Bulk Logging and Port Block Allocation

The Bulk Logging and Port Block Allocation feature allocates a block of ports for translation instead of allocating individual ports. This feature is supported only in carrier-grade Network Address Translation (CGN) mode.

This module provides information about the feature and how to configure it.

Prerequisites for Bulk Logging and Port Block Allocation

  • Enable the carrier-grade Network Address Translation (CGN) mode before enabling the Bulk Logging and Port Block Allocation feature.

  • Enable paired-address pooling for this feature to work.

Restrictions for Bulk Logging and Port Block Allocation

  • The Bulk Logging and Port Block Allocation feature is not supported on interface overload configurations because Network Address Translation (NAT) does not own the port space, the device owns it. You can configure an interface-overload mapping with this feature; however, no messages will be logged for the configuration.

  • Destination information is not logged.

  • Application layer gateways (ALGs) that require consecutive port pairings only work when bulk-port allocation is configured with a step size of one. For more information on step size, see “Bulk Logging and Port Block Allocation Overview.”

  • Only bulk logging of messages is performed when this feature is enabled.

  • ALG ports can be used for bulk-port allocation; however, this can cause degraded performance in sessions associated with these ports. If your configuration does not need ALGs, we recommend that you disable ALGs using the CLI.

  • Syslog is not supported.

  • Low ports, ports below 1024, are not supported; any application that requires a low port does not work with this feature.

  • Bulk-port allocation pools must not overlap with static NAT mappings (particularly static mappings with ports) for this feature to work.

  • The ip nat service full-range command is not supported.

Information About Bulk Logging and Port Block Allocation

Bulk Logging and Port Block Allocation Overview

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 in Bulk Logging and Port Block Allocation

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 default port size is 512 ports, but it can differ based on the configured paired-address pooling limit. The following table provides information of the default port size when various paired-address pooling limits are configured:
Table 1. Default Port Size Based on Paired-Address Pooling Support

Paired-Address Pooling Limit

Default Bulk-Port Allocation Port Size

Maximum Port Step Size

120

512 ports

8

30

2048 ports

2

60

1024 ports

4

250

256 ports

4

500

128 ports

8

1000

64 ports

16

High-Speed Logging in Bulk Logging and Port Block Allocation

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.

The following table provides information about HSL fields, their format and value:
Table 2. HSL Field Description

Field

Format

ID

Value

Source IP address

IPv4 address

8

Varies

Translated source address

IPv4 address

225

Varies

VRF1 ID

32-bit ID

234

Varies

Protocol

8-bit value

4

Varies

Event

8-bit value

230

  • 0—Invalid

  • 1—Add event

  • 2—Delete event

UNIX timestamp in milliseconds

64-bit value

323

Varies

Port block start

16-bit port

361

Varies

Port block step size

16-bit step size

363

Varies

Number of ports in the block

16-bit number

364

Varies
1 virtual routing and forwarding

How to Configure Bulk Logging and Port Block Allocation

Configuring Bulk Logging and Port-Block Allocation

Before you configure bulk logging and port-block allocation, you must:

  • Enable carrier-grade Network Address Translation (CGN) mode.

  • Enable NAT paired-address pooling.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface type number
  4. ip nat inside
  5. exit
  6. interface type number
  7. ip nat outside
  8. exit
  9. ip nat settings mode cgn
  10. ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length}
  11. access-list access-list-number permit source [source-wildcard]
  12. ip nat inside source list access-list-number pool name
  13. ip nat settings pap bpa set-size 512 step-size 8
  14. ip nat log translations flow-export v9 udp destination addr port
  15. end
  16. show ip nat translations

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Device> enable
Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

interface type number

Example:

Device(config)# interface gigabitethernet 0/0/0

Specifies an interface and enters interface configuration mode.

Step 4

ip nat inside

Example:

Device(config-if)# ip nat inside

Connects the interface to the inside network, which is subject to Network Address Translation (NAT).

Step 5

exit

Example:

Device(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 6

interface type number

Example:

Device(config)# interface gigabitethernet 1/0/1

Specifies an interface and enters interface configuration mode.

Step 7

ip nat outside

Example:

Device(config-if)# ip nat outside

Connects the interface to the outside network.

