Module Overview
23.1 VLSM
23.1.1 What is
VLSM and why is it used?
23.1.2 A waste of
space
23.1.3 When to
use VLSM
23.1.4
Calculating subnets with VLSM
23.1.5 Route
aggregation with VLSM
23.1.6
Configuring VLSM
23.2 RIP Version
2
23.2.1 RIP
history
23.2.2 RIP v2
features
23.2.3 Comparing
RIP v1 and v2
23.2.4
Configuring RIP v2
23.2.5 Verifying
RIP v2
23.2.6
Troubleshooting RIP v2
23.2.7 Default
routes
Summary
Overview
Network
administrators must anticipate and manage the physical growth of networks. This
may require them to buy or lease another floor of a building for new network
equipment such as racks, patch panels, switches, and routers. Network designers
must choose address schemes that allow for growth. Variable-length subnet mask
(VLSM) is used to create efficient and scalable address schemes.
Almost every
enterprise must implement an IP address scheme. Many organizations select
TCP/IP as the only routed protocol to run on their networks. Unfortunately, the
architects of TCP/IP did not predict that the protocol would eventually sustain
a global network of information, commerce, and entertainment.
IPv4 offered an
address strategy that was scalable for a time before it resulted in an
inefficient allocation of addresses. IPv4 may soon be replaced with IP version
6 (IPv6) as the dominant protocol of the Internet. IPv6 has virtually unlimited
address space and implementation has begun in some networks. Over the past two
decades, engineers have successfully modified IPv4 so that it can survive the
exponential growth of the Internet. VLSM is one of the modifications that has
helped to bridge the gap between IPv4 and IPv6.
Networks must be
scalable since the needs of users evolve. When a network is scalable it is able
to grow in a logical, efficient, and cost-effective way. The routing protocol
used in a network helps determine the scalability of the network. It is
important to choose the routing protocol wisely. Routing Information Protocol
version 1 (RIP v1) is suitable for small networks. However, it is not scalable
to large networks. RIP version 2 (RIP v2) was developed to overcome these
limitations.
This module
covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.
Students who
complete this module should be able to perform the following tasks:
- Define VLSM and briefly
describe the reasons for its use
- Divide a major network into
subnets of different sizes using VLSM
- Define route aggregation and summarization
as they relate to VLSM
- Configure a router using VLSM
- Identify the key features of
RIP v1 and RIP v2
- Identify the important
differences between RIP v1 and RIP v2
- Configure RIP v2
- Verify and troubleshoot RIP v2
operation
- Configure default routes using
the ip route and ip default-network commands 1.1
VLSM
23.1 VLSM
23.1.1 What is VLSM and why is it used?
As IP subnets
have grown, administrators have looked for ways to use their address space more
efficiently. This page introduces a technique called VLSM. With VLSM, a network
administrator can use a long mask on networks with few hosts, and a short mask
on subnets with many hosts. -
In order to
implement VLSM, a network administrator must use a routing protocol that
supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF),
Integrated IS-IS, Enhanced Interior Gateway Routing Protocol (EIGRP), RIP v2,
and static routing.
VLSM allows an
organization to use more than one subnet mask within the same network address
space. VLSM implementation maximizes address efficiency, and is often referred
to as subnetting a subnet.
Classful routing
protocols require that a single network use the same subnet mask. As an
example, a network with an address of 192.168.187.0 can use just one subnet
mask, such as 255.255.255.0.
A routing
protocol that allows VLSM gives the network administrator freedom to use
different subnet masks for networks within a single autonomous system. Figure
shows an example of how a network administrator can use a 30-bit mask
for network connections, a 24-bit mask for user networks, and even a 22-bit
mask for networks with up to 1000 users.
23.1
VLSM
23.1.2
A waste of space
This page will
explain how certain address schemes can waste address space.
In the past, the
first and last subnet were not supposed to be used. The use of the first
subnet, which was known as subnet zero, was discouraged because of the
confusion that could occur if a network and a subnet had the same address. This
also applied to the use of the last subnet, which was known as the all-ones
subnet. With the evolution of network technologies and IP address depletion,
the use of the first and last subnets have become an acceptable practice in
conjunction with VLSM.
In Figure , the
network management team has borrowed three bits from the host portion of the
Class C address that has been selected for this address scheme.
