Chapter 5 Introducing SDN Control in MPLS Networks - High ...

198 CHAPTER 5. INTRODUCING SDN CONTROL IN MPLS NETWORKSchanges to link-state that come about from the change in the tunnel reservation, requiresonly updating the annotated topology-map and not the switches. The only time that Auto-Bandwidth induced change needs to be propagated down to the data plane is when itcauses one or more tunnels to re-route. In such cases, the Controller processes the changeby removing flow-state for the old tunnel route and replacing it with the new tunnel-routein the affected LSRs. As we have mentioned before, this can cause significant churn in alarge network with many tens-of-thousands of LSPs. But with SDN, the effect-of-churn isminimized as routers no longer have to deal with RSVP or OSPF state-update messages.They only need to process/generate OpenFlow messages that change flow (LSP) routes.In Sec. 5.4, we’ll discuss a way in which Auto-Bandwidth can be eliminated altogetherwith SDN.LSP-Priorities: One final MPLS-TE feature we cover is LSP priorities. TE-LSPscan reserve bandwidth at eight different priority levels (with 0 being the highest priorityand 7 the lowest). Accordingly links advertize un-reserved bandwidth at each of thepriority levels, for both the global pool and the solitary DS-TE sub-pool. LSP prioritiesare used for preemption – a higher priority LSP can force a lower priority LSP to re-routeif there isn’t enough un-reserved bandwidth available for both of them on a link they bothshare. For example, if a link has maximum reservable bandwidth of 900 Mbps; of which500 Mbps has been reserved by a Priority-0 tunnel; and 300 Mbps by a Priority-1 tunnel;then 400Mbps of unreserved bandwidth is available at Priority-0, and only 100Mbps isavailable at Priorities 1-7. But if a new Priority-0 tunnel comes along reserving 200Mbps;then all three tunnels cannot co-exist on the link as their cumulative reservations exceedthe maximum reservable bandwidth on the link. The lower priority-1 tunnel is forced tore-route – the head-end for this tunnel periodically tries to find a new path which satisfiesall its constraints.SDN based LSP-Priorities: As with all other LSP attributes, the LSP-priority is just anumber maintained in the annotated topology map within the Controller. The data-planeswitches have no information on LSP priorities. We maintain tunnel priorities in the