Cisco IOS Software Releases 12.2 T

Table Of Contents

UDP Forwarding Support of IP Redundancy Virtual Router Group

Contents

Information About UDP Forwarding Support for Virtual Router Group

Benefits of the UDP Forwarding Support for Virtual Router Group

VRG Awareness

How to Configure UDP Forwarding Support for Virtual Router Group

Enabling Forwarding Support for VRG

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

ip helper-address

UDP Forwarding Support of IP Redundancy Virtual Router Group

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First Published: March 17, 2003

Last Updated: February 28, 2006

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 IP address. The UDP Forwarding Support of IP Redundancy Virtual Router Group feature enables UDP forwarding to be VRG aware, resulting in forwarding only to the active router in the VRG.

History for the UDP Forwarding Support of IP Redundancy Virtual Router Group Feature

Release	Modification
12.2(15)T	This feature was introduced.
12.2(28)SB	This feature was integrated into Cisco IOS Release 12.2(28)SB.

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

Information About UDP Forwarding Support for Virtual Router Group

How to Configure UDP Forwarding Support for Virtual Router Group

Additional References

Command Reference

Information About UDP Forwarding Support for Virtual Router Group

Before you configure the UDP Forwarding Support for Virtual Router Group feature, you should understand the following concepts:

•Benefits of the UDP Forwarding Support for Virtual Router Group

•VRG Awareness

Benefits of the UDP Forwarding Support for Virtual Router Group

Forwarding is limited to the active router in the VRG instead of all routers within the VRG.

VRG Awareness

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 can cause some DHCP servers to operate incorrectly. By making the UDP forwarding code VRG aware, forwarding will be limited to the active router in the VRG.

VRG awareness is achieved with IP Redundancy Service (IRS). The IRS API provides for notification updates of a specific VRG, addition and deletion of a VRG, and querying of the current state of a VRG. 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 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 only for VRGs that are active.

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Note The UDP Forwarding Support for virtual Router Group feature is available only on platforms that support VRGs.

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How to Configure UDP Forwarding Support for Virtual Router Group

This section contains the following procedure:

•Enabling Forwarding Support for VRG

Enabling Forwarding Support for VRG

This task enables UDP forwarding support for VRG.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip helper-address address redundancy vrg-name

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	ip helper-address address redundancy vrg-name Example: Router(config-if)# ip helper-address 10.1.1.1 redundancy shop	Enables UDP forwarding support for the VRG.

Additional References

The following sections provide references related to UDP Forwarding Support of IP Redundancy Virtual Router Group.

Related Documents

Related Topic	Document Title
UDP Forwarding of broadcast packets	"Configuring IP Addressing" chapter in the Cisco IOS IP Configuration Guide, Release 12.2
Addressing commands: complete command syntax, command mode, defaults, usage guidelines, and examples	"IP Addressing Commands" chapter in the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2

Standards

Standard	Title
None	—

MIBs

MIB	MIBs Link
None	To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs

RFCs

RFC	Title
None	—

Technical Assistance

Description	Link
The Cisco Technical Support & Documentation website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.	http://www.cisco.com/techsupport

Command Reference

This section documents modified command only.

•ip helper-address

ip helper-address

To enable the forwarding of User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address command in interface configuration mode. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.

ip helper-address [vrf name | global] address [redundancy vrg-name]

no ip helper-address [vrf name | global] address [redundancy vrg-name]

Syntax Description

vrf name	(Optional) Enables VPN routing and forwarding (VRF) instance and VRF name.
global	(Optional) Configures a global routing table.
address	Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address per interface.
redundancy vrg-name	(Optional) Defines the VRG group name.

Defaults

Disabled.

Command Modes

Interface configuration

Command History

Release	Modification
10.0	This command was introduced.
12.2(4)B	The vrf name keyword and argument combination was added, and the global keyword was added.
12.2(8)T	This command was integrated into Cisco IOS Release 12.2(8)T.
12.2(15)T	The redundancy vrg-name keyword and argument combination was added.
12.2(28)SB	This command was integrated into Cisco IOS Release 12.2(28)SB.

Usage Guidelines

Combined with the ip forward-protocol command, the ip helper-address command allows you to control which broadcast packets and which protocols are forwarded.

One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP), which is defined in RFC 1531. To enable BOOTP or DHCP broadcast forwarding for a set of clients, configure a helper address on the router interface connected to the client. The helper address should specify the address of the BOOTP or DHCP server. If you have multiple servers, you can configure one helper address for each server.

All of the following conditions must be met in order for a UDP or IP packet to be helpered by the ip helper-address command:

•The MAC address of the received frame must be all-ones broadcast address (ffff.ffff.ffff).

•The IP destination address must be one of the following: all-ones broadcast (255.255.255.255), subnet broadcast for the receiving interface, or major-net broadcast for the receiving interface if the no ip classless command is also configured.

•The IP time-to-live (TTL) value must be at least 2.

•The IP protocol must be UDP (17).

•The UDP destination port must be for TFTP, Domain Name System (DNS), Time, NetBIOS, ND, BOOTP or DHCP packet, or a UDP port specified by the ip forward-protocol udp command in global configuration mode.

If the DHCP server resides in a Virtual Private Network (VPN) or global space that is different from the interface VPN, then the vrf name or global option allows you to specify the name of the VRF or global space in which the DHCP server resides.

The ip helper-address vrf name address option uses the address associated with the VRF name regardless of the VRF of the incoming interface. If the ip helper-address vrf name address command is configured and later the vrf is deleted from the configuration, then all IP helper addresses associated with that VRF name will be removed from the interface configuration.

If the ip helper-address address command is already configured on an interface with no VRF name configured, and later the interface is configured with the ip helper-address vrf name address command, then the previously configured ip helper-address address is considered to be global.

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Note The ip helper-address command does not work on an X.25 interface on a destination router because the router cannot determine if the packet was intended as a physical broadcast.

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Examples

The following example defines an address that acts as a helper address:

interface ethernet 1

 ip helper-address 10.24.43.2

The following example defines an address that acts as a helper address and is associated with the VRF named host1:

interface ethernet 1/0

 ip helper-address vrf host1 10.25.44.2

The following example defines an address that acts as a helper address and is associated with the VRG named group1:

interface ethernet 1/0

 ip helper-address 10.25.45.2 redundancy group1

Related Commands

Command	Description
ip forward-protocol	Specifies which protocols and ports the router forwards when forwarding broadcast packets.

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

© 2003, 2006 Cisco Systems, Inc. All rights reserved.