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• RPL can be enabled only on the main interface.
• RPL may not work if multiple links have same link local address.
• The probing parameters are not configurable in root template in this implementation. The probing parameters are required in root template also.
• The following RPL features listed in RFC6550 are not supported:
  • Secure RPL Control Messages.
  • Non-storing mode.
  • Multicast DAO
  • Unicast DIS
  • Unicast DIO
  • Reception of DAO-ACK
  • Virtual DODAG root
  • Metrics and constraints—Implements a subset of these metrics and constraints only.
  • DIO Destination prefix
  • Non-storing mode for DAO routes—Supports processing of DAO prefixes received from nonstoring nodes but always operate in a full route storing mode.
  • No support for adding tags to DAO prefixes using DAO suboptions.
  • Local confidence
  • Interface level link check for Expected Transmission Count or latency calculation—Uses layer-3 probe process for calculating link.
• You cannot change the values of the Instance-ID, DODAG ID, and OCP options for an RPL after the RPL template is configured. If you want to change the options, you must delete the template from all interfaces.
• The no dao enable command cannot be used when the template is active.

Low power and Lossy Networks (LLNs) are a class of network in which both the routers and their interconnect are constrained: LLN routers typically operate with constraints on (any subset of) processing power, memory and energy (battery), and their interconnects are characterized by (any subset of) high loss rates, low data rates and instability. LLNs are comprised of anything from a few dozen and up to thousands of LLN routers, and support point-to-point traffic (between devices inside the LLN), point-to-multipoint traffic (from a central control point to a subset of devices inside the LLN) and multipoint-to-point traffic (from devices inside the LLN towards a central control point).

The Routing Protocol for Low Power and Lossy Networks feature specifies the IPv6 Routing Protocol for LLNs (RPL), thereby providing a mechanism whereby multipoint-to-point traffic from devices inside the LLN towards a central control point, and point-to-multipoint traffic from the central control point to the devices inside the LLN, is supported. Point-to-point traffic is also supported.

RPL makes use of a wide set of routing attributes that can be used either as constraints or metrics as against IGPs such as IS-IS or OSPF. When used as a constraint, the routing attribute allows pruning the links and nodes from candidate paths that do not respect the constraint; when used as a metric the routing attribute is used to determine the least cost path. Additionally, routing metrics can either be used as aggregated (e.g. path cost equal to the sum of the link metrics) or recorded in which case metrics of all links along the path are recorded and announced along with the path by RPL. Recorded metrics are particularly useful when aggregating metrics implies loosing relevant information for path selection.

Metrics can also be local (exchanged between two neighbors) or global (propagated along the path as the path cost).RPL uses dynamic metrics, which implies the use of low pass filters to preserve routing stability, avoid temporary loops and bind control traffic by limiting the number of advertised RPL messages due to link metrics updates and consequently path cost that may have a global impact by recomputing the routing topology.

Note	Most RPL routing attributes could be used as either metric or constraint. For example, bandwidth can be used to prune link without sufficient capacity or as a metric to find the least cost pathn.

The Routing Protocol for Low Power and Lossy Networks feature supports aggregated metrics only and the following subset of routing metrics or constraints:

• Routing metrics—ETX (link), Latency (link) and DAG rank.
• Routing constraints—Node State and Attribute (NSA), Node Energy (node).

The Objective Function (OF) is used by RPL to specify how the routing metric and constraints should be used to reach specific objectives. For example, the OF may specify that the objective is to find the constrained shortest path where the constraint is related to the node power mode and the metric is the ETX.

Each Destination Oriented Directed Acyclic Graph (DODAG) is identified by:

• RPLInstanceID (potentially multiple DODAG, but one OF)
• DODAGID (set by the DAG Root)
• DODAGVersionNumber (DODAG iteration number)
• Parameters advertised by the DODAG root: DAG metric container content, trickle timers, etc.

The DODAGID will be specified as part of the RPL root template definition via the dag-id command.

