Forwarding DHCP requests

[rolandshum]

Just looking for a confirmation that I haven't left anything out on my config for forwarding DHCP packets.

My DHCP servers will be on 10.1.1.50/24 and 10.1.1.51/24. The users are on 10.1.2.0/24. If I'm to forward the packets from the user segment I have put the following statments on the interface they are connected to;

interface vlan 101

ip helper-address 10.1.1.50

ip helper-address 10.1.1.51

I have also put the following global statements:

ip forward-protocol udp 67

ip forward-protocol udp 68

Is there anything I'm leaving out? This will be on a 6509 running 12.2 x IOS.

[thisisshanky]

Just make sure you dont have overlapping scopes for that subnets on either servers.

[rolandshum]

I'll keep that in mind thanks, I don't have that on my plate though, just need to get the packets where they need to go.

I guess that means I don't need any additional lines in my config.

[thisisshanky]

No extra commands needed.

Note: ports 67 and 68 are by default forwarded when you enter the IP helper-address command. So even if you enable them using "ip forward protocol" command, you probably wont see them in the config, as they are on by default.

[andrew.butterworth]

The only additional steps I do is to stop the other 'default' protocols that are forwarded as they inevitably lead to spurious Windows Networking 'Master Browser' issues, the other protocols I remove because they are generally not needed if all you are providing is DHCP support (UDP 67 & 68):

no ip forward-protocol udp tftp

no ip forward-protocol udp domain

no ip forward-protocol udp time

no ip forward-protocol udp netbios-ns

no ip forward-protocol udp netbios-dgm

no ip forward-protocol udp tacacs

Andy

[glen.grant]

Looks good , make sure on all user ports you have used the "switchport host" command also .

[guoqiang.li]

FYI:

Usage Guidelines

Enabling a helper address or UDP flooding on an interface causes the Cisco IOS software to forward particular broadcast packets. You can use the ip forward-protocol command to specify exactly which types of broadcast packets you would like to have forwarded. A number of commonly forwarded applications are enabled by default. Enabling forwarding for some ports (for example, Routing Information Protocol (RIP) may be hazardous to your network.

If you use the ip forward-protocol command, specifying only UDP without the port enables forwarding and flooding on the default ports.

One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP). DHCP is defined in RFC 1531. DHCP protocol information is carried inside of BOOTP packets. To enable BOOTP broadcast forwarding for a set of clients, configure a helper address on the router interface closest to the client. The helper address should specify the address of the DHCP server. If you have multiple servers, you can configure one helper address for each server. Because BOOTP packets are forwarded by default, DHCP information can now be forwarded by the software. The DHCP server now receives broadcasts from the DHCP clients.

If an IP helper address is defined, UDP forwarding is enabled on default ports. If UDP flooding is configured, UDP flooding is enabled on the default ports.

If a helper address is specified and UDP forwarding is enabled, broadcast packets destined to the following port numbers are forwarded by default:

?Trivial File Transfer Protocol (TFTP) (port 69)

?Domain Naming System (port 53)

?Time service (port 37)

?NetBIOS Name Server (port 137)

?NetBIOS Datagram Server (port 138)

?Boot Protocol (BOOTP) client and server datagrams (ports 67 and 68)

?TACACS service (port 49)

?IEN-116 Name Service (port 42)

!

[guoqiang.li]

rolandshum:

I think your config is enough and right!