Mastering MPLS: Qos Interview Questions

 Mastering MPLS: Qos Interview Questions

MPLS interview Questions
MPLS Sample Topology

Related : Mastering MPLS: Essential Interview Questions
Related : Mastering MPLS: Advanced Interview Questions

⚡ What kinds of QoS protocols does MPLS support?

MPLS supports the same QoS capabilities as IP. These mechanisms are IP Precedence, Committed Access Rate (CAR), Random Early Detection (RED), Weighted RED, Weighted Fair Queuing (WFQ), Class-based WFQ, and Priority Queuing. Proprietary and non-standard QoS mechanisms can also be support but are not guaranteed to interoperate with other vendors. 

Since MPLS also supports reservation of Layer 2 resources, MPLS can deliver finely grained quality of service, much in the same manner as ATM and Frame Relay. 

⚡ How do I integrate MPLS and DiffServ ?

DiffServ can support up to 64 classes while the MPLS shim label supports up to 8 classes. This shim header has a 3-bit field defined for experimental use. This poses a problem. This Exp field is only 3 bits long, whereas the Diff-Serv field is 6 bits. There are different scenarios to work around this problem. 

There are two alternatives that address this problem called Label-LSP and Exp-LSP models. But they introduce complexity into the architecture. The diffserv model essentially defines the interpretation of the TOS bits. As long as the IP precedence bits map to the Exp bits the same interpretation as the diffserv model can be applied to these bits. In the case where additional bits are used in the diffserv model, one can essentially use the label value to interpret the meaning of the remaining bits. Recognizing that 3 bits are sufficient to identify the required number of classes, the remaining bits in the diffserv model are used for identifying the drop priority and these drop priorities can be mapped into an L-LSP in which case the label identifies the drop priority while the exp bits identify the Class that the packet belongs to. 

Many Service Provides have or will add just a few classes. This small enhancement will be hard enough to provision, manage and sell. This would be an effective strategy to get to market quickly with a value-added service. 

The followings classes may be more appropriate for the initial deployment of MPLS QoS: 

  • High-priority, low-latency "Premium" class (Gold Service) 
  • Guaranteed-delivery "Mission-Critical" class (Silver Service) 
  • Low-priority "Best-Effort" class (Bronze Service) 

⚡ What is "Generalized MPLS" or "GMPLS" ?

From "Generalized Multi-Protocol Label Switching Architecture" "Generalized MPLS extends MPLS to encompass time-division (e.g. SONET ADMs), wavelength (optical lambdas) and spatial switching (e.g. incoming port or fiber to outgoing port or fiber)." 

GMPLS represents a natural extension of MPLS to allow MPLS to be used as the control mechanism for configuring not only packet-based paths, but also paths in non-packet based devices such as optical switches, TDM muxes, and SONET/ADMs. 

⚡ What are the components of GMPLS?

GMPLS introduces a new protocol called the "Link Management Protocol" or LMP. LMP runs between adjacent nodes and is responsible for establishing control channel connectivity as well as failure detection. LMP also verifies connectivity between channels.

Additionally, the IETF's "Common Control and Measurement Plane" working group (ccamp) is working on defining extensions to interior gateway routing protocols such as OSPF and IS-IS to enable them to support GMPLS operation.

⚡ What are the features of GMPLS?

GMPLS supports several features including:
  • Link Bundling - the grouping of multiple, independent physical links into a single logical link 

  • Link Hierarchy - the issuing of a suite of labels to support the various requirements of physical and logical devices across a given path 

  • Unnumbered Links - the ability to configure paths without requiring an IP address on every physical or logical interface 

  • Constraint Based Routing - the ability to automatically provision additional bandwidth, or change forwarding behavior based on network conditions such as congestion or demands for additional bandwidth 

⚡ What are the "Peer" and "Overlay" models?

GMPLS supports two methods of operation, peer and overlay. In the peer model, all devices in a given domain share the same control plane. This provides true integration between optical switches and routers. Routers have visibility into the optical topology and routers peer with optical switches. In the overlay model, the optical and routed (IP) layers are separated, with minimal interaction. Think of the overlay model as the equivalent of today's ATM and IP networks, where there is no direct connection between the ATM layer and the IP routing layer.

The peer model is inherently simpler and more scalable, but the overlay model provides fault isolation and separate control mechanisms for the physical and routed network layers, which may be more attractive to some network operators.