Cisco ip helper-address how exactly translation works?

Question

Can anyone explain how exactly works translation from broadcast to unicast? What is changed or modified in IP packet? Already I have read how it works and configured it properly for my network but still main question has left.

Example: I want to create IP helper from whole subnet 192.168.1.0/24(R1) to 192.168.2.0/24(R2) To manage this I have configured:

R1:
- ip forward-protocol 10,
- int vlan100
    ip addr 192.168.1.254
    ip helper-address 192.168.2.255

R2:
- access-list 200 permit udp 192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255 eq 10
- int vlan200
    ip addr 192.168.2.254
    ip directed-broadcast 200

Now when I take a look on wireshark it looks like: PCA sends packet to 192.168.2.255 with source 192.168.1.10 and here is the question how that 192.168.2.255 is translated to unicast? At R2 it's source 192.168.1.10 and dest 255.255.255.255 with ff:ff:ff:ff:ff:ff

Answer 1

There's no translation or modification of a packet.

The router receive a broadcast packet that contains a DHCP request. Then it send a new unicast packet to the configured DHCP server requesting a lease on behalf of the host. Among other thing it contains the MAC address of the host.

The DHCP server send the response to the router. Then the router send an unicast packet to the originator (since it knows its mac address), and so on.

In human language it's something like:

• host: yell "Hello, I'm Joe, I need an IP address"
• router: ear Joe and send a call to DHCP "Joe need an IP address, can you give me one?"
• DHCP: respond to router "Sure, I can give Joe IP address A"
• router: respond to Joe "You can get IP address A, is it ok for you?" etc...

Answer 2

I'll base my answer on what I believe to have understood from your description, which is this:

• R1 has a directly a attached subnet with 192.168.1.254 ( I'll assume /24)
• R2 has a directly a attached subnet with 192.168.2.254 ( I'll assume /24)
• there is some form of connectivity between R1 and R2, distinct from these two subnets.
• adjacent to R1, there is a sender (192.168.1.10), broadcasting "something" to 192.168.1.255 or 255.255.255.255, with udp port 10.
• Goal: you want the sender's udp/10 broadcast to be picked up by R1, be forwarded towards R2, and be disseminated in R2's attached subnet as a broadcast.

Based on the config snippets you gave, this is what will be happening:

• PC1 at R1 broadcasts an udp/10 packet to 255.255.255.255 or 192.168.1.255 (src: 192.168.1.10, dst ...255)
• on R1, interface vlan100with ip helper 192.168.2.255 will pick up that broadcast (because it's been enabled to work on udp/10, by virtue of ip forward protocol 10)
• R1 will rewrite this into a unicast packet with destination address 192.168.2.255 and will send out that unicast, according to its routing table. The packet now has Src 192.168.1.10 and Dst 192.168.2.255. R1 has no knowlegde (and doesn't need it) if 192.168.2.255 happens to be a broadcast address in that remote subnet "somewhere else"
• R2 will receive this packet on its R1 facing interface (still with Src 192.168.1.10 and Dst 192.168.2.255)
• R2, by looking up the routing table (well, maybe CEF table) will decide that this packet needs to be forwarded out through interface vlan 200.
• R2 will detect that the destination address is the given subnet's broadcast address, will check for the presence of ip directed broadcast [ACLofAllowedSourceIPs] and check if 192.168.1.10 is in that ACL
• If checks succeed, R2 will rewrite the packet's destination IP address to 255.255.255.255 (and Dst MAC to ff:ff:ff:ff) and send out that packet/frame through its interface vlan200. And there you are.