Routing/Routed Protocols, Part I - Hal-PC

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Oct 27, 2013 (3 years and 9 months ago)

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Routing/Routed Protocols

Part I

Routed Protocol Definition:


Routed Protocol


used to transmit user
data (packets) through an internetwork.
Routed protocols are assigned to an
interface and determine the method of
packet delivery.


Examples: IP, IPX, AppleTalk, DECNet,
Banyan Vines

Routing Protocol Definition:


Routing protocol


any protocol that
defines algorithms to be used for updating
routing tables between routers. Basically,
a routing protocol determines the path of a
packet through an internetwork.


Examples: RIP, RIPv2, IGRP, EIGRP,
OSPF, IS
-
IS, BGP

Remember:


A
Routed

Protocol


defines logical
addressing. Most notable example on the
test


IP


A
Routing

Protocol


fills the routing table
with routing information. Examples on the
test


RIP, IGRP, EIGRP, OSPF, IS
-
IS


CCNA Exam Tips
--

Routing


Since IP routing is basically what Cisco
routers do, this protocol is the backdrop for
the whole CCNA exam. See prior chapter
notes on “IP Addressing” for more info.


Next, some routing protocols that are
strongly rumored to be on the CCNA 801
will be outlined.


But first, some generalities about routing
protocols…

Main Goals of Routing Protocols


To fill the routing table with current best,
loop
-
free routes


To notice when routes in the table are no
longer valid and remove them from the
routing table


To add new routes or replace lost routes


The time for finding a working route is called
convergence
.

Two Categories of Routing
Protocols


Exterior Routing Protocols


used for use
between different organizations such as
ISPs or ISPs and their customers.


Ex: Border Gateway Protocol (BGP)


Interior Routing Protocols


used to
distribute routing information inside a
single organization.



Ex: RIP, IGRP, EIGRP, OSPF, IS
-
IS

Border Gateway Protocol (BGP)


The most popular
exterior

routing protocol
& the only one on the CCNA 801 exam


ISPs use BGP to exchange routing info
between themselves and other ISPs and
customers.


BGP advertises only
routing

info to
specifically defined peers using TCP.


BGP does not use a
metric

like internal
routing protocols

Terminology of Interior
Routing Protocols

This is not as painful as it sounds.
There are only 6 basic concepts.

TYPE of routing protocol


Each interior routing protocol can be
characterized based on the underlying
logic used by the routing protocol.


The underlying logic is referred to as the
TYPE

of routing protocol.


The three types are:

1)
Distance vector

2)
Link
-
state

3)
Hybrid

Full/partial Update


Full routing updates


entire routing tables
are sent regularly


Partial routing updates


only a subset of
the routing table is sent, typically just
information about changed routes.


Partial routing updates require less
overhead than full routing updates.

Convergence


Convergence refers to the time required
for routers to react to changes in the
network.

Metric


The metric refers to the numeric value that
describes how good a particular route is.


The lower the value, the better the route.


Support for VLSM


Variable
-
length subnet masking (VLSM)
means that, in a single Class A, B, or C
network, multiple subnet masks can be
used.


The advantage of VLSM is that it enables
you to vary the size of each subnet, based
on the needs of that subnet.


Some routing protocols support VLSM,
and some do not

Classless or Classful


Classless routing protocols transmit the
subnet mask along with each route in the
routing updates sent by that protocol.


Classful routing protocols do not transmit
mask information.


Only classless routing protocols support
VLSM. To say that a routing protocol is
classless is to say that it supports VLSM.

Now we apply those terms to
some interior routing protocols.

.

Distance Vector Protocols: RIP and
IGRP


Distance vector protocols advertise
routing information by sending
messages, called
routing updates
, out
the interfaces on a router.


These updates contain a series of
entries, with each entry representing a
subnet and a metric.


Failure to receive updates from a
neighbor in a timely manner results in
the removal of the routes previously
learned from that neighbor.

Distance Vector Protocols: RIP and
IGRP


Routers send periodic full updates and
expect to receive periodic updates from
neighboring routers.


