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andy.saykao at staff

Mar 6, 2008, 11:12 PM

Post #1 of 6 (4930 views)
Permalink
How to stop static routes looping

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 192.168.1.1.
They now want an additional /29 subnet and so through Radius we assign
then a /29 (eg: 192.168.2.0/29).

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 192.168.2.0
Routing entry for 192.168.2.0/29
Known via "static", distance 1, metric 0
Redistributing via ospf 100
Advertised by ospf 100 subnets
Routing Descriptor Blocks:
* 192.168.1.1
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
log-adjacency-changes
redistribute connected subnets
redistribute static subnets
network 203.10.x.x 0.0.0.0 area 0

Thankyou in advance.

--

Regards,

Andy

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oboehmer at cisco

Mar 7, 2008, 12:06 AM

Post #2 of 6 (4566 views)
Permalink
Re: How to stop static routes looping [In reply to]

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 192.168.1.1.
> They now want an additional /29 subnet and so through Radius we
> assign then a /29 (eg: 192.168.2.0/29).
>
> 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 192.168.2.0
> Routing entry for 192.168.2.0/29
> Known via "static", distance 1, metric 0
> Redistributing via ospf 100
> Advertised by ospf 100 subnets
> Routing Descriptor Blocks:
> * 192.168.1.1
> 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..

oli
_______________________________________________
cisco-bba mailing list
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bbc at misn

Mar 7, 2008, 6:13 AM

Post #3 of 6 (4588 views)
Permalink
Re: How to stop static routes looping [In reply to]

Can anyone testify as to whether there are any potential pitfalls of
configuring uRPF (ip verity unicast reverse-path) in this application?

I recall, from the past, that "ip verity unicast reverse-path" can cause
undesirable behavior in certain circumstances. But, that may not be the
case anymore.

Thanks!

Bryan


Oliver Boehmer (oboehmer) wrote:
> 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 192.168.1.1.
>> They now want an additional /29 subnet and so through Radius we
>> assign then a /29 (eg: 192.168.2.0/29).
>>
>> 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 192.168.2.0
>> Routing entry for 192.168.2.0/29
>> Known via "static", distance 1, metric 0
>> Redistributing via ospf 100
>> Advertised by ospf 100 subnets
>> Routing Descriptor Blocks:
>> * 192.168.1.1
>> 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..
>
> oli
> _______________________________________________
> cisco-bba mailing list
> cisco-bba [at] puck
> https://puck.nether.net/mailman/listinfo/cisco-bba
>
>
_______________________________________________
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andy.saykao at staff

Mar 10, 2008, 6:03 PM

Post #4 of 6 (4583 views)
Permalink
Re: How to stop static routes looping [In reply to]

In reply to Message 2 below, the route is injected into the routing
table by radius when the customer successfully logs on. Whether they are
using the additional subnet or not is up to the customer to configure
(the route to the additional subnet is there for them to use). We have
this issue because a customer gets charged for all traffic that comes
into their network. So if you can imagine that if data packets are
bouncing in and out of the customer's network and back into the ISP's
network because they haven't set up their additional subnet, they will
incur this usage when the data packet enters their network again and
will often dispute this usage.

>From the ISP's side, is there something we can do to stop this sort of
routing loop. Oliver has suggested applying uRPF (ip verify unicast
reverse-path) to the virtual-access/virtual-template but I am yet to
test this out.

Cheers.

Andy

-----

Message: 2
Date: Fri, 7 Mar 2008 09:06:58 +0100
From: "Oliver Boehmer (oboehmer)" <oboehmer [at] cisco>
Subject: Re: [cisco-bba] How to stop static routes looping
To: "Andy Saykao" <andy.saykao [at] staff>,
<cisco-bba [at] puck>
Message-ID:

<70B7A1CCBFA5C649BD562B6D9F7ED784051197B1 [at] xmb-ams-333>
Content-Type: text/plain; charset="us-ascii"

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 192.168.1.1.
> They now want an additional /29 subnet and so through Radius we assign

> then a /29 (eg: 192.168.2.0/29).
>
> 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 192.168.2.0
> Routing entry for 192.168.2.0/29
> Known via "static", distance 1, metric 0
> Redistributing via ospf 100
> Advertised by ospf 100 subnets
> Routing Descriptor Blocks:
> * 192.168.1.1
> 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..

oli


------------------------------

Message: 3
Date: Fri, 07 Mar 2008 08:13:02 -0600
From: Bryan Campbell <bbc [at] misn>
Subject: Re: [cisco-bba] How to stop static routes looping
To: "Oliver Boehmer (oboehmer)" <oboehmer [at] cisco>
Cc: cisco-bba [at] puck, Andy Saykao
<andy.saykao [at] staff>
Message-ID: <47D14D6E.7070305 [at] misn>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Can anyone testify as to whether there are any potential pitfalls of
configuring uRPF (ip verity unicast reverse-path) in this application?

I recall, from the past, that "ip verity unicast reverse-path" can cause
undesirable behavior in certain circumstances. But, that may not be the
case anymore.

Thanks!

