Help in understanding an MPLS network (MPLS newbie)

Discussion in 'Cisco' started by ttripp, Nov 12, 2007.

  1. ttripp

    ttripp Guest

    I've inherited an MPLS network, and I've not worked with MPLS before.
    The more I read, the more confused I am about the network I have.

    I have a dozen sites, all connected to a Bellsouth (now AT&T) MPLS
    network. Each site has a T1 circuit, and the circuit is configured as
    frame-relay on each sites router. I understand this part OK.

    Now to the part I don't understand. Each remote site is connected
    back to the main site through VPN tunnels. So, each remote site has
    one VPN tunnel back to the main site, and the main site has a dozen
    tunnels, one to each remote site. Then, each site has a dozen static
    routes, all pointing the their local router's WIC IP address.

    Is this a typical MPLS setup? What I thought what MPLS did was not
    unlike what a traditional frame-relay network did; the customer's
    routers handed the WAN traffic to the provider's network, which
    through various pieces of routing magic delivered the packets to the
    customer's routers on the opposite end. No need to set up VPN
    tunnels, and you use a IGP to handle the routing.

    Each frame-relay interface is point-to-point, with the IP address part
    of a /30 network, with one end being the local router, and the other
    end being the AT&T router. Is this also typical?

    Most MPLS documentation seems to be geared towards how the provider
    sets up their network, not how the customer sets up his part. Any
    help would be greatly appreciated, as I have absolutely no
    documentation on how this setup came to be. Thanks.
    ttripp, Nov 12, 2007
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  2. ttripp

    ttripp Guest

    Actually, I got a detail wrong. Three of the sites (including the
    main site) are connected through frame-relay, but the other ten are
    connected through regular T1. I should point out that AT&T is the
    vendor for most, but not all, of these local connections. The MPLS
    network is pure AT&T.
    ttripp, Nov 12, 2007
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  3. ttripp

    stephen Guest

    do you have documentation?

    if not - thats the 1st thing to fix.
    Yes - or at least that is the way the ones i have used work.
    the serial link carries F/R format packets, but uses it as a point to point
    link to peer to the PE router.
    you only need multiple PVCs if you are either
    1. crossing a "real" F/R network to get to the MPLS bit, or
    2. for some logical structure reason across a point to point serial link (we
    use it for multi VPN since different VPNs can use different PVCs on the same
    WAN link to keep traffic separation).

    but nothing stops you adding tunnels on top of that for some reason, since
    it is just another IP network
    - one i get involved in does it as:
    internal net + OSPF -> GRE -> IPsec -> "cloud" with MPLS.
    well, the carrier should have documentation if they want to be able to fix
    it when it breaks- so get hold of your contact and get that bit.

    you shouldnt need any "MPLS specific" docs for the customer part of an MPLS
    All the label switching happens in the core of the MPLS network, and is
    intended to simulate a private dedicated IP cloud for each VPN.
    So - treat it like a private any to any IP net and ignore the MPLS label on
    the tin........
    sounds like some of your sites have to go via a F/R access network to get to
    the MPLS cloud.

    a lot of carriers do this at lower density parts of the network since they
    have big F/R & ATM networks, they still work, and they are busy taking
    people off those direct services, so spare capacity is there.

    However - you often lose facilities if you go to MPLS via a "real" Frame or
    ATM cloud instead of just using it on the access point to point link - fewer
    or no QoS levels, and maybe limits to how full you can run the circuit.
    stephen, Nov 12, 2007
  4. ttripp

    ttripp Guest

    Thanks. Whoever built out the network didn't leave any documentation,
    and I'm trying to get contact info from my boss, but until then I'm
    just sort of stuck at what looks to me like a very strange network
    ttripp, Nov 12, 2007
  5. ttripp


    Nov 12, 2007
    Likes Received:
    Not to worry.

    If anything, just worry about the circuits themselves. MPLS is more of a carrier-side configuration. That's the nice thing about MPLS, you can have Ethernet, Frame-Relay, ATM, etc. packets going in and it's pretty much transparent to you. The only thing that MPLS cares about is the shim header that it puts in your packet.

    Labels are bound to routes in the routing table...
    Ranak, Nov 12, 2007
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