DLSw+ performance adequate on DSL?

Discussion in 'Cisco' started by Doug Simpson, Oct 19, 2004.

  1. Doug Simpson

    Doug Simpson Guest

    We have a brand new MPLS VPN network from a major provider. We then
    resell these VPNs (VRFs) to our customers.

    We are considering rolling out 300 to 600 legacy SNA/SDLC sites via
    DLSw+ over MPLS/IP. Provider would manage the entire package,
    including both core and edge DLSw+ peer routers. A single MPLS VPN
    (VRF) would be used for the entire DLSw+ network. In all that
    follows, therefore, just read "MPLS" as "IP."

    To speed this migration, we are considering using a null modem serial
    cable to connect the edge router directly to the IBM controller at
    each site. This is an interim step to be followed by slower-paced
    conversion to IP LAN at each site.

    Most customers at these sites are poor government agencies who are
    salivating at the lower cost and higher bandwidth of DSL. Most
    agencies care less about SLA than price/bandwidth. No doubt they will
    change their tune at trouble-time.

    Provider is recommending against DSL for DLSw+ because of latency and
    reliability concerns. They also mentioned the tendency of some
    provider's DSL to timeout during zero traffic conditions. Provider
    says this might cause chronic DLSw+ session drops, though I would
    think that DLSw+ hello packets would prevent zero traffic conditions.

    Provider is concerned about sites that might limp along with existing
    but poor SNA service because of DLSw+ session issues. Stealth
    troubles are indeed bad news and we want to make sure we avoid them.

    Provider recommends a 64K frame circuit to each site. Frame is a
    comfort factor for provider. DSL connection is at arm's length to
    MPLS network, but frame comes straight into the provider's MPLS PE
    router. Problem is that 64K is twice the cost and a fraction of the
    bandwidth of DSL. 64K is ample for SNA traffic, but would need to be
    increased with the advent of IP and real applications.

    With sufficient testing and planning, can DSL serve as an adequate
    transport for DLSw+ at the scale of 300 to 600 sites?

    Options considered:

    o Limit DSL to native IP (tn3270).
    Offer DLSw+ only on 64K frame circuits.

    o Scale up DLSw+/DSL in careful, stepwise pilots.
    Switch trouble sites to 64K frame.

    Vendor uses a mix of Cisco and Juniper in the core and all Cisco at
    the edge. All DLSw+ routers would be Cisco.

    Advice and anecdotes are welcome!

    Doug Simpson
    Network Design Engineer
    Doug Simpson, Oct 19, 2004
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  2. Doug Simpson

    stephen Guest

    i have recently looked at a network using both frame and DSL where there is
    DLSw / SDLC to a local device.

    512k ADSL works better than Frame - the higher speed links lower
    serialisation delay compensates for the latency through the DSL / ATM

    however this is private DSL - ADSL is used as an access line rather than for
    an internet feed with IPsec style VPN.

    most of the scaling issues seem to occur with central DLSw engines - use
    some meaty routers here, allow for backups, and limit the number of peers.

    email me offline if you want to discuss.

    i strongly suggest you ask for QoS for the DSLw with either solution.
    Yes - so long as the DSL is predictable and you control the traffic to some
    you may end up with some locations where you cant get ADSL, so you need a
    backstop anyway.
    stephen, Oct 19, 2004
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