Following post will present you how Cisco router handles broadcast IP packets.
We have two types of IP broadcast address:
- All subnets broadcast IP (255.255.255.255)
- Directed broadcast – specific subnet broadcast IP (e.g. 10.0.12.255 for 10.0.12.0/24 subnet)
It’s worth to add that all subnets broadcast IP type is not directed broadcast, directed means broadcast sent to all hosts in specific subnets (directed to specific group of hosts).
By default Cisco router does not forward IP packets addressed to any type of broadcast address – router simple drops them or in case it’s ICMP echo to router’s directly connected broadcast subnet respond via echo reply to requestor.
Directed broadcast example
Let’s take a look on the first example. I have generated ping message from R1 to 10.0.23.255. Because R2 is directly connected to the 10.0.23.0/24 subnet will respond to echo via echo reply but will not forward the ICMP packet over Fa0/1 link towards R3 so R3 will never get it.
Here you are debug IP packet from R1 after ping:
R1#ping 10.0.23.255 repeat 1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 10.0.23.255, timeout is 2 seconds:
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 60/60/60 ms
R1#
*Mar 1 00:24:54.467: IP: tableid=0, s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), routed via FIB
*Mar 1 00:24:54.471: IP: s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), len 100, sending
*Mar 1 00:24:54.475: ICMP type=8, code=0
*Mar 1 00:24:54.515: IP: tableid=0, s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), routed via RIB
*Mar 1 00:24:54.519: IP: s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), len 100, rcvd 3
*Mar 1 00:24:54.523: ICMP type=0, code=0
As you can see R1 gets just R2’s respond.
Let’s add no ip directed-broadcast under Fa0/1 on R2 and see how th debug looks like now on R1:
R2(config-if)#int fa0/1
R2(config-if)#no ip directed-broadcast
R1#ping 10.0.23.255 repeat 1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 10.0.23.255, timeout is 2 seconds:
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 36/36/36 ms
R1#
*Mar 1 00:03:56.839: IP: tableid=0, s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), routed via FIB
*Mar 1 00:03:56.843: IP: s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), len 100, sending
*Mar 1 00:03:56.847: ICMP type=8, code=0
*Mar 1 00:03:56.863: IP: tableid=0, s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), routed via RIB
*Mar 1 00:03:56.867: IP: s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), len 100, rcvd 3
*Mar 1 00:03:56.871: ICMP type=0, code=0
*Mar 1 00:03:56.931: IP: tableid=0, s=10.0.23.3 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), routed via RIB
R1#
*Mar 1 00:03:56.935: IP: s=10.0.23.3 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), len 100, rcvd 3
*Mar 1 00:03:56.939: ICMP type=0, code=0
As you see R1 now gets response from R2 and R3.
Take a look how it looks like on R2 and R3:
R2#*Mar 1 00:10:16.995: IP: tableid=0, s=10.0.12.1 (FastEthernet0/0), d=10.0.23.255 (FastEthernet0/1), routed via RIB
*Mar 1 00:10:16.999: IP: s=10.0.12.1 (FastEthernet0/0), d=10.0.23.255 (FastEthernet0/1), g=255.255.255.255, len 100, forward directed broadcast
*Mar 1 00:10:17.007: ICMP type=8, code=0
R3#*Mar 1 00:07:20.491: IP: s=10.0.12.1 (FastEthernet0/1), d=255.255.255.255, len 100, rcvd 2
*Mar 1 00:07:20.495: ICMP type=8, code=0
*Mar 1 00:07:20.499: IP: tableid=0, s=10.0.23.3 (local), d=10.0.12.1 (FastEthernet0/1), routed via FIB
*Mar 1 00:07:20.499: IP: s=10.0.23.3 (local), d=10.0.12.1 (FastEthernet0/1), len 100, sending
*Mar 1 00:07:20.503: ICMP type=0, code=0
As you can discovered ip directed-broadcast changes the destination directed broadcast address (10.1.23.255) to all subnet broadcast 255.255.255.255.
What in case we would still send directed broadcast to subnet IP? We can use broadcast-address command for this propose.
R2#show run int fa0/1
interface FastEthernet0/1
ip address 10.0.23.2 255.255.255.0
ip broadcast-address 10.0.23.255
ip directed-broadcast
Now R3 gets ICMP packet directed to subnet broadcast 10.0.23.255.
R3#*Mar 1 00:41:35.391: IP: s=10.0.12.1 (FastEthernet0/1), d=10.0.23.255 (FastEthernet0/1), len 100, rcvd 3
*Mar 1 00:41:35.395: ICMP type=8, code=0
Here you are diagram that shows above tests.
All subnets broadcast example
In the following example I will show you how router handles typical broadcast packets. The best example is the DHCP address allocation process (more about it you can read here). The first message called as DHCP Discovery is sent to 255.255.255.255 broadcast address. By default router will ignore this packet and drop it. To properly handle it and send as unicast IP toward final destination we have to use ip helper-address command under fa0/0 interface on R2, exactly under interface that receives broadcast packets.
Please check following scheme and take a look on the mentioned post. Enjoy 😉
