2 switch maintains a mac address table and every device connected to a switch port will generate an entry in the mac address table. The switch forwards frames using the layer 2 mac address and for this type of forwarding there is no need for an arp table.

On a layer 2 switch there is an arp table and that arp table is used by the management interface of the switch. So if the switch management interface wants to communicate with some other IP device then the switch would arp for that destination and create an entry in its arp table. But if the switch management interface has not been communicating then the arp table would be empty.

[GurpreetSidhu60753]

I am also confused as to why the individual pcs show and arp table when i ping other pcs, since thay are all in the same network wouldn't it be considered layer 2 thus the arp table remains empty

[Giuseppe Larosa]

Hello @GurpreetSidhu60753 ,

the ARP table contains the mapping information between OSI layer 3 addresses (IP addresses) and the corresponding OSI layer 2 MAC addresses

If you have 4 PCs and you ping the other three and they are all in the same VLAN/IP subnet you will see an ARP entry for each of the other three PCs.

The switch provides L2 connectivity between PCs in the same Vlan (10 in my previous example) for doing this it just need to examine the L2 frames and to take note of the L2 source MAC address without examining IP packets carried inside each ethernet frame.

The switch behaves as a transparent bridge: it is not allowed to modify the frames and simply forward them out the port where it learned the destination MAC address as source address in previous frames.

This is the CAM table.

Edit:

going back to my example until the switch misses an SVI in vlan 10 its ARP table will be empty it does not take part in the IP subnet 10.10.10.0/24. When we add an SVI in vlan10 with an IP address in the same IP subnet the switch can ping the PCs and will get 4 ARP entries in its ARP table pointing to interface SVI vlan10.

Hope to help

Giuseppe