If the team
decides to use subnet zero, there will be eight useable subnets. Each subnet
can support 30 hosts. If the team decides to use the no ip subnet-zero command,
there will be seven usable subnets with 30 hosts in each subnet. Cisco routers
with Cisco IOS version 12.0 or later, use subnet zero by default.
In Figure , the
Sydney, Brisbane, Perth, and Melbourne remote offices may each have 30 hosts.
The team realizes that it has to address the three point-to-point WAN links
between Sydney, Brisbane, Perth, and Melbourne. If the team uses the last three
subnets for the WAN links, all of the available addresses will be used and
there will be no room for growth. The team will also have wasted the 28 host
addresses from each subnet to simply address three point-to-point networks.
This address scheme would waste one-third of the potential address space.
Such an address
scheme is fine for a small LAN. However, it is extremely wasteful if
point-to-point connections are used.
23.1
VLSM
23.1.3
When to use VLSM
It is important
to design an address scheme that allows for growth and does not waste
addresses. This page examines how VLSM can be used to prevent the waste of
addresses on point-to-point links.
As shown in
Figure , the network management team has decided to avoid the wasteful use of
the /27 mask on the point-to-point links. The team applies VLSM to the address
problem.
To apply VLSM to
the address problem, the team breaks the Class C address into subnets of
variable sizes. Large subnets are created for LANs. Very small subnets are
created for WAN links and other special cases. A 30-bit mask is used to create
subnets with only two valid host addresses. This is the best solution for the
point-to-point connections. The team will take one of the three subnets they
previously decided to assign to the WAN links, and subnet it again with a
30-bit mask.
In the example,
the team has taken one of the last three subnets, subnet 6, and subnetted it
again. This time the team uses a 30-bit mask. Figures and
illustrate that after using VLSM, the team has eight ranges of addresses
to be used for the point-to-point links.
23.1
VLSM
23.1.4
Calculating subnets with VLSM
VLSM helps to
manage IP addresses. This page will explain how to use VLSM to set subnet masks
that fit the link or segment requirements. A subnet mask should satisfy the
requirements of a LAN with one subnet mask and the requirements of a
point-to-point WAN with another.
The example in
Figure shows a network that requires an
address scheme.
The example
contains a Class B address of 172.16.0.0 and two LANs that require at least 250
hosts each. If the routers use a classful routing protocol, the WAN link must
be a subnet of the same Class B network. Classful routing protocols such as RIP
v1, IGRP, and EGP do not support VLSM. Without VLSM, the WAN link would need the
same subnet mask as the LAN segments. A 24-bit mask of 255.255.255.0 can
support 250 hosts.
The WAN link only
needs two addresses, one for each router. That means that 252 addresses would
be wasted.
If VLSM was used,
a 24-bit mask would still be applied on the LAN segments for the 250 hosts. A
30-bit mask could be used for the WAN link because only two host addresses are
needed.
Figure shows where the subnet addresses can be
applied based on the number of host requirements. The WAN links use subnet
addresses with a prefix of /30. This prefix allows for only two host addresses
which is just enough for a point-to-point connection between a pair of routers.
In Figure , the
subnet addresses used are generated when the 172.16.32.0/20 subnet is divided
into /26 subnets.
To calculate the
subnet addresses used on the WAN links, further subnet one of the unused /26
subnets. In this example, 172.16.33.0/26 is further subnetted with a prefix of
/30. This provides four more subnet bits and therefore 16 (24) subnets for the
WANs. Figure illustrates how to work
through a VLSM system.
VLSM can be used
to subnet an already subnetted address. For example, consider the subnet
address 172.16.32.0/20 and a network that needs ten host addresses. With this
subnet address, there are 212 – 2, or 4094 host addresses, most of which will
be wasted. With VLSM it is possible to subnet 172.16.32.0/20 to create more
network addresses with fewer hosts per network. When 172.16.32.0/20 is
subnetted to 172.16.32.0/26, there is a gain of 26, or 64 subnets. Each subnet
can support 26 – 2, or 62 hosts.
Use the following
steps to apply VLSM to 172.16.32.0/20:
- Write 172.16.32.0 in binary
form.
- Draw a vertical line between
the 20th and 21st bits, as shown in Figure . The original subnet boundary
was /20.
- Draw a vertical line between
the 26th and 27th bits, as shown in Figure . The original /20 subnet
boundary is extended six bits to the right, which becomes /26.
- Calculate the 64 subnet
addresses with the bits between the two vertical lines, from lowest to
highest in value. The figure shows the first five subnets available.
