Configuring UDP Forwarding Support for IP Redundancy Virtual Router Groups

Last Updated: November 16, 2011

User Datagram Protocol (UDP) forwarding is a feature used in Cisco IOS software to forward broadcast and multicast packets received for a specific IP address. Virtual Router Group (VRG) support, implemented with the Hot Standby Routing Protocol (HSRP), allows a set of routers to be grouped as a logical router that answers to a well-known IP address. The UDP Forwarding Support for IP Redundancy Virtual Router Groups feature enables UDP forwarding to be VRG aware; this results in packets getting forwarde only to the active router in the VRG.

This module explains the concepts related UDP forwarding and VRG support and describes how to configure UDP forwarding support for IP Redundancy Virtual Router Groups in a network.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for UDP Forwarding Support for IP Redundancy Virtual Router Groups

  • The UDP Forwarding Support for Virtual Router Groups feature is available only on platforms that support VRGs.

Information About UDP Forwarding Support for IP Redundancy Virtual Router Groups

Benefits of the UDP Forwarding Support for Virtual Router Groups Feature

Forwarding is limited to the active router in the VRG instead of all routers within the VRG. Prior to the implementation of this feature, the only VRG support was HSRP. Within a VRG that is formed by HSRP, the forwarding of UDP-based broadcast and multicast packets is done by all the routers within the VRG. This process can cause some DHCP servers to operate incorrectly. The UDP Forwarding Support for VRGs feature limits forwarding to the active router in the VRG.

VRG awareness is achieved with IP Redundancy Service (IRS). The IRS application programming interface (API) provides notification updates of a specific VRG, addition and deletion of a VRG, and querying of the current state of a VRG. A state change notification is provided to avoid the performance impact of querying the state of the VRG each time it is needed. The UDP forwarding code caches the VRG state for each required helper address that is defined. Each time the UDP forwarding code needs to execute, it checks the current state of the VRG associated with the helper address and forwards packets only to VRGs that are active.

How to Configure UDP Forwarding Support for IP Redundancy Virtual Router Groups

Configuring UDP Forwarding Support for IP Redundancy Virtual Router Groups

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number

4.    no shutdown

5.    ip address ip-address mask

6.    ip helper-address address redundancy vrg-name

7.    standby group-number ip ip-address

8.    standby group-number name group-name

9.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number


Example:

Router(config)# interface fastethernet 0/0

 

Specifies an interface and enters interface configuration mode.

 
Step 4
no shutdown


Example:

Router(config-if)# no shutdown

 

Restarts a disabled interface.

 
Step 5
ip address ip-address mask


Example:

Router(config-if)# ip address 172.16.10.1 255.255.255.0

 

Sets a primary address for the interface.

 
Step 6
ip helper-address address redundancy vrg-name


Example:

Router(config-if)# ip helper-address 10.1.1.1 redundancy vrg1

 

Enables UDP forwarding support for the VRG.

 
Step 7
standby group-number ip ip-address


Example:

Router(config-if)# standby 1 ip 172.16.10.254

 

Activates HSRP.

 
Step 8
standby group-number name group-name


Example:

Router(config-if)# standby 1 name vrg1

 

Configures the name of the standby group.

 
Step 9
end


Example:

Router(config-if)# end

 

Exits the current configuration mode and returns to privileged EXEC mode.

 

Configuration Examples for UDP Forwarding Support for IP Redundancy Virtual Router Groups

Example: Configuring UDP Forwarding Support for IP Redundancy Virtual Router Groups

The following example shows how to configure UDP Forwarding Support for IP Redundancy Virtual Router Groups: 

Router(config)# interface fastethernet 0/0
Router(config-if)# no shutdown
Router(config-if)# ip address 172.16.10.1 255.255.255.0
Router(config-if)# ip helper-address 10.1.1.1 redundancy vrg1
Router(config-if)# standby 1 ip 172.16.10.254
Router(config-if)# standby 1 name vrg1
Router(config-if)# end

Additional References

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

IP application services commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

Cisco IOS IP Application Services Command Reference

Standards

Standard

Title

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MIBs

MIB

MIBs Link

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To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC

Title

No new or modified RFCs are supported, and support for existing RFCs has not been modified

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Technical Assistance

Description

Link

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Feature Information for UDP Forwarding Support for IP Redundancy Virtual Router Groups

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1 Feature Information for UDP Forwarding Support for IP Redundancy Virtual Router Groups

Feature Name

Releases

Feature Information

UDP Forwarding Support for IP Redundancy Virtual Router Group

Cisco IOS XE 3.1.0SG

12.2(15)T

User Datagram Protocol (UDP) forwarding is a feature used in Cisco IOS software to forward broadcast and multicast packets received for a specific IP address. Virtual Router Group (VRG) support is currently implemented with the Hot Standby Routing Protocol (HSRP) and it allows a set of routers to be grouped as a logical router that answers to a well known well-known IP address. The UDP Forwarding Support for IP Redundancy Virtual Router Groups feature enables UDP forwarding to be VRG aware, resulting in forwarding only to the active router in the VRG.

The following command was introduced or modified: ip helper-address.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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