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distinguishing BC in the last subnet from all subnets BC

The command “ip subnet-zero” allows the use of the all-ones and all-zeros subnets. How do routers distinguish 192.168.x.255 (/28 for example) directed BC to the last subnet from all subnets BC in the whole class C network? Or 172.16.255.255 directed BC to the last subnet from all subnets BC (/24 prefix, for example)?

Does the command “ip directed broadcast” distinguish them?

F. Weldon

CCSI 21012

3 REPLIES

Re: distinguishing BC in the last subnet from all subnets BC

The command “ip subnet-zero” allows the use of the all-ones and all-zeros subnets. How do routers distinguish 192.168.x.255 (/28 for example) directed BC to the last subnet from all subnets BC in the whole class C network? Or 172.16.255.255 directed BC to the last subnet from all subnets BC (/24 prefix, for example)?

Does the command “ip directed broadcast” distinguish them?

F. Weldon

CCSI 21012

Hi Weldon,

It's really debatable topic If you see the defination ip directed broadcast an IP directed broadcast is an IP packet whose destination address is a valid broadcast address for some IP subnet, but which originates from a node that is not itself part of that destination subnet.

and if you see the subnet-zero defination It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0.

On the issue of using subnet zero and the all-ones subnet, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.

Hope to Help !!

Ganesh.H

New Member

Re: distinguishing BC in the last subnet from all subnets BC

Thanks for your prompt reply. So the matter is clarified--in theory. Now what I want to know is, how do routers route packets destined for the address 172.16.255.255 /24 in practise? Do they flood it, or do they forward a single copy to the default gateway directly connected to the all-ones subnet? I guess I'll cable up my test lab, put Wireshark off the shortest path to the all-ones subnet, and just see what comes out the router interfaces.

Watch this space.

Cheers,

F.Weldon

New Member

Re: distinguishing BC in the last subnet from all subnets BC

I have a result which shows what routers actually do with "ip subnet-zero" and "ip directed-broadcast" in a network where the all-ones subnet is present. See the network diagram attached. Routers BB1 and BB2 share the all-ones subnet, 10.255.255.0/24; routers R1 and R2 share the penultimate subnet 10.255.254.0/24. OSPF (single area) and EIGRP (no auto-summary) are running. All routers see all subnets in their routing tables.

With "ip directed-broadcast" OFF on all router interfaces, pings from R3 to 10.255.254.0 and 10.255.254.255 return replies from the router (R1) on the shortest path directly attached to the 10.255.254.0 network; Wireshark records no ICMP traffic in the subnet.

Pings from R3 to 10.255.255.0 and 10.255.255.255 return similar replies from the router directly attached to the 10.255.255.0 network.

Now with "ip directed-broadcast" ON (all router interfaces), pings from R3 to 10.255.254.0 and 10.255.254.255 flood the network with 255.255.255.255 ICMPs, recorded by Wireshark.

With "ip directed-broadcast" ON, pings from R3 to 10.255.255.0 and 10.255.255.255 generate no ICMP traffic in the 10.255.254.0 network (Wireshark).

Conclusion, with "ip directed-broadcast" OFF, all-subnets BC is not actually flooded to all hosts on all subnets.

With "ip directed-broadcast" ON, all-subnets BC is converted to local BC only in the target subnet. If the target subnet is the all-ones subnet, only the all-ones subnet registers the traffic (converted to local BC). Since all-subnets BC is actually treated as a directed BC when "ip directed-broadcast" is ON, and is also not flooded to all subnets even when "ip directed-broadcast" is OFF, all-subnets BC appears not to exist anymore in its original function.

Further reference: RFC 1878.

Cheers,

FW

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