Chapter 6 - Elktech.org

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Discovery2v4.0
Routing
Working at a Small
-
to
-
Medium Business or ISP

Chapter 6
2
Discovery2v4.0
Routed Vs. Routing

Routed protocols
are protocols that
are routed over an
internetwork (IP,
AppleTalk, IPX).
They are used
between router to
direct user traffic.

Routing Protocols
allows a router to
share information
about known
networks with other
routers
(RIP,
IGRP). They are
used to maintain
tables.
Routed protocols are used to
carry user data while routing
protocols maintain tables.
3
Discovery2v4.0
Multiprotocol Routing

Routers have the ability to pass traffic from different
routed protocols.
4
Discovery2v4.0
How Routers use Routes

A router uses the destination IP address to make
routing decisions

A routing metric is used to determine how reliable a
route is

If dynamic routing is used, routers have to learn routes
from other routers.

If static routing is used, the administrator configures
this information into the router manually

Updates to a routing table are
made by the administrator.

A network with only one path
out of a router is called
a stub network.
5
Discovery2v4.0
Learning IP Routing Destinations

Static routes
-
manually entered by administrator
(
ip
route
destination network next hop interface)

Dynamic routes
-
router learns of paths to
destinations by receiving periodic updates from
other routers
-
adjust in real time to changing
network circumstances

Default routing
-
can be PART of the static
configuration; a routing table entry that directs
packets to the next hop
6
Discovery2v4.0
Learning IP Routing Destinations
7
Discovery2v4.0
Configuring static routes w/ outgoing interface
Administrative distance of 0
-
default
outgoing
interface
8
Discovery2v4.0
Configuring static routes
w
/ next
-
hop IP address
Administrative distance of 1
-
default
Next
hop
interfac
e
9
Discovery2v4.0
Default Route on non
-
directly
connected networks
10
Discovery2v4.0
Routing Metrics

Routing algorithms use different metrics to determine
the best route.

These metrics answer the questions:
How are routing tables built?
What is the nature of the information the table
contains?
How do routing algorithms determine that one route
is preferable to others?
11
Discovery2v4.0
Types of Routing
Protocols

Distance
V
ector Protocols

Pass periodic copies of its entire routing table to
neighbor routers and accumulate distance vectors.

Routers discover the best path to destinations from
each neighbor.

Have limited information about the entire network

Typically converge more slowly
12
Discovery2v4.0
Types of Routing
Protocols

Link State Protocols

Link
-
state maintains a complex database of topology information
and uses link
-
state advertisements (LSAs). If routers have
different sets of LSAs they become unreachable because routers
disagree about a link.

Link
-
state enables routers to have direct knowledge of all links in
the network and how they are connected.

Flood routing information about
it’s OWN links.

Analyze incoming routing
update messages.

If the message indicates that a network
change has occurred, the routing software
recalculates routes and sends out new routing update messages.

These messages permeate the network, stimulating routers to
rerun their algorithms and change their routing tables accordingly.

After the initial flood, link
-
state requires less bandwidth because
routing updates are triggered by topology changes.
13
Discovery2v4.0
Routing Protocols

Routing Information Protocol (RIP)
was developed by
Xerox Corporation in the early 1980s for use in Xerox
Network Systems (XNS) networks.

Distance Vector Protocol

Uses hop count as its only metric

If the hop count is greater than 15, the packet is
discarded
.

Can use a maximum of 16 routers

Broadcasts routing updates every 30 seconds

Works well in small environments but has serious
limitations when used in larger internetworks.

Is slow to converge, meaning that it takes a relatively
long time for network changes to become known to all
routers.
14
Discovery2v4.0
Routing Protocols

Routing Information Protocol (RIP)

Version 1

Is a
classful
only

Version 2

Updates include a subnet mask

Can be classless

Can use the no auto
-
summary command
15
Discovery2v4.0
Routing Protocols

With
the creation of the
Interior Gateway Routing
Protocol (IGRP)
in the early 1980s, Cisco Systems was
the first company to solve the problems associated with
using RIP to route
datagrams
between interior routers.

Proprietary to Cisco

Distance Vector Protocol

Uses
bandwidth, load, reliability, & delay
as its metrics

Broadcasts routing updates every 90 seconds

Converges faster than RIP, thereby avoiding the
routing loops caused by disagreement over the next
routing hop to be taken.

Enables many large, complex, topologically diverse
internetworks to be deployed
.

Is not supported on new routers
16
Discovery2v4.0
Routing Protocols

EIGRP

Cisco’s enhanced distance vector protocol

Uses some distance vector and some link
-
state
principles

Advantage to hybrid routing is it uses less bandwidth,
less memory, and just generally less resources than
distance
-
vector or link
-
state routing

It uses unequal cost load balancing.

It uses Diffused Update Algorithm (DUAL) to calculate
the shortest path

Routing updates are multicast using 224.0.0.10
triggered by topology changes

Capable of handling AppleTalk and Novell IPX routing
information, as well as IP routing information
17
Discovery2v4.0
Routing Protocols

Open Shortest Path First (OSPF)
was developed by the
Internet Engineering Task Force (IETF) as a replacement for
RIP. OSPF is based on work started by John
McQuillan
in the
late 1970s and continued by
Radia
Perlman and Digital
Equipment Corporation (DEC) in the mid
-
1980s.