Step 8

exit

Example:

Device(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 9

ip nat settings mode cgn

Example:

Device(config)# ip nat settings mode cgn

Enables CGN mode.

Step 10

ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length}

Example:

Device(config)# ip nat pool net-208 192.168.202.129 192.168.202.132 prefix-length 24

Defines a pool of global addresses to be allocated as needed.

Step 11

access-list access-list-number permit source [source-wildcard]

Example:

Device(config)# access-list 1 permit source 192.168.34.0 0.0.0.255

Defines a standard access list that permits addresses that are to be translated.

Step 12

ip nat inside source list access-list-number pool name

Example:

Device(config)# ip nat inside source list 1 pool net-208

Establishes dynamic NAT by specifying the access list and the IP address pool defined in Step 10 and Step 11.

Step 13

ip nat settings pap bpa set-size 512 step-size 8

Example:

Device(config)# ip nat settings pap bpa set-size 512 step-size 8

Configures bulk-port allocation.

Step 14

ip nat log translations flow-export v9 udp destination addr port

Example:

Device(config)# ip nat log translations flow-export v9 udp destination 10.1.1.1 2055

Enables the high-speed logging (HSL) of all NAT translations.

Step 15

end

Example:

Device(config)# end

Exits global configuration mode and returns to privileged EXEC mode.

Step 16

show ip nat translations

Example:

Device# show ip nat translations

Displays active NAT translations.

Configuration Examples for Bulk Logging and Port Block Allocation

Example: Configuring Bulk Logging and Port Block Allocation

In the following example, dynamic carrier-grade NAT (CGN) and paired-address pooling is configured for bulk-port allocation.

Device# configure terminal
Device(config)# interface gigabitethernet 0/0/0
Device(config-if)# ip nat inside
Device(config-if)# exit
Device(config)# interface gigabitethernet 1/0/1
Device(config-if)# ip nat outside
Device(config-if)# exit
Device(config)# ip nat settings mode cgn
Device(config)# ip nat pool net-208 192.168.202.129 192.168.202.132 prefix-length 24
Device(config)# access-list 1 permit source 192.168.34.0 0.0.0.255
Device(config)# ip nat inside source list 1 pool net-208 
Device(config)# ip nat settings pap bpa set-size 512 step-size 8
Device(config)# ip nat log translations flow-export v9 udp destination 10.1.1.1 2055
Device(config)# end

Verifying Bulk Logging and Port Block Allocation

SUMMARY STEPS

  1. show ip nat bpa
  2. show ip nat pool namepool-name

DETAILED STEPS

Step 1

show ip nat bpa

Example:

Device# show ip nat bpa

Displays Network Address Translation (NAT) bulk logging and port-block allocation settings.

The following is sample output from the show ip nat bpa command: 

Device# show ip nat bpa

Paired Address Pooling (PAP)
Limit: 120 local addresses per global address
Bulk Port Allocation (BPA)
Port set size: 1024 ports in each port set allocation
Port step size: 1
Single set: True

Step 2

show ip nat pool namepool-name

Example:

Device# show ip nat pool name pool1

Displays NAT pool and port statistics.

The following is sample output from the show ip nat pool name pool1 command: 

Device# show ip nat pool name pool1

NAT Pool Statistics
Pool name pool1, id 1
Assigned Available
Addresses 0 5
UDP Low Ports 0 0
TCP Low Ports 0 0
UDP High Ports 0 150
TCP High Ports 0 150
(Low ports are less than 1024. High ports are greater than or equal to 1024.)

The following is sample output from the show ip nat pool name pool3 command: 

Device# show ip nat pool name pool3

NAT Pool Statistics
Pool name pool3, id 4
Assigned Available
Addresses 0 9
UDP Low Ports 0 0
TCP Low Ports 0 0
UDP High Ports 0 1080
TCP High Ports 0 1080
(Low ports are less than 1024. High ports are greater than or equal to 1024.)

Additional References for Bulk Logging and Port Block Allocation

Related Documents

Related Topic Document Title

Cisco IOS Commands

Master Command List

NAT commands

Cisco IOS IP Addressing Services Command Reference

Carrier-grade NAT

“Carrier-Grade Network Address Translation” module in the IP Addressing NAT Configuration Guide

Paired-address pooling support

“Paired-Address Pooling Support in NAT” module in the IP Addressing NAT Configuration Guide

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