[cisco-bba] How to stop static routes looping

Andy Saykao andy.saykao at staff.netspace.net.au
Fri Mar 7 02:12:49 EST 2008

Hi There,
Just wondering if there's a way to stop this sort of routing loop from
Say for example we have a customer who has a PPP connection and when
they login they get an IP of
They now want an additional /29 subnet and so through Radius we assign
then a /29 (eg:
Internet -> ISP (LNS) -> Cust Route (PPP) -> Cust additional /29 subnet
I gather the static route for this additional /29 subnet is injected to
the router from Radius becauses there's no hard set "ip route" command
on the LNS and OSPF then restributes this static route using the command
"redistribute static subnets" as seen in the "sh ip route" command
lns#sh ip route
Routing entry for
  Known via "static", distance 1, metric 0
  Redistributing via ospf 100
  Advertised by ospf 100 subnets
  Routing Descriptor Blocks:
      Route metric is 0, traffic share count is 1

My problem is that if the customer doesn't use the additional /29 subnet
and traffic is destined for the additional /29 subnet we get a routing
loop happening because the customer's router sends the packet out it's
default route back to the ISP's LNS and then the ISP's LNS thinking it
has a static route sends it back to the customer's router and round and
round we go til the TTL expires. 
Can this routing loop be stopped from the ISP (LNS) side??? Note that we
are using OSPF on our network with the following config below and the
static route mentioned above I assume is injected from Radius because
there's no hard set "ip route" command for that additional /29 subnet on
the LNS.
router ospf 100
 router-id 203.17.x.x
 redistribute connected subnets
 redistribute static subnets
 network 203.10.x.x area 0

Thankyou in advance.

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