[ip directed-broadcast] - behavior with 255.255.255.255

[NicolasDemonty]

Hi everyone !

I'm just wondering how can be the behavior of the "ip directed-broadcast" command when the interface get a ip packet in 255.255.255.255 ?

I understande that the command allow the "explosion" of a directed-broadcast address in the subnet destination of the broadcast but what happens if the interface get a local broadcast ?

For example :

let's imagine a single router with two vlan interfaces : vlan 101 (192.168.101.0/24) and vlan 201 (192.168.201.0/24) each with the ip directed-broadcast command set. If a device in vlan 101 sends a packet with ip dest = 255.255.255.255 then will the vlan 201 get this traffic ?

Thanks a lot in advance for your clarifications ;-)

++

[Karsten Iwen]

Your understanding of "no ip directed broadcast" is right.

For your example: 255.255.255.255 is a local subnet broadcast and is never transmitted to other subnets regardless of the configuration of "ip directed broadcast".

-- 
Don't stop after you've improved your network! Improve the world by lending money to the working poor:
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[NicolasDemonty]

Thx for your answer !

Just another question now but regarding of the ip help-address.

Imagine now that the source subnet 101 sends a directed broadcast to subnet 201 and that the interface vlan 201 has a ip helper-address + ip directed broadcast commands.

Now the directed broadcast will be routed to subnet 201 and in the subnet 201 the directed broadcast will explose in a loacl broadcast. Interface vlan 201 will get a packet with 255.255.255.255 ip address now ? isn't it ? How will the ip helper-address react ?

[Peter Paluch]

Hello Nicolas,

Just to add to Karsten's perfect response, note that the address of 255.255.255.255 does not contain any unique network part. In fact, this IP address is not routable because it not not clear what IP network should such a packet be delivered to.  Therefore, propagation of such broadcast is always confined to the scope of the network where it was originated.

Also, you have to keep in mind that the ip directed-broadcast command applies only to packets that are sent out an interface. It does not apply to received packets. So unless a packet can be routed through the network and be sent out an interface connected to the destination network whose broadcast IP address is used as the destination address of this packet, the setting of the ip directed-broadcast on that interface is not relevant.

Best regards,

Peter

[NicolasDemonty]

Hello,

ok I understand but now I'm wondering if it is possible to creat a broadcast "ping - pong" just like a loop between two subnets. If I use an ip helper-address on the directed broadcast, is it possible to creat a loop ?

For example, on the same router :

interface vlan 101
ip address 10.1.0.1 255.255.0.0
ip helper-address 10.201.255.255
ip directed-broadcast
interface vlan 201
ip address 10.201.0.1 255.255.0.0
ip helper-address 10.1.255.255
ip directed-broadcast
ip forward-protocol XXX

If a local broadcast is generated in vlan 101 for the protocol XXX, it means that a packet with IP dest = 255.255.255.255 is generated and the ip helper will send a copy to 10.201.255.255. This address will be routed to the 10.201.0.0 subnet and the directed broadcast will "explose" in a 255.255.255.255 ip dest packet. So it means that a local broadcast is now generated in the subnet 201.

My question : will the ip 10.201.0.1 get the new local broadcast and then do its helper function to vlan 101, creating so a loop ? or not ?

Thanks in advance for your answer

[Karsten Iwen]

You won't get any loop. The router will send the broadcast out of his own interface, but to get a loop, the packet has to come back as local broadcast on one of the interfaces. But that will not be the case.

-- 
Don't stop after you've improved your network! Improve the world by lending money to the working poor:
http://www.kiva.org/invitedby/karsteni

[Amit Singh]

Nicolas,

The combination of the the two commands " ip-helper" and " ip directed-broadcast" is the way to forward a particular broadcast traffic on a specific protocol in a different subnet rather than the origination subnet. This is actually what you are doing. You have an application with a protocol XXX in vlan 101 which needs to communicate with the hosts in subnet 201 using a broadcast mechanism. The way you do is using the commnad" ip forward protocol xxx" to send the traffic to vlan 201 using the command "ip helper-address 10.201.255.255" and send it as a brioadcast to the whole subnet listed there. When the broadcast reached Vlan 201, it will use " ip directed-broadcast" to have the broadcast traffic sent out to the local configured subnet. The same process goes back forthe traffic originated from Vlans 201 towards Vlan 101 as you are using the same mechanisam for bi-directional broadcast traffic communications.


Hope this helps.

Cheers,

-amit singh