Eigrp Components - Cisco NCS 6000 Series Configuration Manual

Implementing EIGRP
• Partial updates—EIGRP sends incremental updates when the state of a destination changes, instead of
• Neighbor discovery mechanism—This is a simple hello mechanism used to learn about neighboring
• Variable-length subnet masks (VLSMs).
• Arbitrary route summarization.
• Scaling—EIGRP scales to large networks.
• Support for IPv4 and IPv6 address families.
• Provider Edge (PE)-Customer Edge (CE) protocol support with Site of Origin (SoO) and Border Gateway

EIGRP Components

EIGRP has the following four basic components:
• Neighbor discovery or neighbor recovery
• Reliable transport protocol
• DUAL finite state machine
• Protocol-dependent modules
Neighbor discovery or neighbor recovery is the process that routers use to dynamically learn of other routers
on their directly attached networks. Routers must also discover when their neighbors become unreachable or
inoperative. Neighbor discovery or neighbor recovery is achieved with low overhead by periodically sending
small hello packets. As long as hello packets are received, the Cisco IOS XR software can determine that a
neighbor is alive and functioning. After this status is determined, the neighboring routers can exchange routing
information.
The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors.
Some EIGRP packets must be sent reliably and others need not be. For efficiency, reliability is provided only
when necessary.
The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes
advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient,
loop-free paths. DUAL selects routes to be inserted into a routing table based on a calculation of the feasibility
condition. A successor is a neighboring router used for packet forwarding that has a least-cost path to a
destination that is guaranteed not to be part of a routing loop. When there are no feasible successors but there
are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new
successor is determined. The amount of time required to recompute the route affects the convergence time.
Recomputation is processor intensive; it is advantageous to avoid unneeded recomputation. When a topology
change occurs, DUAL tests for feasible successors. If there are feasible successors, it uses any it finds to avoid
unnecessary recomputation.
The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the
EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IP.
It is also responsible for parsing EIGRP packets and informing DUAL of the new information received. EIGRP
sending the entire contents of the routing table. This feature minimizes the bandwidth required for EIGRP
packets.
routers. It is protocol independent.
Protocol (BGP) cost community support.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
EIGRP Components
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