Intradomain
, Link
-
state routing protocol

It is an open standard routing protocol described in RFC 2328.

The SPF algorithm is used to calculate the lowest cost to a
destination.

Routing updates are flooded as topology changes occur.

All routers in a OSPF domain would then adjust their routes

Initially advertisements are flooded

Supports hierarchical routing within an autonomous system.
Autonomous systems can be divided into routing areas. A routing
area is typically a collection of one or more subnets that are
closely related. All areas must connect to the backbone area.

OSPF provides fast rerouting and supports variable length subnet
masks.
18
Discovery2v4.0
Routing Protocols

Integrated IS
-
IS

Intradomain
, link state, hierarchical routing protocol

Can operate over a variety of
subnetworks
, including
broadcast LANs, WANs, and point
-
to
-
point links

Integrated IS
-
IS is an implementation of IS
-
IS for more
than just OSI protocols. Today, Integrated IS
-
IS
supports both OSI and IP protocols

Like all integrated routing protocols, Integrated IS
-
IS
calls for all routers to run a single routing algorithm.

Link state advertisements sent by routers running
Integrated IS
-
IS include all destinations running either
IP or OSI network
-
layer protocols.

Protocols such as ARP and ICMP for IP and End
System
-
to
-
Intermediate System (ES
-
IS) for OSI must
still be supported by routers running Integrated IS
-
IS
19
Discovery2v4.0
Convergence

A network is consider converged when all routers have
the most current information about the network.

Fast convergence is desirable because routers may
make incorrect forwarding decisions without the latest
correct information.
20
Discovery2v4.0
Autonomous System

An Autonomous
System
is a group of networks
(routers) that use the same internal routing policies and
is controlled by a single administrative authority.

Each autonomous system is assigned its own
autonomous system number (ASN).

ASNs are controlled and registered for Internet use

Some companies may have
more than one ASN when
they use two or more ISPs.
21
Discovery2v4.0
Interior
vs
Exterior
Routing
Protocols

Interior Routing Protocols are used to communicate
within a single autonomous system.

RIP

IGRP

EIGRP

OSPF

IS
-
IS

Exterior Routing Protocols are used between multiple
autonomous systems.

Border Gateway Protocol (BGP)
22
Discovery2v4.0
Routing Protocols

Border Gateway Protocol (BGP)
represents an attempt
to address the most serious of EGP's problems.

A distance vector exterior routing protocol

Used between ISPs or ISPs and clients.

Used to route Internet traffic between autonomous
systems.

created for use in the Internet core routers.

was designed to prevent routing loops in arbitrary
topologies and to allow policy
-
based route selection.

BGP was co
-
authored by a Cisco founder, and Cisco
continues to be very involved in BGP development.

The latest revision of BGP, BGP4, was designed to
handle the scaling problems of the growing Internet
.
23
Discovery2v4.0
Exterior Routing Protocols Across the Internet
24
Discovery2v4.0
Exterior Routing Protocols Across the Internet
25
Discovery2v4.0
IP Routing Configuration Tasks

Tells the router which attached networks participate
in routing
updates
26
Discovery2v4.0
Administrative Distance
Route Source
Default Distance
Values
Connected interface
0
Static route
1
Enhanced Interior Gateway Routing Protocol (EIGRP)
summary route
5
External Border Gateway Protocol (BGP)
20
Internal EIGRP
90
IGRP
100
OSPF
110
Intermediate System
-
to
-
Intermediate System (IS
-
IS)
115
Routing Information Protocol (RIP)
120
Exterior Gateway Protocol (EGP)
140
On Demand Routing (ODR)
160
External EIGRP
170
Internal BGP
200
Unknown
255
27
Discovery2v4.0
Examine a show
ip
route command
Hop count
Administrative
distance
28
Discovery2v4.0
This process illustrates the
count to infinity problem
-
there
are three solutions to this problem:

Define a Maximum

Split Horizon

Hold
-
down Timers
Routing Loop Problem
29
Discovery2v4.0
# 1 Solution:
Define
A Maximum

For hop
-
count distance vectors, a maximum of 15 hops is commonly
used

When the metric value exceeds the maximum, Network 1 is
considered unreachable.
30
Discovery2v4.0
#2
Solution: Split
Horizon
Default for
Cisco Routers
Split Horizon will prevent information from exiting
on the router through the same interface.
31
Discovery2v4.0
#3
Solution: Hold
Down Timers
Hold
-
down timers help prevent a router from immediately
using an alternate route that includes the failed route.
32
Discovery2v4.0
Troubleshooting

show
ip
route

displays the routing table information.
Shows route being learned.

debug
ip
rip

Displays RIP activity on a router. Shows
RIP routing updates as they are sent and received.

show
ip
protocols

Displays routing protocol settings
and configuration

show
cdp
neighbors detail

Displays information
about directly connected Cisco devices
33
Discovery2v4.0