CCIE chapter 8 – EIGRP

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Oct 27, 2013 (3 years and 7 months ago)

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CCIE chapter 8


EIGRP


Resources used:

Cisco

Press

CCNP

Self
-
Study

BSCI
Official

Exam

Certification Guide 3
th

Edition

CCIE

Professional

Deve
lopment Routing TCP
-
IP Volume I

CCIE Routing and
Switching

Exam

Certification

Guide

3
rd

E
dition



Metrics
: lowest
path bandwidth(default), accumulative delay (default) , max load,
reliability,

mtu

Hello interval
, 5 seconds on lan 60 seconds on a wan link

Hold timer
,

3 times the hello interval

Update via
multicast 244.0.0.10

Authentication



MD5

Full routing table up
dates to new neighbours , partial for normal operations

Supports IPX and appletalk

Next
-
hop

the next
-
hop field can be a router other then the source of the route


Is a distance vector routing protocol
, it doesn’t build a tree of the network and relys
on ro
uting by rumour.


Uses a 32 bit metric

Supports
unequal cost load
balancing

Eigrp has 3 tables (
routing, topology, neighbour

)

Eigrp automatically summarizes at the classfull boundary


Eigrp terminology


Neighbour

, a router that is running EIGRP that is

directly connected ( layer 3
perspective)


Neighbour table
, a list of all neighbours , including ip address, the hold timer,
smooth round trip time and uptime

and interface that the neighbours hello’s are heard
on.


Topology table
, a list of all routes
learnt

from neighbours, DUAL acts on this table to
find successor and feasible successor routes and then add them to the routing table.


He
l
lo
, a message to find other neighbours, are sent unreliably
.


Update
, an eigrp packet that contains information abou
t a change in the network
, is
multicast but is responded to with unicast packet to provide reliability


Query
, sent when the path to the route is lost
( and no feasible successor)
, is sent to all
neighbour routers asking if they have a feasible successor.

EIGRP querys will
tra
verse
the AS boundary in a EIGRP to EIGRP redistribution
scenario
.

is multicast but is
responded to with unicast packet to provide reliability


Reply
, a response to a query , if the router doesn’t feasible successor it will ask all its

neighbours via a query
. is multicast but is responded to with unicast packet to provide
reliability


ACK
, a hello packet with no data in it, in response to packets that are sent reliably


Holdtime
, how long a router will wait without hearing a hello befor
e it declears the
router is dead and remove it from the neighbour table
.


Smooth round trip timer ( SRTT)

, the amount of time the router waits to hear an
acknowledgement it is kept in the neighbour table and is used to calculate the RTO.


Retransmission t
imeout (RTO)
, how long the router waits without an ACK before
retransmitting the packet.


Advertise distance (AD)
, the cost of the path from the local routers neightbour ( next
hop) so AD cost plus cost to the next hop would equal the total path cost

(fea
sible
distance)
.


Feasible distance (FD)

the lowest cost distance to a remote network



Feasible condition (FC)

when the AD of a second path is lower then the FD


Feasible successor (FS)

if a route matches the FC then it becomes a feasible
successor these

routes are used incase of a route failure, it can be used without a
routing table recalculation/network reconvergence.


Successor
, the lowest cost path to the destination network, gets installed in the
routing table


Stuck in active
,

State reached when

a router has sent out network packets
(query)

and
is

waiting for ACKs from all its neighbors. The route is active until

all the ACKs have
been received. If they do not appear after a

certain time
( 3 minutes default)
, the router
is SIA for the route.


Query

scoping
,
Network design to limit the scope of the query range, that is, how

far the query is allowed to propagate in search of a feasible

successor. This is necessary to prevent SIA, which can cause

multiple problems for the network.


Active
, a mode DUAL
enters when it is calculating a new path to a network

Passive
, when routes aren’t being calculated
, when a successor route is lost and there
is a feasible successor then the route stays passive!




Updates are sent using “reliable multicast” this works by

the receiving router acking
using unicast hello packets that are empty.


NBMA networks are easily supported
using

EIGRP







Neighbour adjacencies
,


A eigrp router will multicast hello packets out its interfaces
.


If a hello is received and its
AS number
, and its
metric settings

are the same then
that router is considered a neighbour.




Topology table
,


The topology table contains all known routes.

Weather to route is currently active or passive

Prefix/masks for remote networks

The metric of the FD

The
metric for the AD


The next hop address

The outgoing interface


Topology table changes,


A new network is detected

A cost is changed on a directly connected interface

When a neighbour becomes unavailable



Events for DUAL


Input Event

Description

IE1 Any
input event for which FC is satisfied or the destination is unreachable

IE2 Query received from the successor; FC not satisfied

IE3 Input event other than a query from the successor; FC not satisfied

IE4 Input event other than last reply or a query from th
e successor

IE5 Input event other than last reply, a query from the successor, or an increase in
distance to destination

IE6 Input event other than last reply

IE7 Input event other than last reply or increase in distance to destination

IE8 Increase in dist
ance to destination

IE9 Last reply received; FC not met with current FD

IE10 Query received from the successor

IE11 Last reply received; FC met with current FD

IE12 Last reply received; set FD to infinity




Removing a Path or Router from the Topology Tab
le

The process of removing a path or router from the topology table is far more complex
and gets to

the crux of EIGRP. The following process uses Figure 13
-
3 and focuses on
Router D:

1.
If a network connected to Router A is disconnected, Router A updates i
ts topology
and routing

table and sends an update to its neighbors.

