Just checking default action of a router when getting ip address from dhcp

migz1234migz1234 ■□□□□□□□□□ Posts: 4Registered Users ■□□□□□□□□□
Help peoples. I am just checking. Say I have Pca Connected to a Router A. This router is connected to a Router C. Both Router B and C are connected to A in a triangle formation. Router B is connected to a DHCP server. Now if PCA is wanting to get an IP address from dhcp. is the default action of Router A to send a broadcast request to routers B and C. My understanding is it would not know where to send the packet so it would need to check Router B and C to see if that is the network the packet wishes to take.


  • networker050184networker050184 Mod Posts: 11,962Mod Mod
    The default behavior is to not forward broadcasts to other routers.
    An expert is a man who has made all the mistakes which can be made.
  • migz1234migz1234 ■□□□□□□□□□ Posts: 4Registered Users ■□□□□□□□□□
    So what is its default behaviour
  • nice343nice343 Posts: 391Member
    Routers don't forward broadcasts. In fact one of the main purposes of routers are to stop/drop broadcasts.

    To answer your question when PCA sends the dhcp request/discover broadcast message, the router will forward the packet as a unicast message(bootp forwarder) to the DHCP server using the source address of the interface connected to PCA. That's how the DHCP server knows which subnet is requesting an IP address.

    The ip helper-address is used to accomplish this in Cisco. And it's not just DHCP, there are a decent amount of applications that can be forwarded with that command

    TFTP - port 69
    Domain Name System (DNS) - port 53
    Time service - port 37
    NetBIOS Name Server - port 137
    NetBIOS Datagram Server - port 138
    Bootstrap Protocol (BOOTP)/DHCP - port 67
    TACACS - port 49

    I have some what of a decent explanation here
    Configuring Cisco router as a DHCP server and relay | Tech Intuition
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  • james43026james43026 Posts: 303Member
    Also, the only routing protocol that would ever ask other routers about a network, is EIGRP, and that only happens if both the successor and feasible successor routes go down, and EIGRP needs to try to re-converge, by sending a query to all neighbors in its neighbor table, asking if they have a route to the lost network. As others have explained above, broadcasts are not something any router would forward by default, and Nice343 did a great job explaining how you could have a router interact with broadcasts. Another great use of the ip helper command, is for PXE booting in your network.
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