Eigrp Components - Cisco Catalyst 9500 series Configuration Manual

Configuring IP Unicast Routing
• Variable-length subnet masks (VLSMs).
• Arbitrary route summarization.
• EIGRP scales to large networks.

EIGRP Components

EIGRP has these four basic components:
• Neighbor discovery and recovery is the process that routers use to dynamically learn of other routers on
• The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all
• The DUAL finite state machine embodies the decision process for all route computations. It tracks all
• The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example
their directly attached networks. Routers must also discover when their neighbors become unreachable
or inoperative. Neighbor discovery and recovery is achieved with low overhead by periodically sending
small hello packets. As long as hello packets are received, the Cisco IOS software can learn that a
neighbor is alive and functioning. When this status is determined, the neighboring routers can exchange
routing information.
neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets
must be sent reliably, and others need not be. For efficiency, reliability is provided only when necessary.
For example, on a multiaccess network that has multicast capabilities (such as Ethernet), it is not necessary
to send hellos reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello
with an indication in the packet informing the receivers that the packet need not be acknowledged. Other
types of packets (such as updates) require acknowledgment, which is shown in the packet. The reliable
transport has a provision to send multicast packets quickly when there are unacknowledged packets
pending. Doing so helps ensure that convergence time remains low in the presence of varying speed
links.
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 feasible
successors. 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 it takes to recompute the route affects the
convergence time. Recomputation is processor-intensive; it is advantageous to avoid recomputation if
it is not necessary. When a topology change occurs, DUAL tests for feasible successors. If there are
feasible successors, it uses any it finds to avoid unnecessary recomputation.
is the IP 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 asks DUAL to make routing decisions, but the results are stored in the IP
routing table. EIGRP is also responsible for redistributing routes learned by other IP routing protocols.
To enable EIGRP, the Device or stack master must be running the Network Advantage
Note
license.
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
EIGRP Components
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