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How does EXOS build its ARP table?


We are using an extreme x450 switch as an access switch with pretty much basic configuration. I observed that the arp table on this switch had many entries that wasn't needed there. In the EXOS user guide, i came across ARP Learning which is enabled by default. From it I understood that the extreme switch builds its own ARP table also by observing the arp request and replies that are passing through it and not actually destined/originating to/from it. Have i understood it right?? And could that be reason of the unusually large arp-table that i see? Any help is appreciated as I'm "extremely" new to EXOS environment.

4 replies

Userlevel 6
Hello Ak,

That is correct. Any VLAN that has an IP address on it will learn ARP entries. This is going to be true even if the IP is not a default gateway. If this switch is acting as a L2 only switch then you really only need one IP on one of the VLANs for switch management.
Patrick Voss wrote:

Hello Ak,

That is correct. Any VLAN that has an IP address on it will learn ARP entries. This is going to be true even if the IP is not a default gateway. If this switch is acting as a L2 only switch then you really only need one IP on one of the VLANs for switch management.

Hello Patrick,

Many thanks for your feedback. In my case, the switch is acting as a L2 switch and there is only a single IP on one vlan for the switch management.
Userlevel 6
It has been our experience that newer XOS switches only learn arps from ip's that are in the subnet provisioned on the vlan but the older ones would learn any arp address if any ip were configured on a vlan. So if you overlapped multiple subnets over a common layer 2 vlan and put an ip on that vlan it would see and learn all the arps.

The new switches do not do this and you would see the other sugnet arps show up as a Rejected ip
EtherMAN wrote:

It has been our experience that newer XOS switches only learn arps from ip's that are in the subnet provisioned on the vlan but the older ones would learn any arp address if any ip were configured on a vlan. So if you overlapped multiple subnets over a common layer 2 vlan and put an ip on that vlan it would see and learn all the arps.

The new switches do not do this and you would see the other sugnet arps show up as a Rejected ip

Hi EtehrMAN,

Many thanks for your feedback. My switch is running version 15.3, so i think it is quite old.

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