When possible, routers use broadcasts
or multicasts to send routing updates.
This way, all neighbors on a LAN can
receive the same routing information in
a single update.


If a router learns multiple routes to the
same subnet, the router chooses the
best route based on the metric.

Routing Information Protocol (RIP)


Been around 15+ years for use with IP
networks.


Easier to use than some newer routing
protocols, but severely limited in
comparison.


Basic RIP Summary


Based on distance vector logic


Uses hop count for the metric


Hop count = number of routers between
two points


Sends periodic full routing updates every 30
seconds


Converges slowly, often taking 3 to 5
minutes


Does not support VLSM, also making it a
classful routing protocol

Interior Gateway Routing Protocol
(IGRP)


IGRP is a Cisco
-
proprietary IP routing
protocol created to provide a better
distance vector protocol.


The most obvious difference between
RIP
-
1and IGRP is the metric.


IGRP advertises up to five parameters that
describe the metric for each route,
although, by default only two are used


bandwidth and delay.

IGRP


Other three possible parameters used
to describe IGRP metric can include:
reliability, load, and MTU (maximum
transmission unit).


IGRP calculates the metric based on a
mathematical formula that “you do not
really need to know for the exam.”
(Wendell Odom, CCNA INTRO, p.415)




Distance Vector Protocols

Feature

RIP (v1)

IGRP

Update timer for
full routing
updates

30 seconds

90 seconds

Metric

Hop Count

Function of
bandwidth and
delay (default)

Supports VLSM

No

No

Infinite
-
metric
value

16

4,294,967,295

Convergence

Slow

Slow

Link
-
State Protocols: OSPF and
Integrated IS
-
IS


The goal of link
-
state protocols is to fill the
routing tables with the current best routes.


Link
-
state advertises a large amount of
topological info about the network


Discovers neighbor routers before
exchanging routing information.


A router running a link
-
state protocol uses
more memory and more processing cycles
than do distance vector protocols.

Link
-
State Protocols: OSPF and
Integrated IS
-
IS


To figure out the current best routes, a
router processes the link
-
state topology
database using an algorithm called the
Dijkstra Shortest Path First (SPF)
algorithm.


This info helps link
-
state protocols avoid
loops & converge quickly.


Quick convergence


often less than 10
seconds.

Open Shortest Path First (OSPF)


OSPF is the most popular link
-
state IP
routing protocol today.


Because OSPF does not send full updates
on a regular short interval (like RIP), the
overall number of bytes sent for routing
information is typically smaller.

Open Shortest Path First (OSPF)


OSPF uses a concept called
cost
for the
metric. Each link is considered to have a
cost; a route’s cost is the sum of the cost
for each link.


By default, Cisco derives the cost value for
a link from the bandwidth.


OSPF supports VLSM.

Integrated IS
-
IS


OSI defines a network layer protocol
called the Connectionless Network
Protocol (CLNP). It also defines a routing
protocol


a routing protocol used to
advertise CLNP routes, called
Intermediate System
-
to
-
Intermediate
System (IS
-
IS). IS
-
IS advertises CLNP
routes between “intermediate systems,”
which is what OSI calls routers.

Integrated IS
-
IS


Integrated IS
-
IS
has the capability to
advertise IP routes as well as CLNP
routes.


“…most installations could care less about
CLNP.” (Wendell Odom, CCNA INTRO,
p.419)


Supports VLSM

Balanced Hybrid Protocols:
Enhanced IGRP


EIGRP uses features similar to link
-
state
protocols, and others similar to distance
vector protocols, and yet others unlike
either of the two.


The internal workings of EIGRP depend
on an algorithm called the
Diffusing
Update Algorithm (DUAL).


Requires less processing than the Dijkstra
SPF algorithm.

EIGRP Summary


A balanced hybrid protocol


Converges in less than 3 seconds


Discovers neighbors (via Hello packets)
before sending them information.


Requires little design effort


Supports VLSM


Cisco proprietary


Metric based on bandwidth & delay, scaled
by multiples of 256.


That’s IT

Next week, May 3, to be discussed:


--

Routing Protocol Configuration
Commands


--

Routing Protocol Logic


--

VLSM