It is important
to remember that only unused subnets can be further subnetted. If any address
from a subnet is used, that subnet cannot be further subnetted. In Figure ,
four subnet numbers are used on the LANs. The unused 172.16.33.0/26 subnet is
further subnetted for use on the WAN links.
23.1
VLSM
23.1.5
Route aggregation with VLSM
This page will
explain the benefits of route aggregation with VLSM.
When VLSM is
used, it is important to keep the subnetwork numbers grouped together in the
network to allow for aggregation. For example, networks like 172.16.14.0 and
172.16.15.0 should be near one another so that the routers only carry a route
for 172.16.14.0/23.
The use of
classless interdomain routing (CIDR) and VLSM prevents address waste and
promotes route aggregation, or summarization. Without route summarization,
Internet backbone routing would likely have collapsed sometime before 1997.
Figure illustrates how route summarization reduces
the burden on upstream routers. This complex hierarchy of variable-sized
networks and subnetworks is summarized at various points with a prefix address,
until the entire network is advertised as a single aggregate route of
200.199.48.0/20. Route summarization, or supernetting, is only possible if the
routers of a network use a classless routing protocol, such as OSPF or EIGRP.
Classless routing protocols carry a prefix that consists of a 32-bit IP address
and bit mask in the routing updates. In Figure , the summary route that
eventually reaches the provider contains a 20-bit prefix common to all of the
addresses in the organization. That address is 200.199.48.0/22 or
11001000.11000111.0011. For summarization to work, addresses should be
carefully assigned in a hierarchical fashion so that summarized addresses will
share the same high-order bits.
The following are
important rules to remember:
- A router must know in detail
the subnet numbers attached to it.
- A router does not need to
inform other routers about each subnet if the router can send one
aggregate route for a set of routes.
- A router that uses aggregate
routes has fewer entries in its routing table.
VLSM increases
route summarization flexibility because it uses the higher-order bits shared on
the left, even if the networks are not contiguous.
Figure shows that the addresses share the first 20
bits. These bits are colored red. The 21st bit is not the same for all the
routes. Therefore the prefix for the summary route will be 20 bits long. This
is used to calculate the network number of the summary route.
Figure shows that the addresses share the first 21
bits. These bits are colored red. The 22nd bit is not the same for all the
routes. Therefore the prefix for the summary route will be 21 bits long. This
is used to calculate the network number of the summary route.
23.1
VLSM
23.1.6
Configuring VLSM
This page will
teach students how to calculate and configure VLSM. If VLSM is the scheme
chosen, it must then be calculated and configured correctly.
The following are
VLSM calculations for the LAN connections in Figure :
- Network address: 192.168.10.0
- The Perth router has to support
60 hosts. That means a minimum of six bits are needed in the host portion
of the address. Six bits will yield 26 – 2, or 62 possible host addresses.
The LAN connection for the Perth router is assigned the 192.168.10.0/26
subnet.
- The Sydney and Singapore
routers have to support 12 hosts each. That means a minimum of four bits
are needed in the host portion of the address. Four bits will yield 24 –
2, or 14 possible host addresses. The LAN connection for the Sydney router
is assigned the 192.168.10.96/28 subnet and the LAN connection for the
Singapore router is assigned the 192.168.10.112/28 subnet.
- The KL router has to support 28
hosts. That means a minimum of five bits are needed in the host portion of
the address. Five bits will yield 25 – 2, or 30 possible host addresses.
The LAN connection for the KL router is assigned the 192.168.10.64/27
subnet.
The following are
VLSM calculations for the point-to-point connections in Figure :
- Perth to KL
The
connection from Perth to KL requires only two host addresses. That means a
minimum of two bits are needed in the host portion of the address. Two bits
will yield 22 – 2, or 2 possible host addresses. The Perth to KL connection is
assigned the 192.168.10.128/30 subnet.
- Sydney to KL
The
connection from Sydney to KL requires only two host addresses. That means a
minimum of two bits are needed in the host portion of the address. Two bits
will yield 22 – 2, or 2 possible host addresses. The Sydney to KL connection is
assigned the 192.168.10.132/30 subnet.
- Singapore to KL
The
connection from Singapore to KL requires only two host addresses. That means a
minimum of two bits are needed in the host portion of the address. Two bits
will yield 22 – 2, or 2 possible host addresses. The Singapore to KL connection
is assigned the 192.168.10.136/30 subnet.
