Conference 7.286::fddi

640.0. "Bridge queries" by LARVAE::HARVEY (Baldly going into the unknown...) Thu Jul 09 1992 09:06

    A couple of queries I need to clarify for a customer and myself....
  
    "Are all LAN Bridges created equal ??"
  
    Scenario:-
  
    2 x Ethernet LAN segments are linked via DECbridge 510s/FDDI backbone as 
    well as a single LANBridge 200 in standby mode.
  
    I have "created" the above setup by establishing the FDDI backbone first, 
    changing the DECbridge 510 Priorities to a value of 20 (from the default of 
    128) and then introducing the LANbridge 200 with its standard default 
    values.
  
    I appreciate that this is a simple LAN configuration but in an informal 
    training session with the customer we got talking about more complex "mesh" 
    topologies which may occur in the future. At this stage it became apparent 
    that some undesirable topologies may occur if we were to rely on Priority 
    numbers alone.
  
    My understandings of S/Tree etc. are that the lowest priority bridge 
    numbers take precedence in any reconfiguration in the event of failures 
    etc. 
  
    Therefore if I were to configure the above 2 x ethernet segments together 
    using the same equipment with default values set, am I likely to see the 
    LANbridge 200 form the link in precedence to the FDDI backbone ????
    ie. the Priorities are the same, the Line Costs are the same, therefore the 
    "cost" of going through 2 x FDDI bridges makes it the most expensive route.
  
    If I were to rely on setting Priorities alone I can envisage similar 
    "holes" occuring in a larger, more complex LAN. Is it good practice to set 
    Line Cost parameters as a matter of course ? I understand how these can 
    determine the desired standard and failover configurations.
  
    What I'd also like to understand is whether the Line Cost values for a 
    LANbridge 150/200 and DECbridge 5xx/6xx are set the same on all circuits - 
    NI and FI. Or is FDDI costed at a lesser value so that by default FDDI 
    circuits would (more) naturally become the default route(s) ?
  
    In terms of the network id value for the bridges, are the FDDI devices in a 
    different range of numbers from the ethernet range and therefore also 
    "naturally" lower in value/higher in priority ?
  
    I don't have ready access to such devices at the present so I cannot 
    research this for myself.....
  
    Thanks in advance.
  
    Rog
  

    Default priority of all of our bridges is 128; default cost of each
    line is 10.  So they are indeed created equal.  Addresses of newer
    bridges are typically numerically greater (regardless of type of bridge)
    since addresses in ROMs assigned at manufacturing increase in value
    with time.
    
    Changing priority will affect the location of the Root, and
    changing line costs will affect the topology only in a redundantly
    configured network.  In small networks, it may be easier to let the
    network configure by itself.  In larger ones, or ones with standby
    bridges, it may be a good idea to look at the resulting topology and
    influence it if necessary by tuning priorities and costs in a couple of
    bridges.  In your example, you may have had to modify the priority to
    prevent the LB200 from becoming the Root since Roots don't like to
    put any of their links into the Backup state -- and change the cost so
    that the LB200 doesn't even become a Designated Bridge.
    
    The new Spanning Tree mapping facility in MCC allows you to map the
    existing topology, which helps decide if it should be tuned.
    
    Anil