Implementing EIGRP on CiscoIOS XR Softwa re
Information About Implementing EIGRP on CiscoIOS XR Software
RC-138
Cisco IOS XR Routing Configuration Guide
OL-14356-01
•Provider Edge (PE)-Customer Edge (CE) protocol support with Site of Origin (SoO) and Border
Gateway Protocol (BGP) cost community support.
•PECE protocol support for MPLS and L2TPv3-based-IP L3VPNs.
EIGRP Components
EIGRP has the following four basic components:
•Neighbor discovery of 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. 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 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 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.