Content Overview Enhanced Interior
Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing
protocol. It has been described as a hybrid routing protocol,
because it combines the best of distance vector routing
protocols with link-state algorithms. EIGRP offers benefits
over historical distance vector routing protocols, such as
Routing Information Protocol (RIP) v1 and IGRP, by offering
rapid convergence, lower bandwidth utilization, support for
multiple routed protocols (for example, IP, Internetwork Packet
Exchange [IPX], and AppleTalk), and support for classless
interdomain routing (CIDR) and variable length subnet masking
(VLSM). To achieve some of these benefits, EIGRP relies on
features commonly associated with link-state protocols. For
example, EIGRP uses the best traits of Open Shortest Path First
(OSPF) Protocol, such as partial updates and neighbor
discovery. EIGRP offers benefits over OSPF in large networks
built primarily on Cisco routers. In large networks, OSPF is
deployed in a multi-area hierarchical design, which is
administratively complex. EIGRP converges faster than OSPF and
is easier to implement and maintain. In addition, unlike OSPF,
which only supports IP, EIGRP supports multiple routed
protocols. This module describes how EIGRP works and how to
implement and verify EIGRP operations. Advanced topics, such as
route summarization, load balancing, bandwidth usage, and
authentication, are also explored. The module concludes with a
discussion of EIGRP issues and problems and how to correct
them. Web Links EIGRP Support Page
http://www.cisco.com/en/US/tech/tk365/tk207/
tsd_technology_support_sub-protocol_home.html
Content
2.1 EIGRP Fundamentals and Features
2.1.1 EIGRP Capabilities and Attributes The
addition of several link-state features, such as dynamic
neighbor discovery, makes EIGRP an advanced distance vector
protocol. EIGRP includes the following key features: -
Fast convergence: A router running EIGRP stores all its
neighbors’ routing tables so that it can quickly adapt to
alternate routes if a preferred route disappears. If an
appropriate route does not exist, EIGRP queries its neighbors
to discover an alternate route. These queries propagate until
an alternate route is found.
- VLSM support:
EIGRP is a classless routing protocol, which means that it
advertises a subnet mask for each destination network. Support
for VLSM allows for different subnetmasks within the same
network and support for non-contiguous subnetworks. EIGRP
routes are automatically summarized at the major network number
boundary. However, EIGRP can be configured to summarize on any
bit boundary on any router interface.
- Partial
updates: EIGRP does not send periodic updates. Instead, it
sends partial triggered updates. The updates are sent only when
the path or the metric changes for a route, and they contain
only information about the changed routes. Propagation of
partial updates is automatically bounded so that only those
routers that need the information are updated. Partial updates
are handled by using multicast and unicast packets instead of
broadcast packets. As a result, EIGRP consumes significantly
less bandwidth than IGRP. This behavior is different than
link-state protocols, in which an update is transmitted to all
link-state routers within an area.
- Multiple
network-layer protocol support: EIGRP supports IP,
AppleTalk, and Novell NetWare IPX through the use of
protocol-dependent modules. These modules are responsible for
protocol requirements specific to the network layer. The rapid
convergence and sophisticated metric structure of EIGRP offers
superior performance and stability when implemented in IPX and
AppleTalk networks.
Note
This course
covers only the TCP/IP implementation of EIGRP. Other EIGRP
features include the following: - Seamless
connectivity across all data link layer protocols and
topologies: EIGRP does not require special configuration to
work across any Layer 2 protocols. Other routing protocols,
such as OSPF, use different configurations for different Layer
2 protocols, such as Ethernet and Frame Relay. EIGRP operates
effectively in both LAN and WAN environments. WAN support for
dedicated point-to-point links and nonbroadcast multiaccess
(NBMA) topologies is standard for EIGRP. EIGRP accommodates
differences in media types and speed when neighbor adjacencies
form across WAN links and can be configured to limit the amount
of bandwidth that the protocol uses on WAN links.
-
Sophisticated metric: The EIGRP metric is based on
bandwidth and delay. However, it can also be configured to
consider reliability, load, and maximum transmission unit
(MTU).
- Multicast and unicast: To establish
neighbor relationships, EIGRP uses multicast and unicast
addressing rather than broadcast addressing to send and
acknowledge routing updates. The EIGRP multicast address is
224.0.0.10. The use of multicast and unicast also helps reduce
bandwidth requirements.
Web Links
Introduction to EIGRP
http://www.cisco.com/en/US/tech/tk365/
technologies_tech_note09186a0080093f07.shtml
EIGRP
White Paper
http://www.cisco.com/warp/public/103/eigrp-toc.h
tml
Content 2.1 EIGRP Fundamentals and
Features 2.1.2 Underlying Process and
Technologies EIGRP employs four key components that
differentiate it from other routing technologies :
- Protocol-dependent modules
- Reliable Transport
Protocol (RTP)
- Neighbor discovery and recovery
- Diffusing Update Algorithm (DUAL) finite-state machine
Content 2.1 EIGRP
Fundamentals and Features 2.1.3
Protocol-dependent Modules Protocol-dependent modules
(PDMs) are responsible for network layer protocol-specific
requirements. EIGRP supports IP, AppleTalk, and Novell NetWare.
Each protocol has its own EIGRP module and operates
independently from any of the others that may be running. The
IP-EIGRP module, for example, is responsible for sending and
receiving EIGRP packets that are encapsulated in IP. IP-EIGRP
is also responsible for parsing EIGRP packets and informing
DUAL of the new information that has been received. IP EIGRP
asks DUAL to make routing decisions. These results are then
stored in the IP routing table. IP-EIGRP is responsible for
redistributing routes learned by other IP routing protocols.
Content 2.1 EIGRP Fundamentals and
Features 2.1.4 Reliable Transport Protocol
RTP is responsible for guaranteed ordered delivery and
reception of EIGRP packets to all neighbors. RTP supports
intermixed transmission of multicast and unicast packets. For
efficiency, only certain EIGRP packets are transmitted
reliably. For example, since Ethernet is a multiaccess network
with multicast capabilities, it is not necessary to send hello
packets reliably to all neighbors individually. Instead, EIGRP
sends a single multicast hello packet containing an indicator
that informs the receivers that the packet does not need to be
acknowledged. Other types of packets, such as update packets,
indicate that acknowledgment is required. RTP contains a
provision for sending multicast packets quickly, even when
unacknowledged packets are pending, which helps ensure that
convergence time remains low when there are links of varying