Blocking broadcast traffic on Cisco routers.

Cisco Networking

CISCO – Blocking broadcast traffic.

We’ve had some requests for examples of how to filter broadcast traffic.
As with a lot of things in the computer world, “there is more than one way to do it.”

If your network is composed *only* of /24 allocations (ie you’re not supernetting or subnetting anywhere on class C’s, or all your class A/B networks are subnetted as /24’s), then you can do all the filtering with one filter on your *inbound* Internet interfaces.

(note that you should filter both the all ones and the all zeros addresses, as they are both recognized by some IP stacks.)

! filter broadcast packets from the outside world into our network
access-list 109 deny ip any
access-list 109 deny ip any
access-list 109 permit ip any any
in se 0
ip access-group 109 in

Another way to do it is to only filter to the broadcast addresses *outgoing* on your directly connected interfaces.
For example, if your router has an ethernet interface with half a dozen as5200’s or Max 4004’s on it, and your ethernet interface had address, then you’d do this:

! filter broadcast packets from anywhere into my directly attached
! terminal server network
access-list 110 deny ip any host
access-list 110 deny ip any host
access-list 110 permit ip any any
in e 0
ip access-group 110 out

I’ve just been made aware of a command for ciscos, ‘ip directed-broadcast’.

Specifically, the ‘no’ form of the command will not convert broadcast packets (all ones, I think) into broadcast ethernet packets, on the final, directly connected interface.

From Cisco’s online documentation:

To enable the translation of directed broadcast to physical broadcasts, use the ip directed-broadcast interface configuration command. To disable this function, use the no form of this command.

What I take this to mean is that ‘no ip directed-broadcast’ will prevent the mapping of broadcast packets (I don’t know what your cisco will guess ‘broadcast packets’ are) to broadcast ethernet framing. I think this will help… although I don’t know all the ramifications, because I haven’t used it, and don’t know anyone who has.

And a final note: there are very few applications which depend on the routing of broadcast packets. You may know of one such application; if it’s a popular one that you think lots of people are using, speak up. So you should feel safe in blocking broadcast traffic in your network. Popular applications which depend on *non*-routed broadcast traffic include RIP and netbios (Microsoft’s networking protocol). Putting filter access-lists on your interfaces should not interfere with non-routed broadcast traffic.