The
following configuration is for the Singapore to KL point-to-point connection:
Singapore(config)#interface
serial 0
Singapore(config-if)#ip
address 192.168.10.137 255.255.255.252
KualaLumpur(config)#interface
serial 1
KualaLumpur(config-if)#ip
address 192.168.10.138 255.255.255.252
23.2
RIP Version 2
23.2.1
RIP history
This page will
explain the functions and limitations of RIP. The Internet is a collection of
autonomous systems (AS). Each AS is generally administered by a single entity.
Each AS has a routing technology which can differ from other autonomous
systems. The routing protocol used within an AS is referred to as an Interior
Gateway Protocol (IGP). A separate protocol used to transfer routing
information between autonomous systems is referred to as an Exterior Gateway
Protocol (EGP). RIP is designed to work as an IGP in a moderate-sized AS. It is
not intended for use in more complex environments.
RIP v1 is
considered a classful IGP. RIP v1 is a
distance vector protocol that broadcasts the entire routing table to each
neighbor router at predetermined intervals. The default interval is 30 seconds.
RIP uses hop count as a metric, with 15 as the maximum number of hops.
If the router
receives information about a network, and the receiving interface belongs to
the same network but is on a different subnet, the router applies the one
subnet mask that is configured on the receiving interface:
- For Class A addresses, the
default classful mask is 255.0.0.0.
- For Class B addresses, the
default classful mask is 255.255.0.0.
- For Class C addresses, the
default classful mask is 255.255.255.0.
RIP v1 is a
popular routing protocol because virtually all IP routers support it. The
popularity of RIP v1 is based on the simplicity and the universal compatibility
it demonstrates. RIP v1 is capable of load balancing over as many as six
equal-cost paths, with four paths as the default.
RIP v1 has the
following limitations:
- It does not send subnet mask
information in its updates.
- It sends updates as broadcasts
on 255.255.255.255.
- It does not support
authentication.
- It is not able to support VLSM
or classless interdomain routing (CIDR).
23.2
RIP Version 2
23.2.2
RIP v2 features
This page will
discuss RIP v2, which is an improved version of RIP v1. Both versions of RIP
share the following features:
- It is a distance vector
protocol that uses a hop count metric.
- It uses holddown timers to
prevent routing loops – default is 180 seconds.
- It uses split horizon to
prevent routing loops.
- It uses 16 hops as a metric for
infinite distance.
RIP v2 provides
prefix routing, which allows it to send out subnet mask information with the
route update. Therefore, RIP v2 supports the use of classless routing in which
different subnets within the same network can use different subnet masks, as in
VLSM.
RIP v2 provides
for authentication in its updates. A set of keys can be used on an interface as
an authentication check. RIP v2 allows for a choice of the type of
authentication to be used in RIP v2 packets. The choice can be either clear
text or Message-Digest 5 (MD5) encryption. Clear text is the default. MD5 can
be used to authenticate the source of a routing update. MD5 is typically used
to encrypt enable secret passwords and it has no known reversal.
RIP v2 multicasts
routing updates using the Class D address 224.0.0.9, which provides for better
efficiency.
23.2
RIP Version 2
23.2.3
Comparing RIP v1 and v2
This page will
provide some more information about how RIP works. It will also describe the
differences between RIP v1 and RIP v2. RIP uses distance vector algorithms to
determine the direction and distance to any link in the internetwork. If there
are multiple paths to a destination, RIP selects the path with the least number
of hops. However, because hop count is the only routing metric used by RIP, it
does not necessarily select the fastest path to a destination.
RIP v1 allows
routers to update their routing tables at programmable intervals. The default
interval is 30 seconds. The continual sending of routing updates by RIP v1
means that network traffic builds up quickly.
To prevent a packet from looping infinitely, RIP allows a maximum hop
count of 15. If the destination network is more than 15 routers away, the
network is considered unreachable and the packet is dropped. This situation
creates a scalability issue when routing in large heterogeneous networks. RIP
v1 uses split horizon to prevent loops. This means that RIP v1 advertises
routes out an interface only if the routes were not learned from updates
entering that interface. It uses holddown timers to prevent routing loops.
Holddowns ignore any new information about a subnet indicating a poorer metric
for a time equal to the holddown timer.
Figure summarizes the behavior of RIP v1 when used
by a router.
