DHCP clients; where does the preffered routes come from?

Discussion in 'Server Networking' started by vidro, Nov 18, 2005.

  1. vidro

    vidro Guest

    Where do my DHCP clients pick up the gate way to point to the 192.168.2.5?

    Is the default gateway that is configure in the IP address of the DNS
    server passed on to the DHCP clients?

    I have 3 router on a network some clients have configured different IP
    address for their Default gateway.

    Scenario:
    Subnet 1: 192.168.1.x has DHCP server and clients
    subnet 2: 192.168.2.x has DNS server
    Two routers on subnet two 192.168.2.5 and 192.168.2.10.
    The DHCP clients on subnet 1 can attach to shared resource of those clients
    on subnet 2 that have been configured with a default gateway of 192.168.2.5
    but the DHCP clients can not access client on subnet 2 with default gateway
    of 192.168.2.10.
    If the clients with Gateways of 192.168.2.10 are reconfigured with a GW of
    192.168.2.5 the DHCP clients can access the resources.
    If the clients with a GW of 192.168.2.10 have a "Route add" pointing to the
    gate way ( not shown in this scenario) to get to the 192.168.1.x network.
    Than the DHCP clients on 192.168.1.x can access the resources.

    My question than is it looks like my DHCP clients are picking up a preferred
    gateway of 192.168.2.5 where is this value coming from, if my DHCP clients
    are on a subnet 192.168.1.X?
     
    vidro, Nov 18, 2005
    #1
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  2. vidro

    Bill Grant Guest

    Your DHCP clients in the 192.168.1 subnet will be getting a default
    gateway from DHCP. That default gateway must be a 192.168.1.x address. The
    default gateway must be "reachable" - ie it must be on the same segment and
    in the same IP subnet as the host machine.

    The problem you are seeing will be caused by the default gateway
    settings on the machines in the 192.168.2 subnet. The traffic from a
    machine in the 192.168.1 subnet will go to the router which is defined as
    the default gateway for the subnet. This router will deliver the packet to
    the target machine in the 192.168.2 subnet.

    The problem is then how does that machine send a reply back to the
    192.168.1 subnet? As you say, if you give them a default gateway of
    192.168.2.5 it will work. It will also work if you add a route to the target
    machine to send traffic for 192.168.1 to that address.

    I presume you want to leave the machines in 192.168.2 pointing to
    192.168.2.10 as their default gateway but still be able to "see" the
    machines in 192.168.1 ? The simplest way to achieve this is to add a static
    route to the router at 192.168.2.10 . All traffic goes to that router by
    default. Add a static route to it to "bounce" the local traffic to the
    router at 192.168.2.5 . eg

    192.168.1.0 255.255.255.0 192.168.2.5
     
    Bill Grant, Nov 19, 2005
    #2
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  3. vidro

    vidro Guest

    I may not have articulated my concern correctly.

    I understand that my DHCP clients are going to get a default gateway in
    their same subnet. GW 192.168.1.10

    The gateway to get to the 192.168.1.x subnet from 192.168.2x subnet is
    192.168.2.20 but I have 2 other interfaces 192.168.2.5 and 192.168.2.10 that
    are actually my default gateways. I have gateways, plural because I am
    testing performance and bottle necks, both gateways go to the same place just
    different interfaces.

    None of the computers in the 192.168.2.x subnet will point to 192.168.2.20
    as a default gateway.
    They will either point to 192.168.2.5 or 192.168.2.10 for GW

    Again, to get to the 192.168.1.x subnet you have to go through 192.168.2.20.

    Now this being the case why are those computers pointing to 192.168.2.5 able
    to be access by the DHCP clients on 192.168.1.x subnet and those computer
    that are configured with 192.168.2.10 GW can not be accessed by the DHCP
    clients except with a manually added â€route addâ€?

    I know that these computers are picking up routing tables, I guess that
    question is more of where?
    Both these interface that are used as default gateways have the same values
    in their routing tables.
     
    vidro, Nov 21, 2005
    #3
  4. vidro

    Bill Grant Guest

    You cannot have two active default gateways. Even if you set two at the
    same priority, the system will only use one of them. So you need to select
    which one you want to use as the default, then use static routiong for any
    traffic which needs to use some other gateway.
     