2.
When Router D receives the update, it updates the neighbor table and the topology
table.

3.
As a router, D is programmed to find an alternative route to the remote network. It
examines
the

topology table for alternatives. Because there is only one path to the
remote network, no

alternatives are found.

4.
The router then sends out a query to its neighbors requesting that they look in their
tables for

paths to the remote network. The route

is marked active in the topology
table at this time.

5.
The query is tracked, and when all the replies are in, the neighbor and topology
tables are

updated.

6.
DUAL, which starts to compute as soon as a network change is registered, runs to
determine

the
best path, which is placed in the routing table.

7.
Because no alternative route is available, the neighbors reply to the query stating
that they have

no path.

8.
Before they respond, they query their own neighbors; in this way, the search for an
alternati
ve

path extends or diffuses throughout the organization.

9.
When no router can supply a path to the network, all the routers remove the
network from their

routing and topology tables.



ALSO READ PAGE 210 to 218 a lot from TCP/IP VOL 1!!!!



Route types,


Internal, routes learnt via EIGRP

External, Routes redistributed into EIGRP

Summary routes,
internal
routes that are summarized


Summary routes are added to the routing table sourced from the Null0 interface
not the interface that it was configured on, if
two routers in a line are
summarizing the same route then the furthest from the routes one will loose
connectivity to those summary routes because the administrative distance of the
local summary (5) will beat the summary route learnt via eigrp (90)

thus b
lack
holing those routes to null0.


Default metric equation

metric = [(10000000


smallest bandwidth kbps) + sum of delays] * 256


k values,


K1 bandwidth

K2 loading ( based on statistics of the out going interface)

K3 delay, accumulative delay of a route
, measued in 10 micro second blocks

K4 reliability, based on the keepalives on out going interfaces

K5

MTU , the smallest found mtu on the path


Default k values=
K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0




SIA avoidance


The best way to avoid SIA is to sum
marize, if a route within a summary is lost

the
query’s will stop at routers

that don’t know about the exact route( exact prefix)
. If
there is no summarization then every router will know about

the route and all routers
with
in the EIGRP AS ( and even in ot
her redistributed EIGRP AS’s
)

will be

queried
for an alternate route
.


EIGRP stub router

Eigrp stub router can only have one neighbour, t
hat neighbour will only send them
routes as definded by the exact stub configuration. There are the following choices
m
ore then one can be used at a time:

receive
-
only
(Optional) Sets the router as a receive
-
only neighbor

connected
(Optional) Advertises connected routes

static
(Optional) Advertises static routes

summary
(Optional) Advertises summary routes

redistributed ,
advertises redistributed routes


The default is connected + summary



UN
equal cost

load
balancing
,


The vaule used to config unequal cost is called the variance, it can be set from 1 to 128. this value is
multiplied be the FD of the route if other routes
fall within the FD x variance

and re feasible successors

then they can be used as links for load balancing. There is a maximum of 6 links that can be load
balanced for a route
.



Hello packet and what frequency on what types of links




High bandwidth links (every 5 seconds):



Broadcast media, such as Ethernet, Token Ring, and FDDI



Point
-
to
-
point serial links, such as PPP or HDLC leased circuits, Frame Relay
point
-
to
-

point subinterfaces, and ATM



Point
-
to
-
point subinterfaces



High

bandwidth (greater than T1) multipoint circuits, such as ISDN PRI and
Frame

Relay



Low bandwidth links (every 60 seconds):



Multipoint circuits T1 bandwidth or slower, such as Frame Relay multipoint

interfaces, ATM multipoint interfaces, and ATM



Switc
hed virtual circuits and ISDN BRIs



Bandwidth statement,


The ip bandwidth
-
percent
-
eigrp command interacts with the bandwidth command on
the interface. You would use this command primarily because in your network, the
bandwidth command does not reflect th
e true speed of the link. The bandwidth
command might have been altered to manipulate the routing metric and path selection
of a routing protocol, such as IGRP or OSPF. It might be bet
ter to use other methods
of controlling the routing metric and return the bandwidth to a true value. Otherwise,
the ip bandwidth
-
percent eigrp command is available. It is possible to set a bandwidth
percent that is larger than the stated bandwidth. This is

with the understanding that,
although the bandwidth might be stated to be 56 kbps, the link is in fact 256 kbps.



TLV,

TLV type defines what type of eigrp packet this packet is.

0x0001

EIGRP Parameters

0x0003

Sequence

0x0004

Software Version[12]

0x0005

Next Multicast Sequence


IP
-
Specific TLV Types

0x0102

IP Internal Routes

0x0103

IP External Routes


AppleTalk
-
Specific TLV Types

0x0202

AppleTalk Internal Routes

0X0203

AppleTalk External Routes

0x0204 AppleTalk Cable Configuration


IPX
-
Specific TLV Types

0x0302

IPX Internal Routes

0x0303

IPX External Routes






TLV example