Hi ssj100, I personally know zip about this stuff. But, if CIS doesn't do something with regards to ARPs, then something must be wrong. Because looking at CIS's help, there's an awful lot of nothing there.
Protect the ARP Cache
Checking this option makes Comodo Firewall to start performing stateful inspection of ARP (Address Resolution Protocol) connections. This will block spoof ARP requests and protect your computer from ARP cache poisoning attacks.
The ARP Cache (or ARP Table) is a record of IP addresses stored on your computer that is used to map IP addresses to MAC addresses. Stateful inspection involves the analysis of data within the lowest levels of the protocol stack and comparing the current session to previous ones in order to detect suspicious activity.
Background - Every device on a network has two addresses: a MAC (Media Access Control) address and an IP (Internet Protocol) address. The MAC address is the address of the physical network interface card inside the device, and never changes for the life of the device (in other words, the network card inside your PC has a hard coded MAC address that it will keep even if you install it in a different machine.) On the other hand, the IP address can change if the machine moves to another part of the network or the network uses DHCP to assign dynamic IP addresses. In order to correctly route a packet of data from a host to the destination network card it is essential to maintain a record of the correlation between a device's IP address and it's MAC address. The Address Resolution Protocol performs this function by matching an IP address to its appropriate MAC address (and vice versa). The ARP cache is a record of all the IP and MAC addresses that your computer has matched together.
Hackers can potentially alter a computer's ARP cache of matching IP/MAC address pairs to launch a variety of attacks including, Denial of Service attacks, Man in the Middle attacks and MAC address flooding and ARP request spoofing. It should be noted, that a successful ARP attack is almost always dependent on the hacker having physical access to your network or direct control of a machine on your network - therefore this setting is of more relevance to network administrators than home users.
Block gratuitous ARP frames
A gratuitous ARP frame is an ARP Reply that is broadcast to all machines in a network and is not in response to any ARP Request. When an ARP Reply is broadcast, all hosts are required to update their local ARP caches, whether or not the ARP Reply was in response to an ARP Request they had issued. Gratuitous ARP frames are important as they update your machine's ARP cache whenever there is a change to another machine on the network (for example, if a network card is replaced in a machine on the network, then a gratuitous ARP frame will inform your machine of this change and request to update your ARP cache so that data can be correctly routed). Enabling this setting you will block such requests - protecting the ARP cache from potentially malicious updates.
PS I formatted it like the Help had it.