Subject: IGP shortcut in OSPF

From: Santosh Esale <[email-masked]>
Date: Thu, 9 Feb 2006 18:27:14 +0530

Hi All,

        Want to know the solutions for below problem:

Consider the below topology from RFC 3906 and all ospf link cost as 10 and
two TE-tunnels T1: (RTA-RTD) and T2: (RTA to RTE via rtrC)

     rtrA -- rtrB -- rtrC

               |       |

              rtrD -- rtrE

  In OSPF, after calcualting SPT till rtrB, suppose we choose rtrD from
candidate list to put into SPT, we first modify rtrD routers nexthop to T1
with cost 1 which is a fixed cost assigned to all TE-Tunnel by MPLS in my
algorthim.Then we choose rtrE(cost 11, compare to rtrC with cost 20) from
candidate list and modify the nexthop to T2 with cost of 1.

Now, to reach rtrC, the cost from rtrE to rtrC is 11 because of TE-tunnel
T2, while from rtrB is 20, hence we modify rtrC's nexthop to TE tunnel T2.

This is wrong as all traffic going to rtrC, will go to rtrE via tunnel T2
(via rtrC), then routed back to rtrC.

i though of checking with the help of mpls, while modifying nexthop of rtrC
to tunnel T2, whether any of RTC's address is present in nexthop list of  TE
Tunnel T2, basically avoiding a loop, but in some cases mpls will not have
nexthop list(no RROs), hence stuck ?

so can anyone think of any other solutions ?

Regards

Santosh

---

From: Ehsan Rezaaifar <[email-masked]>
Date: Thu, 9 Feb 2006 09:49:26 -0500

Hi,

Well one way is to keep the cost of the routes the same as IGP metric of 
the native IP.

My understanding of the RFC is that if you choose to change the cost of 
the route, you should
only modify it when it's added to the routing table (or as the RFC calls 
it the Private IGP RIB).
I am not quite sure how to implement this, but I think what it means is 
that the IGP SCs would not affect
how the SPT looks like, (SPT is built based on the IGP metrics), but 
when the routes are added
to the "private IGP RIB" the load balancing is done based on the cost of 
the IGP SCs. Basically
when we are adding the routes to the Routing RIB, we compare the ECMPs 
based on their IGP SC
cost.
So in your case it should not affect the toute to the rtrC, (still 
through rtrB).

Take care,
Ehsan

Santosh Esale wrote:

> Consider the below topology from RFC 3906 and all ospf link cost as 10 
> and two TE-tunnels T1: (RTA-RTD) and T2: (RTA to RTE via rtrC)
>
>      rtrA -- rtrB -- rtrC
>
>               |       |
>
>
>              rtrD -- rtrE
>
>   In OSPF, after calcualting SPT till rtrB, suppose we choose rtrD 
> from candidate list to put into SPT, we first modify rtrD routers 
> nexthop to T1 with cost 1 which is a fixed cost assigned to all 
> TE-Tunnel by MPLS in my algorthim.Then we choose rtrE(cost 11, compare 
> to rtrC with cost 20) from candidate list and modify the nexthop to T2 
> with cost of 1.
>
> Now, to reach rtrC, the cost from rtrE to rtrC is 11 because of 
> TE-tunnel T2, while from rtrB is 20, hence we modify rtrC's nexthop to 
> TE tunnel T2.
>
> This is wrong as all traffic going to rtrC, will go to rtrE via tunnel 
> T2 (via rtrC), then routed back to rtrC.
>
> i though of checking with the help of mpls, while modifying nexthop of 
> rtrC to tunnel T2, whether any of RTC's address is present in nexthop 
> list of  TE Tunnel T2, basically avoiding a loop, but in some cases 
> mpls will not have nexthop list(no RROs), hence stuck ?
>