    Bill Grant, Nov 22, 2005
    #4
  5. vidro

    vidro Guest

    BINGO you said the word I can hone in on "system". What would be considered
    the "system".
    My DHCP clients get the default gateway that points to the interface
    (192.168.1.10) that let's them get to the 192.168.2.x subnet.
    Now all the clients on the 192.168.2.x subnet need to know to get to
    192.168.1x they have to go to 192.168.2.20
    two ways of doing this
    1.either manually with "route add"
    2. or the router that is configure as the default gateway has a route table
    that tells the client were to go.

    Really any router interface heading out the 192.168.2.x network could be
    considered a default gateway. That preference is up to the Network engineer
    (as long as the clients can get to where they need to go).

    My question is my DHCP client want to hop 2 subnets, they know what the
    local gateway is 192.168.1.10 How do they know what the default gateway is
     
    vidro, Nov 23, 2005
    #5
  6. vidro

    Bill Grant Guest

    The gateway which DHCP hands out gives the clients the default gateway
    for the local subnet. When a client needs to send a packet the decision is
    usually simple. If it is on the local subnet, it is delivered directly (on
    the wire using hardware addressing). If it is not on the local subnet, the
    packet is sent to the default router (or gateway). The client does not know
    or care what the next hop is.

    The gateway router examines its routing table and redirects the packet
    based on that information. If there is a specific route for the packet, that
    is used. If there is not, the router's default route is used (and so on for
    as many hops as it takes!).
     
    Bill Grant, Nov 23, 2005
    #6
  7. vidro

    vidro Guest

    Negative. Problem does not lie there.

    The verbiage is not clear in the explanation.
    The DHCP clients picks up the correct default gateway on their subnet
    (192.168.1.x) this gets them over to the 192.168.2. x subnet.

    If the Internet gateway is on the 192.168.2.x subnet where is my DHCP
    clients on the 192.168.1 x subnet picking up the information.


    Client is on the 192.168.1.x subnet
    DHCP server is on the 192.168.1.x subnet
    DNS is on the 192.168.2.x subnet
    I-net gateway is on the 192.168.2.x subnet
     
    vidro, Dec 7, 2005
    #7
  8. They don't. And they don't need to. The only thing they know is what to use
    as a Gateway to leave their own Subnet,...they are not aware, and never will
    be aware, of what happens after that. Once they pass the traffic to their
    local Gateway the LAN's Routing Scheme takes over and the Clients do not
    have any role in the process at all.

    --
    Phillip Windell [MCP, MVP, CCNA]
    www.wandtv.com
    -----------------------------------------------------
    Understanding the ISA 2004 Access Rule Processing
    http://www.isaserver.org/articles/ISA2004_AccessRules.html

    Microsoft Internet Security & Acceleration Server: Guidance
    http://www.microsoft.com/isaserver/techinfo/Guidance/2004.asp
    http://www.microsoft.com/isaserver/techinfo/Guidance/2000.asp

    Microsoft Internet Security & Acceleration Server: Partners
    http://www.microsoft.com/isaserver/partners/default.asp

    Deployment Guidelines for ISA Server 2004 Enterprise Edition
    http://www.microsoft.com/technet/prodtechnol/isa/2004/deploy/dgisaserver.mspx
    -----------------------------------------------------
     
    Phillip Windell, Dec 7, 2005
    #8
  9. vidro

    vidro Guest

    Phillip, thank you.
    Can you help me on this?
    You have used the term "routing scheme" which is good verbiage to what my
    thinking but I was not able to put my finger on it.
    Now that being said, What components are involved in the "Routing scheme" ?

    I'm trying to figure out why a client with a static IP on subnet 192.168.2.x
    can ping a DHCP client on 192.168.1.x when it (the static client) is
    configured with a gateway of 192.168.2.5, but if I change the gateway on
    this static client to IP 192.168.2.10 he can not ping the DHCP unless a
    "route add" is done.

    None of these gateway IP used on the static client are any way associate
    with the interface that is directly connected to the 192.168.1.x subnet, that
    interface IP is 192.168.2.20.
    The route add comment would be as such "route add 192.168.1.x mask x.x.x.x
    192.168.2.20"

    This is what I believe is happening.

    When the the static client on 192.168.2.x is configured with a gateway of
    192.168.2.5 it can ping the DHCP client on the other subnet without any
    intervention i.e. "route add", this to me is a sign that the DHCP clients
    once crossing into the 192.168.2.x subnet are picking up 192.168.2.5 as a
    default gateway on that subnet.

