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DHCP Relay agent help

Discussion in 'Network Infrastructure' started by DC Pr0Mo, Dec 17, 2011.

  1. DC Pr0Mo

    DC Pr0Mo Kilobyte Poster

    Hi Guys, I'm working through my lab for the 291 exam and trying to get DHCP relay agent working, My lab has 3 subnets, A , B and C, DHCP server is in in Subnet A, and DHCP client in Subnet C

    [(DHCP Server) Subnet A] - Router1 - [Subnet B] - Router2 - [Subnet C (DHCP Client)]

    • If I configure static addresses the server and client can ping each other, so routing is fine
    • Both routers are windows server 2003 r2 RRAS
    • I have set up a superscope consisting of two scopes
    • Scope 1 IP settings match the Router internal network card of Subnet A NIC
    • Scope 2 IP settings match the Router internal network card of Subnet C NIC

    As far the relay agent config go, i have set up the following

    DHCP relay agent on Router 1, Listening on NIC connected to Subnet B forwarding request to DHCP server in Subnet A

    I'm, confused as how to setup the DHCP relay agent on Router 2, It must listen for request on the NIC connected to Subnet C

    Should this relay agent forward request straight to the DHCP server on Subnet A, or to another relay agent on Router 1, which will forward to the DHCP router on Subnet A?

    Hop count on both relays are set to 4 hops.
    Certifications: MCDST | BSc Network Computing
    WIP: 70-291 | 70-293 | 70-294 | 70-297
  2. onoski

    onoski Terabyte Poster


    Have a look and read through here RRAS Setup Wizard as RRAS needs to be configured on all intended routers.
    Certifications: MCSE: 2003, MCSA: 2003 Messaging, MCP, HNC BIT, ITIL Fdn V3, SDI Fdn, VCP 4 & VCP 5
    WIP: MCTS:70-236, PowerShell
  3. derkit

    derkit Gigabyte Poster

    You need to set up the DHCP Relay Agent on Router 2 so it listens on Subnet C, but in the properties of the Interface you set the DHCP Server's IP address. The Relay Agent will add a new destination and source IP to each packet so the it knows where to find a DHCP server and where to send the responses back to.

    The DHCP packets will be directed by the Routing table already set up on Router 2, so it'll know how to reach Subnet A.
    If you don't have any DHCP clients on Subnet B, you can remove the DHCP Relay Agent on Router 1 as it isn't used to allow communications between DHCP Server and Client.

    The hop count means how many hops the packets will be allowed to travel across the network until the TTL expires. I believe if you set the Hop Count to 1 on Router 2, you probably won't see any DHCP packets being received by the DHCP Server as its had to go through 2 hops to get there. This is a worthwhile exercise as well once you have it up and running and use the Network Tracert app to see the DHCP packets move backwards and forwards.
    Last edited: Dec 20, 2011
    Certifications: MBCS, BSc(Hons), Cert(Maths), A+, Net+, MCDST, ITIL-F v3, MCSA
    WIP: 70-293

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