blocks. Each block is one route. There is also an
asterisk (*) next to one of the block entries. This corresponds
to the active route that is used for new traffic.
Content
7.2 RIP 7.2.9 Load balancing across
multiple paths Load-balancing describes the ability of a
router to transmit packets to a destination IP address over
more than one path. Load balancing is a concept that allows a
router to take advantage of multiple best paths to a given
destination. The paths are derived either statically or with
dynamic protocols, such as RIP, EIGRP, OSPF, and IGRP.When a
router learns multiple routes to a specific network, the route
with the lowest administrative distance is installed in the
routing table. Sometimes the router must select a route from
among many, learned via the same routing process with the same
administrative distance. In this case, the router chooses the
path with the lowest cost or metric to the destination. Each
routing process calculates its cost differently and the costs
may need to be manually configured in order to achieve load
balancing. If the router receives and installs multiple paths
with the same administrative distance and cost to a
destination, load-balancing can occur. There can be up to six
equal cost routes (a limit imposed by Cisco IOS on the routing
table), but some Interior Gateway Protocols (IGPs) have their
own limitation. EIGRP allows up to four equal cost routes. By
default, most IP routing protocols install a maximum of four
parallel routes in a routing table. Static routes always
install six routes. The exception is BGP, which by default
allows only one path to a destination. The range of maximum
paths is one to six paths. To change the maximum number of
parallel paths allowed, use the following command in router
configuration mode: Router(config-router)#maximum-paths
[number] IGRP can load balance up to six unequal links.
RIP networks must have the same hop count to load balance,
whereas IGRP uses bandwidth to determine how to load balance.
Three ways to get to Network X: - E to B to A with a
metric of 30
- E to C to A with a metric of 20
- E to D to A with a metric of 45
Router E chooses
the second path above, E-C-A with a metric of 20 as it is a
lower cost than 30 and 45. When routing IP, the Cisco IOS
offers two methods of load balancing, per-packet and
per-destination load balancing. If process switching is
enabled, the router will alternate paths on a per-packet basis.
If fast switching is enabled, only one of the alternate routes
will be cached for the destination address, so all packets in
the packet stream bound for a specific host will take the same
path. Packets bound for a different host on the same network
may use an alternate route, traffic is load balanced on a
per-destination basis. Lab Activity Lab Exercise: Load
Balancing Across Multiple Paths This lab is to configure Load
balance across multiple paths. Lab Activity e-Lab
Activity: Load Balancing Across Multiple Paths In this lab, the
students will load balance across multiple paths and observe
load balancing. Interactive Media Activity Drag and
Drop: Administrative Distances After completing this activity,
the student will be able to administrative distances. Web
Links How Does Load Balancing Work?
http://www.cisco.com/en/US/tech/
tk826/tk365/
technologies_tech_note09186a0080094820.shtml
Content 7.2 RIP 7.2.10
Integrating static routes with RIP Static routes are
user-defined routes that force packets moving between a source
and a destination to take a specific path. Static routes become
very important if the Cisco IOS software does not learn a route
to a particular destination. They are also useful for
specifying a “gateway of last resort”, commonly referred to as
a default route. If a packet is destined for a subnet that is
not explicitly listed in the routing table, the packet is
forwarded to the default route. A router running RIP can
receive a default route via an update from another router
running RIP. Another option is for the router to generate the
default route itself. The static routes can be removed using
the no ip route global configuration command. The
administrator can override a static route with dynamic routing
information by adjusting the administrative distance values.
Each dynamic routing protocol has a default administrative
distance (AD). A static route can be defined as less desirable
than a dynamically learned route, as long as the AD of the
static route is higher than that of the dynamic route. Static
routes that point out an interface will be advertised via the
RIP router that owns the static route, and propagated
throughout the internetwork. This is because static routes that
point to an interface are considered in the routing table to be
connected and thus lose their static nature in the update. If a
static route is assigned to an interface that is not defined in
the RIP process, via a network command, RIP will not
advertise the route unless a redistribute static command
is specified in the RIP process. When an interface goes down,
all static routes pointing out that interface are removed from
the IP routing table. Likewise, when the software can no longer
find a valid next hop for the address specified in the static
route, then the static route is removed from the IP routing
table. In Figure a static route has been configured on the GAD
router to take the place of the RIP route in the event that the
RIP routing process fails. This is referred to as a floating
static route. The floating static route was configured by
defining an AD on the static route (130) greater than the
default AD of RIP (120). The BHM router would also need to be
configured with a default route. To configure a static route,
use the Figure command in global configuration mode. Lab
Activity e-Lab Activity: Integrating Static Routes with
RIP In this lab, the students will enable RIP to propagate
static routes.
Content 7.3 IGRP
7.3.1 IGRP features IGRP is a distance vector
Interior Gateway Protocol (IGP). Distance vector routing
protocols mathematically compare routes by measuring distances.
This measurement is known as the distance vector. Routers using
distance vector protocols must send all or a portion of their
routing table in a routing update message at regular intervals
to each of their neighboring routers. As routing information
spreads throughout the network, routers perform the following
functions: - Identify new destinations
- Learn of
failures
IGRP is a distance vector routing protocol
developed by Cisco. IGRP sends routing updates at 90 second
intervals, advertising networks for a particular autonomous
system. Key design characteristics of IGRP are a follows:
- The versatility to automatically handle indefinite, complex
topologies
- The flexibility needed to segment with
different bandwidth and delay characteristics
- Scalability for functioning in very large networks
By default, the IGRP routing protocol uses bandwidth and
delay as metrics. Additionally, IGRP can be configured to use a
combination of variables to determine a composite metric. Those
variables include: - Bandwidth
- Delay
- Load
- Reliability
Interactive Media
Activity Checkbox: RIP versus IGRP After completing this
activity, the student will be able to understand RIP and IGRP.
Web Links Specifying a Next Hop IP Address for Static
Routes http://www.cisco.com/en/US/tech/tk826/
tk365/technologies_tech_ note09186a00800ef7b2.shtml
Content 7.3 IGRP 7.3.2 IGRP
metrics The show ip protocols command displays
parameters, filters, and network information concerning the
routing protocols in use on the router. The algorithm used to
calculate the routing metric for IGRP is shown in the graphic.
It defines the value of the K1-K5 metrics and provides
information concerning the maximum hop count. The metric K1