The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all 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 hello packets
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 indicated in the packet. The reliable transport has a provision to
send multicast packets quickly when unacknowledged packets are pending. This provision helps to ensure
that convergence time remains low in the presence of various speed links.
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
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.
EIGRP Configuration Grouping
Cisco IOS XR software groups all EIGRP configuration under router EIGRP configuration mode, including
interface configuration portions associated with EIGRP. To display EIGRP configuration in its entirety, use
the show running-config router eigrp command. The command output displays the running configuration
for the configured EIGRP instance, including the interface assignments and interface attributes.
EIGRP Configuration Modes
The following examples show how to enter each of the configuration modes. From a mode, you can enter the
? command to display the commands available in that mode.
Router Configuration Mode
The following example shows how to enter router configuration mode:
RP/0/RSP0/CPU0:router# configuration
RP/0/RSP0/CPU0:router(config)# router eigrp 100
RP/0/RSP0/CPU0:router(config-eigrp)#
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
242 OL-30423-03
Implementing EIGRP
EIGRP Configuration Grouping
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