The Routing Protocol for Low power and Lossy Networks (RPL) feature supports the Objective functions (OF)—OF0, OF1 (ETX) and OF1 (latency). These functions constitute a large set to cover a wide spectrum of uses in Smart Cities. OF0 is used where the objective is to build a DODAG that simply provides connectivity, while OF1 is used to build a reliable network and optimized DODAGs without delay requirements, for example, smart metering and lighting management, and traffic management in a smart city. Each OF specifies a different parent selection criteria and strictly follow the rules for parent selection as specified in the RFC for loop avoidance.

Use the step-rank in the RPL template definition to configure the objective function.

RPL supports redistribution of routes into a limited set (OSPF, IS-IS, BGP) of routing protocols. The route redistribution happens at the root, which will redistribute routes into other routing protocol that are directly connected as well as routes learnt from DAO messages. Routes learnt from other routing protocols will not be redistributed into the RPL network domain. The root updates the application VRFs with the routing update information. Use the redistribute rpl command to specify redistribution in an RPL network.

Management is critical in all networking solution but particularly in environments comprising a large number of networking devices. The DODAG root (a Cisco router) plays a central role in the DODAG management. The Routing Protocol for Low power and Lossy Networks (RPL) feature provides an extensive set of configuration options via CLI (general and service internal) to tune the behavior in the field.

The unique characteristics of LLNs provide several design choices for RPL. RPL routing performance is dependent on the routing environments. If convergence time is of utmost importance for a routing protocol operating in a “classic” IP networks, the prime performance objective in LLNs is scalability and stability, which are the main challenges in LLNs where links and nodes are potentially highly unstable. Another objective is to bound the control traffic to save bandwidth and energy—the control traffic is negligible compared to the traffic that is considerably lower in “typical” IP networks. The following metrics can be used to evaluate the performances of a routing protocol:

RPL configuration in Cisco IOS software is simple. The Routing Protocol for Low power and Lossy Networks (RPL) feature provides a configuration template to define the behavior. Use this template to configure and fine tune the parameters as needed. Most of the parameters are available with default values and states and hence will be optional unless there is a need to override the default values or behavior. After defining the template, the template can used to enable the protocol on a set of interfaces.

Router> enable

Router# configure terminal

Router(config)# ipv6 router rpl rpl_ip1

Router(config-rtr)# redistribute connected

Router(config-rtr)# definition root

Router(config-rtr)# instance 125 non-storing

Router(config-rtr)# dag 15

Router(config-rtr)# ocp 1

Router(config-rtr)# fast-connect

Router(config-rtr)# no send-dis

Router(config-rtr)# preference 0

Router(config-rtr)# trickle-timer minimum 10

Router(config-rtr)# rank maximum-increase 0

Router(config-rtr)# global-repair interval 30

Router(config-rtr)# floating

Router(config-rtr)# no dao enable

Router(config-rtr)# dao solicit-interval 10

Router(config-rtr)# load-balance p2mp 10

Router(config-rtr)# step-of-rank 1

Router(config-rtr)# end

Proceed to Enabling RPL section to enable Routing Protocol for Low Power and Lossy Networks.

Router> enable

Router# configure terminal

Router(config)# ipv6 router rpl rpl_ip1

Router(config-rtr)# redistribute connected

Router(config-rtr)# definition root

Router(config-rtr)# instance 125 non-storing

Router(config-rtr)# ocp 1

Router(config-rtr)# fast-connect

Router(config-rtr)# no send-dis

Router(config-rtr)# no dao enable

Router(config-rtr)# dao solicit-interval 10

Router(config-rtr)# load-balance p2mp 10

Router(config-rtr)# end

Proceed to Enabling RPL task to enable Routing Protocol for Low Power and Lossy Networks.

DAG—Directed Acyclic Graph. A directed graph having the property that all edges are oriented in such a way that no cycles exist. All edges are contained in paths oriented toward and terminating at one or more root nodes.

DAG root—A DAG root is a node within the DAG that has no outgoing edges. Because the graph is acyclic, by definition all DAGs must have at least one DAG root and all paths terminate at a DAG root.