RIP v2 is an
improved version of RIP v1. It has many of the same features of RIP v1. RIP v2
is also a distance vector protocol that uses hop count, holddown timers, and
split horizon. Figure compares and
contrasts RIP v1 and RIP v2. The TTL field in the IP packet forces the packet
to be dropped. When the hop count reaches 15 routers, the network is considered
unreachable, and the packet is dropped because the router doesn't have a route
to the destination network.
The first Lab
Activity on this page will show students how to set up and configure RIP on
routers. The second Lab Activity will review the basic configuration of
routers. The Interactive Media Activity will help students understand the
differences between RIP v1 and RIP v2.
23.2
RIP Version 2
23.2.4
Configuring RIP v2
This page will
teach students how to configure RIP v2. RIP v2 is a dynamic routing protocol
that is configured by naming the routing protocol RIP Version 2, and then
assigning IP network numbers without specifying subnet values. This section
describes the basic commands used to configure RIP v2 on a Cisco router.
To enable a
dynamic routing protocol, the following tasks must be completed:
- Select a routing protocol, such
as RIP v2.
- Assign the IP network numbers
without specifying the subnet values.
- Assign the network or subnet
addresses and the appropriate subnet mask to the interfaces.
RIP v2 uses
multicasts to communicate with other routers. The routing metric helps the
routers find the best path to each network or subnet.
The router
command starts the routing process. The
network command causes the implementation of the following three functions:
- The routing updates are
multicast out an interface.
- The routing updates are
processed if they enter that same interface.
- The subnet that is directly
connected to that interface is advertised.
The network
command is required because it allows the routing process to determine which
interfaces will participate in the sending and receiving of routing updates.
The network command starts up the routing protocol on all interfaces that the
router has in the specified network. The network command also allows the router
to advertise that network.
The router rip
and version 2 commands combined specify RIP v2 as the routing protocol, while
the network command identifies a participating attached network.
In this example,
the configuration of Router A includes the following:
- router rip – Enables RIP as the
routing protocol
- version 2 – Identifies version
2 as the version of RIP being used
- network 172.16.0.0 – Specifies
a directly connected network
- network 10.0.0.0 – Specifies a
directly connected network
The interfaces on
Router A connected to networks 172.16.0.0 and 10.0.0.0, or their subnets, will
send and receive RIP v2 updates. These routing updates allow the router to
learn the network topology. Routers B and C have similar RIP configurations but
with different network numbers specified.
23.2
RIP Version 2
23.2.5
Verifying RIP v2
The show ip
protocols and show ip route commands display information about routing
protocols and the routing table. This
page explains how show commands are used to verify a RIP configuration.
The show ip
protocols command displays values about routing protocols and routing protocol
timer information associated with the router. In the example, the router is
configured with RIP and sends updated routing table information every 30
seconds. This interval is configurable. If a router running RIP does not receive
an update from another router for 180 seconds or more, the first router marks
the routes served by the non-updating router as being invalid. In Figure , the
holddown timer is set to 180 seconds. Therefore, an update to a route that was
down and is now up could stay in the holddown state until the full 180 seconds
have passed.
If there is still
no update after 240 seconds the router removes the routing table entries. The
router is injecting routes for the networks listed following the Routing for Networks
line. The router is receiving routes from the neighboring RIP routers listed
following the Routing Information Sources line. The distance default of 120
refers to the administrative distance for a RIP route.
The show ip
interface brief command can also be used to list a summary of the information
and status of an interface.
The show ip route
command displays the contents of the IP routing table. The routing table contains entries for all
known networks and subnetworks, and contains a code that indicates how that
information was learned.
Examine the
output to see if the routing table is populated with routing information. If
entries are missing, routing information is not being exchanged. Use the show
running-config or show ip protocols Privileged EXEC commands on the router to
check for a possible misconfigured routing protocol.
23.2
RIP Version 2
23.2.6
Troubleshooting RIP v2
This page
explains the use of the debug ip rip command.
Use the debug ip
rip command to display RIP routing updates as they are sent and received. The no debug all or undebug all commands will
turn off all debugging.
The example shows
that the router being debugged has received updates from one router at source
address 10.1.1.2. The router at source
address 10.1.1.2 sent information about two destinations in the routing table
update. The router being debugged also sent updates, in both cases to the
multicast address 224.0.0.9 as the destination. The number in parentheses is
the source address encapsulated into the IP header.
