to a destination are maintained in the topology
table. The topology table includes the following fields:
- Feasible distance (FD is 2195456) 200.10.10.10 –
The feasible distance (FD) is the lowest calculated metric to
each destination. For example, the feasible distance to
32.0.0.0 is 90 as indicated by FD is equal 90.
-
Route source (via 200.10.10.10) – The source of the
route is the identification number of the router that
originally advertised that route. This field is populated only
for routes learned externally from the EIGRP network. Route
tagging can be particularly useful with policy-based routing.
For example, the route source to 32.0.0.0 is 200.10.10.10 via
200.10.10.10.
- Reported distance (FD/RD) – The
reported distance (RD) of the path is the distance reported by
an adjacent neighbor to a specific destination. For example,
the reported distance to 32.0.0.0 is 2195456 as indicated by
(90/2195456).
- Interface information – The
interface through which the destination is reachable
-
Route status – Routes are identified as being either
passive (P), which means that the route is stable and ready for
use, or active (A), which means that the route is in the
process of being recomputed by DUAL.
The EIGRP
routing table holds the best routes to a destination. This
information is retrieved from the topology table. Each EIGRP
router maintains a routing table for each network protocol. A
successor is a route selected as the primary route to use to
reach a destination. DUAL identifies this route from the
information contained in the neighbor and topology tables and
places it in the routing table. There can be up to four
successor routes for any particular route. These can be of
equal or unequal cost and are identified as the best loop-free
paths to a given destination. A copy of the successor routes is
also placed in the topology table. A feasible successor (FS) is
a backup route. These routes are identified at the same time
the successors are identified, but they are only kept in the
topology table. Multiple feasible successors for a destination
can be retained in the topology table although it is not
mandatory. A router views its feasible successors as neighbors
downstream, or closer to the destination than it is. Feasible
successor cost is computed by the advertised cost of the
neighbor router to the destination. If a successor route goes
down, the router will look for an identified feasible
successor. This route will be promoted to successor status. A
feasible successor must have a lower advertised cost than the
existing successor cost to the destination. If a feasible
successor is not identified from the existing information, the
router places an Active status on a route and sends out query
packets to all neighbors in order to recompute the current
topology. The router can identify any new successor or feasible
successor routes from the new data that is received from the
reply packets that answer the query requests. The router will
then place a Passive status on the route. The topology table
can record additional information about each route. EIGRP
classifies routes as either internal or external. EIGRP adds a
route tag to each route to identify this classification.
Internal routes originate from within the EIGRP autonomous
system (AS). External routes originate outside the EIGRP AS.
Routes learned or redistributed from other routing protocols,
such as Routing Information Protocol (RIP), OSPF, and IGRP, are
external. Static routes originating outside the EIGRP AS are
external. The tag can be configured to a number between 0-255
to customize the tag.
Content 3.1 EIGRP
Concepts 3.1.3 EIGRP design features EIGRP operates
quite differently from IGRP. EIGRP is an advanced distance
vector routing protocol and acts as a link-state protocol when
updating neighbors and maintaining routing information. The
advantages of EIGRP over simple distance vector protocols
include the following: - Rapid convergence
-
Efficient use of bandwidth
- Support for
variable-length subnet mask (VLSM) and classless interdomain
routing (CIDR). Unlike IGRP, EIGRP offers full support for
classless IP by exchanging subnet masks in routing
updates.
- Multiple network-layer support
-
Independence from routed protocols. Protocol-dependent modules
(PDMs) protect EIGRP from lengthy revision. Evolving routed
protocols, such as IP, may require a new protocol module but
not necessarily a reworking of EIGRP itself.
EIGRP
routers converge quickly because they rely on DUAL. DUAL
guarantees loop-free operation at every instant throughout a
route computation allowing all routers involved in a topology
change to synchronize at the same time. EIGRP makes efficient
use of bandwidth by sending partial, bounded updates and its
minimal consumption of bandwidth when the network is stable.
EIGRP routers make partial, incremental updates rather than
sending their complete tables. This is similar to OSPF
operation, but unlike OSPF routers, EIGRP routers send these
partial updates only to the routers that need the information,
not to all routers in an area. For this reason, they are called
bounded updates. Instead of using timed routing updates, EIGRP
routers keep in touch with each other using small hello
packets. Though exchanged regularly, hello packets do not use
up a significant amount of bandwidth. EIGRP supports IP, IPX,
and AppleTalk through protocol-dependent modules (PDMs). EIGRP
can redistribute IPX RIP and SAP information to improve overall
performance. In effect, EIGRP can take over for these two
protocols. An EIGRP router will receive routing and service
updates, updating other routers only when changes in the SAP or
routing tables occur. Routing updates occur as they would in
any EIGRP network, using partial updates. EIGRP can also take
over for the AppleTalk Routing Table Maintenance Protocol
(RTMP). As a distance vector routing protocol, RTMP relies on
periodic and complete exchanges of routing information. To
reduce overhead, EIGRP redistributes AppleTalk routing
information using event-driven updates. EIGRP also uses a
configurable composite metric to determine the best route to an
AppleTalk network. RTMP uses hop count, which can result in
suboptimal routing. AppleTalk clients expect RTMP information
from local routers, so EIGRP for AppleTalk should be run only
on a clientless network, such as a wide-area network (WAN)
link.
Content 3.1 EIGRP Concepts
3.1.4 EIGRP technologies EIGRP includes many new
technologies, each of which represents an improvement in
operating efficiency, speed of convergence, or functionality
relative to IGRP and other routing protocols. These
technologies fall into one of the following four categories:
- Neighbor discovery and recovery
- Reliable
Transport Protocol
- DUAL finite-state machine
algorithm
- Protocol-dependent modules
Simple distance vector routers do not establish any
relationship with their neighbors. RIP and IGRP routers merely
broadcast or multicast updates on configured interfaces. In
contrast, EIGRP routers actively establish relationships with
their neighbors, much the same way that OSPF routers do. EIGRP
routers establish adjacencies as described in Figure . EIGRP
routers establish adjacencies with neighbor routers by using
small hello packets. Hellos are sent by default every five
seconds. An EIGRP router assumes that as long as it is
receiving hello packets from known neighbors, those neighbors
and their routes remain viable or passive. By forming
adjacencies, EIGRP routers do the following: -
Dynamically learn of new routes that join their network
- Identify routers that become either unreachable or
inoperable
- Rediscover routers that had previously
been unreachable
Reliable Transport Protocol (RTP)
is a transport-layer protocol that can guarantee ordered
delivery of EIGRP packets to all neighbors. On an IP network,
hosts use TCP to sequence packets and ensure their timely