Broadcasts between multiple routers using IP HELPER

[gserghiou]

Hello everyone,

I newly CCNA qualified and i need some help-clarification regarding ip helper command.

A few words about the setup of the network:

The place were i work has multiple routers connected to a central router. For example I have router A and Router B. I need a PCs on the LAN segment of router A to be able to broadcast packets to a server (Domain Controller) located on the LAN of router B. I also want broadcasts from the server to travel freely to the LAN of the PC on router A. These two routers are connected with 2Mbps FR.

My configurations work, but i am not sure if i configured unnecessary commands. Can you please give me a sample configuration stating the interfaces were i need to configure IP HELPER-ADDRESS , and if i need the IP DIRECTED-BROADCAST command at all.

Also i have used IP FORWARD-PROTOCOL UDP command.

Thank you very much,

George

[Kevin Dorrell]

George,

OK. You want broadcasts from PCs on LAN A to go to the servers on LAN B. I shall presume that you do not know how many PCs are on LAN B; you just want the broadcast to be seen by all of them. So, suppose the address of LAN B is 192.168.30.0/24, you should go to the interface of LAN A, and configure ip helper-address 192.168.30.255. This will forward brodcasts from LAN A to LAN B. By default, when they arrive at LAN B, the router there would try and send them to a host 192.168.30.255 - but this is not what we want. So on LAN B, you will need the command ip directed-broadcast. This will recognise the packet 192.168.30.255 as a directed broadcast, and it will make sure it gets sent as a MAC broadcast, rather than a host unicast.

In the other direction, just for illustration, I shall assume that you are only interested in passing the PC broadcasts to the specific server in LAN A, let us say 192.168.20.45. In this case, go to the interface of LAN B, and configure ip helper-address 192.168.20.45. This will see the PC broadcasts, and send them to the server on LAN A. In this case, it is not necessary to configure ip directed-broadcast on LAN A, because, it is enough to send the packets to just that server.

Of course, if you want the PC broadcasts to go to all servers in LAN A, you have two options: either you send them to 192.168.20.255 and enable directed broadcasts on LAN A, or you make a list of servers, and have an ip helper-address on LAN B for each one.

As for ip forward-protocol, it may not be necessary. There is a list of protocols that the ip helper-address command forwards by default. You only need the ip forward-protocol command if you want to deviate from the list. See doc for details:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipras_r/1rfipadr.htm#wp1018606 http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipras_r/1rfipadr.htm#wp1018318

I hope this clarifies things for you.

Kevin Dorrell

Luxembourg

[gserghiou]

Hi kevin,

Thanks for your fast reply.

I now have cleared a couple of things in my head.

One question that remains unanswerred (perhaps because i didn`t explain it well) is that LAN A is on one router (router A) and LAN B on Router B. These two routers are conencted by a serial connection FR. The broadcasts have to travel through both routers.

Will I in this case need to configure IP HELPER-ADDRESS on the incoming interfaces on both routers and if so can you shed some more light on this please.

Thank you,

George

[Kevin Dorrell · Accepted solution]

No need. As far as the FR link is concerned it is just forwarding a packet to host 192.168.30.255. The link does not know or care that it is a directed broadcast. That is how you can do end-to-end directed broadcasts across a network that you do not control. Only the source LAN of the broadcast (ip helper-address), and the destination LAN (ip directed-broadcasts) are concerned with the fact that it is a broadcast.

Kevin Dorrell

Luxembourg

[gserghiou]

Hi Kevin,

Thanks for the help.

Your replies answer my question fully.

Once again thanks for the help,

George