Why isn't my switch doing arp???

Robbo777 · September 2015

I've successfully configured a router on a stick topology in GNS3 but my switch connecting to the router and the pc's is not displaying the arps of the other devices when I use the show arp command! I find this odd because all the other devices are displaying the arp's and everything is working to! Any idea as to why this not working??

Thanks for the help!

EdTheLad · September 2015

"show arp" will show the contents or the arp cache, the arp cache contains the mapping of "destination ip address" to "destination mac". When does the switch need to know the destination mac address? when its routing i.e. a layer 3 switch. As a layer 2 switch it will examine the destination ip address, check the mac table for the exit interface and forward the frame, the ip address never comes into play, so the arp cache is never updated.

volfkhat · September 2015

Wow,
Excelente Explicación, mi Amigo :]

Robbo777 · September 2015

I thought that it still needed to know the mac address of the connected devices to know where it's going to send the frames? If it doesn't keep track of the mac address then how does it know where to send the frames?

Deathmage · September 2015

If a device that is connected to the switch have a MAC Address on the Nic of the device, as it should, then the switch should automatically store the mac address once it's plugged in.

If it doesn't it's most of the time a bad port, wrong cable, incorrectly terminated cable, port could be shutdown and finally a bad nic overall.

networker050184 · September 2015

A switch does keep track of the MAC's, but the mechanism isn't ARP.

Robbo777 · September 2015

Oh right thanks, so is there a command on the switch that would enable me to view the MAC addresses stored on the switch?

theodoxa · September 2015

A Layer 2 switch will use the CAM or MAC Address Table to determine the egress port.

1. Switch receives frame destined to MAC Address 0123.4567.890A
2. Switch looks for the Destination MAC Address in its CAM Table for the VLAN the frame was received on.
3A. If 0123.4567.890A is not listed in there, the switch will "Flood" the frame out all ports except the one it was received on.
3B. If 0123.4567.890A is listed there, the switch will forward the frame out the port listed for that MAC Address.
4. If the switch "Flooded" the frame and assuming it makes it to its destination, the receiver will likely send something back to the sender (be it an acknowledgedment, requested data, or whatever). When that frame is received at the switch, it will add the MAC Address and port it was received on to its CAM Table for the VLAN it was received on. Subsequent packets can now be forwarded out only the port leading to that MAC Address.

BRSW#show mac address-table dynamic

          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 172    0011.d9**.****    DYNAMIC     Gi0/1
 172    0011.d9**.****    DYNAMIC     Gi0/1
 172    0014.d1**.****    DYNAMIC     Gi0/1
 172    001b.a9**.****    DYNAMIC     Gi0/4
 172    001f.ca**.****    DYNAMIC     Gi0/1
 172    0040.9d**.****    DYNAMIC     Gi0/3
 172    6c33.a9**.****    DYNAMIC     Gi0/2
 172    7081.05**.****    DYNAMIC     Gi0/1
 172    c033.5e**.****    DYNAMIC     Gi0/1
  11    000c.29**.****    DYNAMIC     Gi0/15
  20    000c.29**.****    DYNAMIC     Gi0/15
Total Mac Addresses for this criterion: 11

Robbo777 · September 2015

in my topology I have 2 different "sites" set up in GNS3. I have a router in the middle acting as an isp and I have a router at both end points of the sites. I would like to know what is the best way I enable them to communicate across the "isp"?.
I can post a pic of the config if you guys want but I'm not sure if I can do it on this website.

P.S....For the "isp" what type of device should I be using? Should I be using a router iso with some modifications to the config?

Thanks again for the help!

Why isn't my switch doing arp???

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