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New Member

limiting dhcp broadcasts over the wan

Hi All,

This is my first post here. Basically we have a primary dhcp server in Brisbane and have a secondary dhcp server in Sydney on a different subnet therefore I will configure the ip helper cmd on the Brisbane router. Hence when the Brisbane dhcp server goes down the dhcp broadcasts will traverse the wan to the Sydney server. This would be ok during the failover scenario however when the Brisbane dhcp server is up I want to limit the broadcasts on the Brisbane lan only so that the Bris to Syd wan throughput is not affected. Therefore is there any way to achieve this i.e to stop the broadcasts from traversing the wan each time a Brisbane device renews its ip or requests an ip address.My internal routing at Bris and Syd is static. Brisbane wan is FR with static routing and Syd wan is BGP on a MPLS tail.The servers are QIP on Windows platform. The routers are Cisco.

Vimal.

5 REPLIES
New Member

Re: limiting dhcp broadcasts over the wan

Vimal,

Try "no ip direcrted broadcast" cmd

May this help u.

Rgrds,

hardik

New Member

Re: limiting dhcp broadcasts over the wan

Hardik thanks for the reply and my apologies for phrasing my intial query a bit ambiguously. Your suggestion is good however when the primary Brisbane lan dhcp server is down then I would want these dhcp broadcasts to be forwarded by the Brisbane router over the wan to the secondary server in Syd. Infact the Brisbane router will convert it to unicasts and direct them to the Syd dhcp server via the ip-helper cmd. The problem is that when the brisbane primary dhcp lan server is up these unicasts still traverse the wan and onto the Syd secondary server. I am trying to formulate a way so that the devices get their ip addresses from the primary server and only send dhcp packets (be it broadcast or unicast) to the secondary only if the primary is down. I have a faint feeling that the only soln would be via some scripting etc or maybe just let the dhcp packets through the wan since they would not consume significant b.w.

Re: limiting dhcp broadcasts over the wan

Hardik,

I dont agree with you on this. "no ip directed broad-cast" will not help in this. This command is useful only on the interface which is connected to the destination subnet.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hiad_r/adr_i1h.htm#wp1175909

In a DHCP scenario when we have a primary and a secondry DHCP server configured on the the Layer-3 device i.e router in this case and you have ip-helper address configured, the router convert that DHCP broadcast to a Unicast packet and sends that query directly to the DHCP server.

Vimal, in this case even if you have multiple dhcp servers I guess the subnet at brisbane will always get the DHCP address from the local server unless that Server is down.When the Brisbane DHCP server is up, I do understand that router will also try sending the DHCP unicast request to the secondry DHCP server in Sydney as it has the IP-helper address configured.The same way it works with multiple DHCP servers over a LAN.

Guys, have anyone of you have implemented any workaround for the same.

-amit singh

Re: limiting dhcp broadcasts over the wan

Vimal,

I do agree on the same with you. Either some scirpt will help us or have the packets traverse over the link since they use a little bandwidth.

I think someone should have definetly implemented some script out there to resolve this problem.Let us wait if someone has some soltuion. :-))

HTH,

-amit singh

New Member

Re: limiting dhcp broadcasts over the wan

Thanks Amit. I will see if someone can advise on any script. I will probably check with my Microsoft mates as well.

- Vimal

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