Other outputs
sometimes seen from the debug ip rip command includes entries such as the
following:
RIP: broadcasting
general request on Ethernet0
RIP: broadcasting
general request on Ethernet1
These outputs
appear at startup or when an event occurs such as an interface transition or a
user manually clears the routing table.
An entry, such as
the following, is most likely caused by a malformed packet from the
transmitter:
RIP: bad version
128 from 160.89.80.43
Examples of debug
ip rip outputs and meanings are shown in Figure .
23.2
RIP Version 2
23.2.7
Default routes
This page will
describe default routes and explain how they are configured.
By default,
routers learn paths to destinations three different ways:
- Static routes – The system
administrator manually defines the static routes as the next hop to a
destination. Static routes are useful for security and traffic reduction,
as no other route is known.
- Default routes – The system
administrator also manually defines default routes as the path to take
when there is no known route to the destination. Default routes keep
routing tables shorter. When an entry for a destination network does not
exist in a routing table, the packet is sent to the default network.
- Dynamic routes – Dynamic
routing means that the router learns of paths to destinations by receiving
periodic updates from other routers.
In Figure , the
static route is indicated by the following command:
Router(config)#ip
route 172.16.1.0 255.255.255.0 17.16.2.1
The ip
default-network command establishes a default route in networks using dynamic
routing protocols:
Router(config)#ip
default-network 192.168.20.0
Generally after
the routing table has been set to handle all the networks that must be
configured, it is often useful to ensure that all other packets go to a
specific location. This is called the default route for the router. One example
is a router that connects to the Internet. All the packets that are not defined
in the routing table will go to the nominated interface of the default router.
The ip
default-network command is usually configured on the routers that connect to a
router with a static default route.
In Figure , Hong
Kong 2 and Hong Kong 3 would use Hong Kong 4 as the default gateway. Hong Kong
4 would use interface 192.168.19.2 as its default gateway. Hong Kong 1 would
route packets to the Internet for all internal hosts. To allow Hong Kong 1 to
route these packets it is necessary to configure a default route as:
HongKong1(config)#ip
route 0.0.0.0 0.0.0.0 s0/0
The zeros in the
IP address and mask portions of the command represent any destination network
with any mask. Default routes are referred to as quad zero routes. In the
diagram, the only way Hong Kong 1 can go to the Internet is through interface
s0/0.
Summary
This page
summarizes the topics discussed in this module.
Variable-Length
Subnet Masks (VLSM), often referred to as "subnetting a subnet", is
used to maximize addressing efficiency. It is a feature that allows a single
autonomous system to have networks with different subnet masks. The network
administrator is able to use a long mask on networks with few hosts, and a
short mask on subnets with many hosts.
It is important
to design an addressing scheme that allows for growth and does not involve
wasting addresses. To apply VLSM to the addressing problem, large subnets are
created for addressing LANs. Very small subnets are created for WAN links and
other special cases.
VLSM helps to
manage IP addresses. VLSM allows for the setting of a subnet mask that suits
the link or the segment requirements. A subnet mask should satisfy the
requirements of a LAN with one subnet mask and the requirements of a
point-to-point WAN with another.
Addresses are
assigned in a hierarchical fashion so that summarized addresses will share the
same high-order bits. There are specific rules for a router. It must know in
detail the subnet numbers attached to it and it does not need to tell other
routers about each individual subnet if the router can send an aggregate route
for a set of routers. A router using aggregate routes would have fewer entries
in its routing tables.
If VLSM is the
scheme chosen, it must then be calculated and configured correctly.
RIP v1 is
considered an interior gateway protocol that is classful. RIP v1 is a distance
vector protocol that broadcasts its entire routing table to each neighbor
router at predetermined intervals. The default interval is 30 seconds. RIP uses
hop count as a metric, with 15 as the maximum number of hops.
To enable a
dynamic routing protocol, select a routing protocol, such as RIP v2, assign the
IP network numbers without specifying the subnet values, and then assign the
network or subnet addresses and the appropriate subnet mask to the interfaces.
In RIP v2, the router command starts the routing process. The network command
causes the implementation of three functions. The routing updates are multicast
out an interface, the routing updates are processed if they enter that same
interface, and the subnet that is directly connected to that interface is
advertised. The version 2 command enables RIP v2.
The show ip
protocols command displays values about routing protocols and routing protocol
timer information associated with the router. Use the debug ip rip command to
display RIP routing updates as they are sent and received. The no debug all or
undebug all commands will turn off all debugging.
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