Information Technology Reference
In-Depth Information
Default
Hello
interval depends on the bandwidth:
■
≤
1.544 Mbps =
60
sec.
Hello
interval (
180
hold time)
■
> 1.544 Mbps =
5
sec.
Hello
interval (
15
hold time)
■
Update
packets:
Sent (
reliably
/unreliably), there are 2 types:
(
Unicast
/multicast) to new neighbor discovered; contains routing table
■
(Unicast/
multicast
) to all neighbors when topology changes
■
Query
packets
: Queries are (unicast/
multicast
) (
reliably
/unreliably) during route recomputation, ask-
ing neighbors for a new successor to a lost route.
Reply
packets:
Neighbors (
unicast
/multicast) a reply to a query whether or not they have a route.
Acknowledgment
packets
: “Dataless” (
unicast
/multicast) packets that acknowledge the receipt of
packets that were sent reliably. This type is actually a Hello packet with a nonzero value in the
acknowledgment field.
Hello Protocol
An EIGRP router assumes that as long as it is receiving Hello packets from a neighbor, the neighbor
and its routes remain viable.
Hold time
tells the router the maximum time the router should wait to
receive the next Hello before declaring that neighbor as
unreachable
. By default, this waiting period is
three
times the
Hello
interval, or
15
seconds on most networks and
180
seconds on networks with
speeds of T1 or slower. If the time expires, EIGRP will declare the route as down and DUAL will
search for a new path by sending out queries.
EIGRP Bounded Updates
Unlike RIP, EIGRP does not send periodic updates. Instead, EIGRP sends its updates only when the
metric for a route changes.
The term
partial
means that the update only includes information about the route changes.
The term
bounded
refers to the propagation of partial updates sent only to those routers that are affect-
ed by the change.
DUAL: An Introduction
Diffusing Update Algorithm
(DUAL) is the convergence algorithm used by EIGRP to calculate best
routes and prevent routing loops.
The decision process for all route computations is done by the DUAL
Finite State Machine
(FSM).
The DUAL FSM tracks all routes; uses its metric to select efficient, loop-free paths; and selects the
routes with the least-cost path to insert into the routing table.
Administrative Distance
EIGRP has a default administrative distance of
90
for internal routes and
170
for routes imported from
an external source, such as default routes. EIGRP has a third AD value of
5
for summary routes.
Basic EIGRP Configuration
The following sections review the basics of EIGRP configuration. There are many similarities with the
commands used in configuring other routing protocols such as RIP. In these sections of the Study
Guide, you will design an addressing scheme and answer questions related to its implementation using
EIGRP as the routing protocol.
