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Ip Directed broadcast and ip helper

Hi All,

What would happen if you configure an SVI eg VLAN 10 with an ip helper and then configure a VLAN 11 without ip helper, but with a physical dhcp server attached in that vlan. You then configure ip directed broadcast on both SVI's.

interface Vlan10

ip address 10.0.0.253 255.255.255.0

ip helper-address 192.168.1.1

ip helper-address 192.168.1.1

no ip redirects

ip directed-broadcast

end

interface Vlan11

ip address 172.16.1.1 255.255.255.0

no ip redirects

ip directed-broadcast

end

Would it be possible that a dhcp request of a device in vlan 10 is picked up by the dhcp server in vlan 11 and that you get ip adresses assigned in vlan 10 from the range of vlan 11?

Thanks,

Best Regards,

Joris

Everyone's tags (1)
2 REPLIES
Cisco Employee

Ip Directed broadcast and ip helper

Hi Joris,

The ip directed-broadcast has no meaning in your setup because DHCP does not communicate using directed broadcasts (i.e. packets addressed to the specific broadcast address of a particular IP subnet). DHCP either uses link-local broadcast of 255.255.255.255 or unicast communication, neither of which is influenced by the ip directed-broadcast. What this command does is actually allowing an outside device send a packet to the broadcast address of a particular network. Without this command, such packets would be dropped. However, as DHCP does not make use of directed broadcasts, this command would have no effect.

Would it be possible that a dhcp request of a device in vlan 10 is  picked up by the dhcp server in vlan 11 and that you get ip adresses  assigned in vlan 10 from the range of vlan 11?

A DHCP request in VLAN 10 will be forwarded only to the DHCP servers specified in the ip helper-address commands. Now, your example contains twice the same IP address 192.168.1.1. That was probably not intentional. In any case, once a DHCP server receives a relayed packet from a client, it will choose the appropriate pool to serve the client using the IP address of the DHCP relay agent that will be recorded in the relayed packet - in this case, 10.0.0.253. Using this information, a DHCP server is always able to exactly identify the network from which the request originated, and choose the correct pool. If clients in VLAN 10 are receiving wrong IP addresses then most probably, the DHCP server is misconfigured or misbehaving.

Best regards,

Peter

Cisco Employee

Ip Directed broadcast and ip helper

Hi Joris,

To interpret it...

Broadcast received on Vlan 10 SVI will be forwarded as unicast to the ip helper Ip address as listed and should be able to reach the DHCP server. I don't see any issues if you put ip directed-broadcast in vlan 11 as this is unicast traffic coming from vlan 10. (ip helper IP should be on same segment with vlan 11.. )

Better disable ip directed-broadcast on that vlan which could cause DOS attacks.. unless otherwise you use Wake on Lan functionality..

Best...

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