Overload with port-block-allocation CGN IP pool

Overload with Port block allocation (PBA) reduces CGNAT logging overhead by creating a log entry only when a client first establishes a network connection and is assigned a port block. The number of log entries are reduced because a log entry is created when the port block is assigned, and not for each client connection. Overload causes FortiOS to re-use ports within a block, allowing for more possible connections before running out of ports.

When all of the client sessions have ended, FortiOS releases the port block and writes another log message. You can also configure logging to only write a log message when the port block is released. See Configuring hardware logging.

In general, because each customer environment is different, different configurations may be required to achieve optimal performance.

PBA allocates a contiguous set of source translation endpoints called port blocks. These port blocks are associated to a client by one IP address and a block of ports. Port blocks are allocated on-demand and have a fixed size.

Choose these settings carefully to adequately and efficiently service clients that may require a different number of simultaneous connections. Careful analysis and testing is required to find optimal values for the traffic conditions on your network.

From the GUI

Go to Policy & Objects > IP Pools.
Select IP Pool (for IPv4 IP pools) or IPv6 IP Pool.
Select Create New.
Give the IP pool a Name.
Set Type to CGN Resource Allocation.
Set Mode to Overload (Port Block Allocation).
Configure the External IP Range to specify the set of translation IP addresses available in the pool as a collection of IP prefixes with their prefix lengths. These are typically public-side addresses.
Optionally Exclude IPs from the External IP Range. You can include multiple single IP addresses.
Configure the Start port and End port to define the source port range for the IP pool.
Configure the Block Size to set the number of ports allocated in a block.
You can enable NAT64 to make this a NAT64 IP pool.
Enable or disable ARP reply to reply to ARP requests for addresses in the external address range.

From the CLI

Use the following command to configure Overload PBA CGN IP pools from the CLI:

config firewall ippool

edit <name>

set type cgn-resource-allocation

set startip <ip>

set endip <ip>

set arp-reply {disable | enable}

set arp-intf <interface-name>

set associated-interface <interface-name>

set cgn-spa disable

set cgn-overload enable

set cgn-client-ipv6shift <shift>

set cgn-block-size <number-of-ports>

set cgn-port-start <port>

set cgn-port-end <port>

set utilization-alarm-raise <usage-threshold>

set utilization-alarm-clear <usage-threshold>

set nat64 {disable | enable}

set exclude-ip <ip>, <ip>, <ip> ...

end

You can define an overload port-block allocation IP pool by configuring the following:

External IP range (start-ip and end-ip). Specifies the set of translation IP addresses available in the pool as a collection of IP prefixes with their prefix lengths. These are typically public-side addresses.
Exclude IPs (exclude-ip). Specify external IP addresses that the CGN IP pool will not allocate. This is a security feature that allows you to exclude one or more IP addresses from being allocated if the IP pool could assign addresses that have been targeted by external attackers. You can only add single IP addresses. You cannot add IP address ranges. From the CLI you can use the ? to see how many IP addresses you can add. The limit depends on the FortiGate model.
Start port (cgn-port-start). The lowest port number in the port range. The default value is 5117.
End port (cgn-port-end). The highest possible port number in the port range. The default value is 65530.
Block size (cgn-block-size). The number of ports allocated in a block. The default value is 128. Use a smaller port block size to conserve available ports.
NAT64 (nat64). Enable to make this a NAT64 IP pool.
ARP reply (arp-reply). Enable to reply to ARP requests for addresses in the external address range.

CLI-only options:

Optionally specify the interface (arp-intf) that replies to ARP requests.
Optionally specify the interface associated with this IP pool (associated-interface).
For NAT64 IP pools, you can use the cgn-client-ipv6shift option to limit the matching of IPv6 client addresses. By default, in an IP pool, IPv6 addresses are matched based on all 128 bits of the address. You can use this option if you want client IPv6 IP addresses to be matched on fewer bits in the IP address. For example, if you want IPv6 addresses to match based on the lower 32 bits of the IPv6 address to match, you can set cgn-client-ipv6shift to 32.
Generate an SNMP trap when the usage of the resources defined by an IP pool exceeds a threshold (utilization-alarm-raise). The range is 50 to 100 per cent.
Generate an SNMP trap when the usage of the resources defined by an IP pool falls below a threshold (utilization-alarm-clear). The range is 40 to 100 per cent.