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Re: IGPs and services?

  • From: ww
  • Date: Wed May 17 01:40:52 2000

>>>>> "nicholas" == nicholas harteau <> writes:

    nicholas> I'm looking  for some advice on IGP  tricks such that we
    nicholas>  can  give out  the  same  IPs  for a  specific  service
    nicholas> (say...DNS)  to all  of our customers  despite geography
    nicholas>  and have  my  IGP  route those  packets  to a  regional
    nicholas> server,  thus accomplishing some inherent  level of load
    nicholas> balancing,  and even eventually returning  data based on
    nicholas> client geography.

Assign a unique (or several  uniqe...) address that the customers will
use, say Configure this address onto  the loopback interface
of each  of the servers as a   secondary address. Have the  host run a
routing  protocol (OSPF  lends itself  to this  nicely since  the area
construct us usefull)  and have it inject  into the IGP. 

Then arrange  so that the  IGP will direct  all traffic to  the server
within  the the  same area  as the  source of  the traffic.  There are
several ways to do this:

- filter   routing  updates   for   at  the   area  border
routers. This is not so good  as it would probably be good to redirect
traffic to a  server in another area should the local  one go down for
some reason.

- with OSPF,  configure the  cost parameter on  inter area links  to a
value larger  than that  associated with any  intra area path  -- this
will make sure  that a routes from one area  are always preferred over
inter area routes. In fact I'm pretty sure OSPF does this automatially
anyways, but it's nice to  have things explicitly configured like this
-- your "serving"  area  needn't  necessarily correspond with  an OSPF
area  in  this case  (a  single  OSPF area  may  be  divided into  two
"serving areas" , for example).

You can achieve load balancing in a more fault tolerant manner in this
way too  -- say you want to  load balance across two  web servers, but
you  want  one  to  take  over  all  of  the  load  should  the  other
fail. Configure the dns to round robin on and, say and
configure  both  of  these   addresses  as  aliases  on  the  loopback
interfaces  of both  servers. The  first server  advertises  the first
address with  a lower  cost than  the second and  the second  does the
reverse.  The difference  in  cost  should be  greater  than the  cost
associated with the best path between the servers.

What  is  the  general  feeling  about running  routing  protocols  on
web/dns/mail servers?

Will Waites \________\____________________________
Idiosyntactix Ministry of Research and Development\

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