How can I equal-cost load balancing via route-map when next-hop is by DSCP?

[Nadav]

Hi everyone,

I have two ASR1k routers which connect to each other via 4 GRE tunnels. Let's call the 4 tunnels: T1,T2,T3,T4. Between these routers we have a 3rd party network which routes our traffic.

The tunnels are advertised via OSPF over area 0. The tunnels are at present the same OSPF cost, although if the solution to this discussion requires changing the cost then that's doable.

1) Is it possible to configure it so that for certain DSCP markings the traffic will go to T1 and T2 (Pair1), and for all other DSCP markings the traffic will go to T3 and T4 (Pair 2)? 

2) Can this traffic be load balanced within Pair 1 and within Pair 2? Each pair with their relevant mapped DSCPs.  The load balancing should be per-session and not per-packet.

3) Can an automatic failover be configured so that if Pair 1 isn't available only then will their traffic will be load balanced over Pair 2, and vica versa?  

Our client isn't interested in using PFR (Performance Routing) for this solution due to the complexity of changing our WAN infrastructure.

I imagine this is possible with PBR yet I haven't seen how the traffic can be load balanced between pairs. "set ip next-hop" doesn't round robin sessions as far as I can tell, but rather the second next-hop is only used if the first is unavailable.

Thanks for your help!

[pigallo]

Hello,

you have the ability to do that through MPLS TE.
This should be supported in IOS XE.
You could test that on your own infrastructure or ask your ISP to enable it.
Technically the selection happens based on exp bits of MPLS but you could define table maps before traffic enters on your PE device so that your DSCP values are correctly mapped accordingly.

[Nadav]

Hello,

The 3rd party network is composed of a few simple routers and I can't make demands of them other than to provide network basic L3 connectivity, no TE is available.  

The best I can do is to route traffic by some policy through a different sub-interface on the single WAN interface of each router. Each of the four sub-interfaces has a different GRE tunnel assigned to it. Two of the GRE's make up Pair 1, and the other two make up Pair 2.

[Nadav]

At present I haven't found an easy route-map based solution for this issue.

The best I've been able to come up with is to split the load-balanced paths between the two routers at each site and by route-map check on each router:

1) If local tunnel is reachable, send DSCP pair over specific local GRE tunnel

2) If local tunnel not reachable, check if second local router's DSCP-specific tunnel is available via IP SLA. If so, send it to second local router and let him route it out his local tunnel. 

3) If neither local tunnel nor neighbor's tunnel is reachable, just route by RIB.