[cisco-bba] How to stop static routes looping

Oliver Boehmer (oboehmer) oboehmer at cisco.com
Fri Mar 7 03:06:58 EST 2008

Andy Saykao <> wrote on Friday, March 07, 2008 8:13 AM:

> Hi There,
> Just wondering if there's a way to stop this sort of routing loop
> from happening. 
> 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 below.    
> 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.    

Right. But why do you route it if the customer doesn't use it? Then the
loop isn't bad ;-)

> Can this routing loop be stopped from the ISP (LNS) side???

Well, not really. However, you want to consider applying uRPF (ip verify
unicast reverse-path) to the virtual-access/virtual-template which will
cause the "looped" packet to be dropped as it is sourced from an IP not
being reachable over this interface. You want to do this anyway to
prevent spoofing..


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