    If this is correct where are they picking up this information?
    If I wanted to change this value what network component is most likely to be
    broadcasting or distributing it?

    My goal or question is to try and figure out why certain clients are acting
    the way they do.


     
    vidro, Dec 7, 2005
    #9
  10. ?

    The Routing Scheme is the product of the LAN Router(s) routing table(s) and
    the Default Gateway Settings of the Hosts on the LAN.

    The only thing the Hosts (Clients) do is send any traffic that doesn't
    belong in thier own subnet to the Device listed as their Default Gateway.
    Once they do that they are no longer part of the process.

    The LAN Routers repeat the process over and over until the traffic reaches
    its destination. If a LAN Router has a "route" in the routing table that
    matches the Destination it will send the traffic to the Gateway listed in
    the "route". If it has no matching route then it sends the traffic to its
    own Default Gateway. The process repeats over and over. The Default
    Gateway is nothing more than the "routing trashcan" for destinations that is
    does not know what else to do with it,...in a sense it is "passing the buck"
    to the next router and "hopes" that it will know what to do with it,...if it
    doesn't know then is "passes the buck" on to the next router. The last
    router within your environement will always be your Internet Sharing Device
    (NAT Firewall, Proxy Server, whatever),...even though these devices (in most
    cases) are not really routers, the LAN will still treat them as if they are.

    --
    Phillip Windell [MCP, MVP, CCNA]
    www.wandtv.com
    -----------------------------------------------------
    Understanding the ISA 2004 Access Rule Processing
    http://www.isaserver.org/articles/ISA2004_AccessRules.html

    Microsoft Internet Security & Acceleration Server: Guidance
    http://www.microsoft.com/isaserver/techinfo/Guidance/2004.asp
    http://www.microsoft.com/isaserver/techinfo/Guidance/2000.asp

    Microsoft Internet Security & Acceleration Server: Partners
    http://www.microsoft.com/isaserver/partners/default.asp

    Deployment Guidelines for ISA Server 2004 Enterprise Edition
    http://www.microsoft.com/technet/prodtechnol/isa/2004/deploy/dgisaserver.mspx
    -----------------------------------------------------
     
    Phillip Windell, Dec 7, 2005
    #10
  11. vidro

    vidro Guest

    I hope I'm not coming acrossed as hardeded
    But for a client to ping another client on a differnet subnet don't they
    both have to be config with the same defualt gateway?(which would hold the
    route table of the netwrok).

    They both have to know where to go to find a routing table.


     
    vidro, Dec 7, 2005
    #11
  12. No. They are only configed with the Default Gateway on their own segment.
    You can't use a Gatway on another segment,...how could you get there?...you
    would need a gateway to get to the gateway,...it won't work.
    The routers usually know about other routers (when in the same LAN) so the
    Client simply has to get it to the router on their own segment and let the
    router worry about it.

    If you send snail-mail to someone,...do you drive to their house and stick
    it in their mail box or do you stick it in your own mail box (or a curb-side
    mailbox) and let the Mailman worry about it? The LAN Routers are the
    "mailmen",...that's what they do for a living.

    On the Public Internet, most of the Routers don't know about other routers.
    The Internet works almost entirely by sending from one Default Gateway to
    the Next until it hits a router that is high enough in the "food chain" to
    be aware of a "general route" to the destination and sends it to the next
    router towards that path and the process repeats and "narrows down" until it
    gets to the correct subnet hosting the destination

    --
    Phillip Windell [MCP, MVP, CCNA]
    www.wandtv.com
    -----------------------------------------------------
    Understanding the ISA 2004 Access Rule Processing
    http://www.isaserver.org/articles/ISA2004_AccessRules.html

    Microsoft Internet Security & Acceleration Server: Guidance
    http://www.microsoft.com/isaserver/techinfo/Guidance/2004.asp
    http://www.microsoft.com/isaserver/techinfo/Guidance/2000.asp

    Microsoft Internet Security & Acceleration Server: Partners
    http://www.microsoft.com/isaserver/partners/default.asp

    Deployment Guidelines for ISA Server 2004 Enterprise Edition
    http://www.microsoft.com/technet/prodtechnol/isa/2004/deploy/dgisaserver.mspx
    -----------------------------------------------------
     
    Phillip Windell, Dec 7, 2005
    #12
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