The Bulk Logging and Port Block Allocation feature allocates ports to users in blocks, instead of allocating individual ports. When a session is started from inside the network, instead of allocating a single global IP address and a global port, multiple global ports of a single global IP address are allocated for Network Address Translation (NAT) of traffic. Based on the volume of translations, additional blocks of ports can be allocated.

To allocate port sets, you can use either the consecutive port-set method or the scattered port-set method. In the consecutive port-set method, a user is allocated a set of ports with consecutive port numbers. It is easy to determine the port numbers in the consecutive method and this as a result, can be a security threat.

The Bulk Logging and Port Block Allocation feature uses the scattered port-set method, which allows you to define a start port number, a step value, and the number of ports to allocate. For example, if the starting port number is 4000, the step value is four, and the number of ports is 512, then the step value of four is added to 4000 to get the second port number. Four is added again to 4004 to get the third port number and this process repeats until you have 512 ports in the port set. This method of port-set allocation provides better security.

Some application layer gateways (ALGs) require two consecutive global ports to operate correctly. These ALGs are supported with this feature only when a step value of one is configured, which allocates a consecutive port set.

You must enable NAT paired-address pooling support for this feature to work. This feature also supports Point-to-Point Tunneling Protocol (PPTP).

Note

This feature is supported only in carrier-grade NAT (CGN) mode; therefore only source information is logged when this feature is configured. Destination information is not logged. For more information about CGN, see the “Carrier-Grade Network Address Translation" module in IP Addressing: NAT Configuration Guide.

Port size is configurable and determines the number of ports allocated in each port set. However, ports below 1024, also known as low ports, will not work when bulk logging and port-block allocation is configured.

The first port that is allocated is always the first port in the set. Initially, ports are likely to be allocated in a linear method; however, as sessions are released and ports are freed, the allocation is semi-random. A port set is freed when the last session referencing it is freed.

A few port sets are reserved for users using a specific global IP address. Therefore, when allocated ports are used up, a session can use a reserved port set. If all reserved port sets are used, the session is dropped.

The Bulk Logging and Port Block Allocation feature reduces the volume of Network Address Translation (NAT) high-speed logging (HSL). The reduction is accomplished by dynamically allocating a block of global ports instead of a single global port.

Messages are usually logged when a session is created and destroyed. In bulk port allocation, messages are logged when a port set is allocated or freed.