Bryan


Oliver Boehmer (oboehmer) wrote:
> 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 192.168.1.1.
>> They now want an additional /29 subnet and so through Radius we
>> assign then a /29 (eg: 192.168.2.0/29).
>>
>> 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 192.168.2.0
>> Routing entry for 192.168.2.0/29
>> Known via "static", distance 1, metric 0
>> Redistributing via ospf 100
>> Advertised by ospf 100 subnets
>> Routing Descriptor Blocks:
>> * 192.168.1.1
>> 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..
>
> oli
> _______________________________________________
> cisco-bba mailing list
> cisco-bba [at] puck
> https://puck.nether.net/mailman/listinfo/cisco-bba
>
>


------------------------------

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https://puck.nether.net/mailman/listinfo/cisco-bba

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****************************************

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andy.saykao at staff

Mar 13, 2008, 5:01 PM

Post #5 of 6 (4594 views)
Permalink
Re: How to stop static routes looping [In reply to]

Thanks to Oli, I implemented the command "ip verify unicast
reverse-path" to our Virtual-Access template and it has stopped the
routing loop.

Interestingly enough I could only replicate the problem when the
customer was using a Cisco product (eg: Cisco 827) and did not configure
the additional subnet on their Ethernet interface. This resulted in a
loop back and forth between the customer's network and our LNS. This was
before I used the "ip verify unicast reverse-path" command.

However, when using non-cisco product like a Netcomm NB1300 or Siemens
6520, and not having configured the Ethernet interface with the
additional subnet on these devices, they did not show the same routing
loop problem. Somehow these devices stopped the routing loop from
happening.

Why would this be the case whereby the routing loops are only occuring
on the cisco 827 as illustrated above?

----------------------------------------------------------------------

Message: 2
Date: Fri, 7 Mar 2008 09:06:58 +0100
From: "Oliver Boehmer (oboehmer)" <oboehmer [at] cisco>
Subject: Re: [cisco-bba] How to stop static routes looping
To: "Andy Saykao" <andy.saykao [at] staff>,
<cisco-bba [at] puck>
Message-ID:

<70B7A1CCBFA5C649BD562B6D9F7ED784051197B1 [at] xmb-ams-333>
Content-Type: text/plain; charset="us-ascii"

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 192.168.1.1.
> They now want an additional /29 subnet and so through Radius we assign

> then a /29 (eg: 192.168.2.0/29).
>
> 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 192.168.2.0
> Routing entry for 192.168.2.0/29
> Known via "static", distance 1, metric 0
> Redistributing via ospf 100
> Advertised by ospf 100 subnets
> Routing Descriptor Blocks:
> * 192.168.1.1
> 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..

oli


------------------------------

Message: 3
Date: Fri, 07 Mar 2008 08:13:02 -0600
From: Bryan Campbell <bbc [at] misn>
Subject: Re: [cisco-bba] How to stop static routes looping
To: "Oliver Boehmer (oboehmer)" <oboehmer [at] cisco>
Cc: cisco-bba [at] puck, Andy Saykao
<andy.saykao [at] staff>
Message-ID: <47D14D6E.7070305 [at] misn>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Can anyone testify as to whether there are any potential pitfalls of
configuring uRPF (ip verity unicast reverse-path) in this application?

I recall, from the past, that "ip verity unicast reverse-path" can cause
undesirable behavior in certain circumstances. But, that may not be the
case anymore.

Thanks!

Bryan


Oliver Boehmer (oboehmer) wrote:
> 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 192.168.1.1.
>> They now want an additional /29 subnet and so through Radius we
>> assign then a /29 (eg: 192.168.2.0/29).
>>
>> 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 192.168.2.0
>> Routing entry for 192.168.2.0/29
>> Known via "static", distance 1, metric 0
>> Redistributing via ospf 100
>> Advertised by ospf 100 subnets
>> Routing Descriptor Blocks:
>> * 192.168.1.1
>> 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..
>
> oli
> _______________________________________________
> cisco-bba mailing list
> cisco-bba [at] puck
> https://puck.nether.net/mailman/listinfo/cisco-bba
>
>


------------------------------

_______________________________________________
cisco-bba mailing list
cisco-bba [at] puck
https://puck.nether.net/mailman/listinfo/cisco-bba

End of cisco-bba Digest, Vol 57, Issue 5
****************************************

______________________________________________________________________
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For more information please visit http://www.messagelabs.com/email
______________________________________________________________________

This email and any files transmitted with it are confidential and
intended solely for the use of the individual or entity to whom they are
addressed. Please notify the sender immediately by email if you have
received this email by mistake and delete this email from your system.
Please note that any views or opinions presented in this email are
solely those of the author and do not necessarily represent those of
the organisation.
Finally, the recipient should check this email and any attachments for
the presence of viruses. The organisation accepts no liability for any
damage caused by any virus transmitted by this email.



------------------------------

_______________________________________________
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****************************************

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oboehmer at cisco

Mar 14, 2008, 12:44 AM

Post #6 of 6 (4560 views)
Permalink
Re: How to stop static routes looping [In reply to]

Andy Saykao <> wrote on Friday, March 14, 2008 1:01 AM:

> Thanks to Oli, I implemented the command "ip verify unicast
> reverse-path" to our Virtual-Access template and it has stopped the
> routing loop.
>
> Interestingly enough I could only replicate the problem when the
> customer was using a Cisco product (eg: Cisco 827) and did not
> configure the additional subnet on their Ethernet interface. This
> resulted in a loop back and forth between the customer's network and
> our LNS. This was before I used the "ip verify unicast reverse-path"
> command.
>
> However, when using non-cisco product like a Netcomm NB1300 or Siemens
> 6520, and not having configured the Ethernet interface with the
> additional subnet on these devices, they did not show the same routing
> loop problem. Somehow these devices stopped the routing loop from
> happening.
>
> Why would this be the case whereby the routing loops are only occuring
> on the cisco 827 as illustrated above?

I have no clue. It sounds like an "intelligent" behaviour, but still
strange as I would expect a router to consult its routing table and act
accordingly.

oli
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