DODAG—Destination Oriented DAG. A DAG rooted at a single destination, i.e. at a single DAG root (the DODAG root) with no outgoing edges.

DODAGID—The identifier of a DODAG root. The DODAGID must be unique within the scope of a RPL Instance in the LLN.

DODAG Iteration—A specific version number iteration (version) of a DODAG with a given DODAGID.

DODAG parent—A parent of a node within a DODAG is one of the immediate successors of the node on a path towards the DODAG root. The DODAG parent of a node will have a lower rank than the node itself.

DODAG root—A DODAG root is the DAG root of a DODAG.

DODAG sibling—A sibling of a node within a DODAG is defined in this specification to be any neighboring node which is located at the same rank within a DODAG. The siblings defined in this manner do not necessarily share a common DODAG parent.

DODAGVersionNumber—A sequential counter that is incremented by the root to form a new Version of a DODAG. A DODAG Version is identified uniquely by the (RPLInstanceID, DODAGID, DODAGVersionNumber) tuple.

Down—The direction from DODAG roots towards leaf nodes, going against the orientation of the edges within the DODAG.

Goal—A host or set of hosts that satisfy a particular application objective / OF. Whether or not a DODAG can provide connectivity to a goal is a property of the DODAG. For example, a goal might be a host serving as a data collection point, or a gateway providing connectivity to an external infrastructure.

Grounded—A DODAG is said to be grounded, when the root can reach the Goal of the objective function.

Floating—A DODAG is floating if it is not Grounded. A floating DODAG is not expected to reach the Goal defined for the OF. As they form networks, LLN devices often mix the roles of 'host' and 'router' when compared to traditional IP networks. The host refers to an LLN device that can generate but does not forward RPL traffic, router refers to an LLN device that can forward as well as generate RPL traffic, and node refers to any RPL device, either a host or a router.

LBR—Low power and lossy network Border Router. A device that connects the Low power and Lossy Network to another routing domain such as a Local Area Network (LAN), Wide Area Network (WAN) or the Internet where a possibly different routing protocol is in operation. In some cases, in addition to acting as a routing device, the LBR may also be a host performing functions such as data collector or aggregator.

LLN—Low power and Lossy Networks. These networks are typically composed of many embedded devices with limited power, memory, and processing resources interconnected by a variety of links, such as IEEE 802.15.4 or Low Power WiFi. There is a wide scope of application areas for LLNs, including industrial monitoring, building automation (HVAC, lighting, access control, fire), connected home, healthcare, environmental monitoring, urban sensor networks, energy management, assets tracking and refrigeration.

OCP—Objective Code Point. An identifier, used to indicate which Objective Function is in use for forming a DODAG. The Objective Code Point is further described in METRIC.

OF—Objective Function. Defines which routing metrics, optimization objectives, and related functions are in use in a DODAG. The Objective Function is further described in METRIC.

Rank—The rank of a node in a DAG identifies the nodes position with respect to a DODAG root. The farther away a node is from a DODAG root, the higher is the rank of that node. The rank of a node may be a simple topological distance, or may more commonly be calculated as a function of other properties as described later.

RPL Instance—A set of possibly multiple DODAGs. A network may have more than one RPL Instance, and a RPL node can participate in multiple RPL Instances. Each RPL Instance operates independently of other RPL Instances. This document describes operation within a single RPL Instance. In RPL, a node can belong to at most one DODAG per RPL Instance. The tuple (RPLInstanceID, DODAGID) uniquely identifies a DODAG.

RPLInstanceID—Unique identifier of a RPL Instance.

Sub-DODAG—The set of other nodes in the DODAG that might use a path towards the DODAG root that contains that node. Nodes in the sub-DODAG of a node have a greater rank than that node itself (although not all nodes of greater rank are necessarily in the sub-DODAG of that node).

Up—The direction from leaf nodes towards DODAG roots, following the orientation of the edges within the DODAG.