6-2 IP Broadcast Handling | Cisco Field Manual[c] Router Configuration

Directed broadcasts are packets that are sent to all hosts on a specific network or subnet. By default, directed broadcasts are not forwarded on router interfaces.
The router can forward certain UDP broadcasts to specific "helper" IP addresses. The helper address is substituted for the broadcast destination address, and the packets are forwarded normally.

Configuration

(Optional) Enable directed broadcasts:
```
 (interface)  ip directed-broadcast  [  access-list  ] 
```
By default, directed broadcasts are not enabled on any router interface. Packets destined for directed broadcast addresses are dropped. If necessary, they can be enabled per interface. When they are enabled, only directed broadcasts containing protocols that are defined by the ip forward-protocol command are actually forwarded. An optional access list can be used to permit only specific packets to be forwarded.
(Optional) Define an IP broadcast address:
```
 (interface)  ip broadcast-address  [  ip-address  ] 
```
By default, the broadcast address is defined as 255.255.255.255. If necessary, this address can be changed to any other IP address on a per-interface basis.

NOTE

The "all-1s" broadcast address 255.255.255.255 is used as a function of two bits (10 and 14) in the router configuration register. These bits can be set to define the following broadcast address as a combination of 1s or 0s in the network and host portions of the address, as shown in the following table:

Bit 14 Bit 10 Broadcast Address <net>.<host>

<1s>.<1s>

1 <0s>.<0s>

1 <network>.<0s>

1 1 <network>.<1s>
(Optional) Configure broadcast flooding on bridged interfaces.
1. Flood broadcasts using the spanning-tree database:
```
 (global)  ip forward-protocol spanning-tree  
```
  Broadcast packets can be flooded or sent out all network interfaces on a router. Flooding is performed according to the spanning-tree database such that only interfaces in the forwarding STP state can actually forward the broadcast packets. These packets must be destined for broadcast addresses (either MAC or IP broadcasts), and they contain TFTP, DNS, Time, NetBIOS, Network Disk, BOOTP, or UDP protocols allowed by the ip forward-protocol udp command. The packet's time- to-live (TTL) must be at least 2.
2. (Optional) Increase flooding performance:
```
 (global)  ip forward-protocol turbo-flood  
```
  Broadcast flooding occurs more rapidly on Ethernet (ARPA encapsulation), FDDI, and Serial (HDLC encapsulation) interfaces.
(Optional) Configure UDP broadcast forwarding.
1. Enable UDP broadcast forwarding to an IP address:
```
 (interface)  ip helper-address   ip-address  
```
  By default, this command forwards BOOTP (and DHCP) packets received on an interface to the specified IP address. Additional UDP ports can be specified for forwarding with the ip forward-protocol command. Multiple helper addresses can be configured on a single interface.
2. (Optional) Specify UDP broadcast port numbers to forward:
```
 (global)  ip forward-protocol  {  udp  [  port  ]  nd   sdns  } 
```
  UDP broadcast packets are identified by port number, by nd (the Network Disk protocol, used by diskless Sun hosts), or by sdns (Secure Data Network Service). If udp is specified without the optional port number, the following broadcast protocols are forwarded:
  - Trivial File Transfer Protocol (TFTP) UDP port 69
  - Domain Name System (DNS) UDP port 53
  - Time service UDP port 37
  - NetBIOS Name Server UDP port 137
  - NetBIOS Datagram Server UDP port 138
  - Boot protocol (BOOTP and DHCP) client and server UDP ports 67 and 68
  - Terminal Access Controller Access Control System (TACACS) UDP port 49
  Specific UDP ports can be given as either numbers or a protocol name. You can see a list of common UDP broadcast protocols using the ip forward-protocol udp ? command.

Bit 14	Bit 10	Broadcast Address <net>.<host>
		<1s>.<1s>
	1	<0s>.<0s>
1		<network>.<0s>
1	1	<network>.<1s>

Example

Interface ethernet 0 on a router is left at the default setting of not forwarding directed broadcasts. IP helper addresses 192.168.75.4 and 192.168.99.16 are configured to forward the default set of UDP broadcasts, including BOOTP/DHCP. However, the NetBIOS services on UDP ports 137 and 138 are not to be forwarded:

  interface ethernet 0   no ip directed-broadcast   ip address 192.168.16.1 255.255.255.0   ip helper-address 192.168.75.4   ip helper-address 192.168.99.16   no ip forward-protocol udp netbios-dgm   no ip forward-protocol udp netbios-ns