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Routing with a distance
vector protocol
Introducing Routing and Switching in the Enterprise – Chapter 5.1
Copyleft 2012 Vincenzo Bruno (www.vincenzobruno.it)
Released under Crative Commons License 3.0 By-Sa
Cisco name, logo and materials are Copyright Cisco Systems Inc. 1
Overview
Hierarchical networks
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The organization must create a hierarchy to meet the different
network requirements of each part of the company.
Crucial information and services typically reside near the top of
the hierarchy, in secured server farms or on storage area
networks.
Enterprise networks provide a high level of reliability and
services. To ensure this, network professionals:
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Design networks to provide redundant links to use in case a
primary data path fails.
Deploy Quality of Service (QoS) to ensure critical data receives
priority treatment.
Use packet filtering to deny certain types of packets, maximize
available bandwidth, and protect the network from attacks.
Network topologies
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Star
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Extended star
Partial mesh
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Full mesh
Routing Table
Exit Interface
Routing
Administrative
source
Distance and Metric
information Destination
Network and
Subnet mask
Next Hop
Stub network
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Network with only a single connection to a router
Static route is
used in stub
networks
Static and dynamic routing
Configure static routes
If an exit interface is disabled, static routes disappear from the routing
table. The routing table reinstalls the routes when the interface is reenabled.
Recursive lookup
Static routes configured with a next hop interface require two steps
to determine the exit interface. This is called a recursive lookup.
In a recursive loopkup:
1) The router search the destination network
2) It matches the next hop IP address of the static route to entries in its
routing table to determine which interface to use.
Destination IP: 192.168.3.8
1)
2)
Route summarization
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Summarizing several static routes as a single entry reduces the
size of the routing table and makes the lookup process more
efficient.
A single static route summarizes multiple static routes if:
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The destination networks summarize into a single network
address.
All of the static routes use the same exit interface or next-hop IP
address.
Without summary routes, routing tables within Internet core
routers become unmanageable. Enterprise networks encounter
the same problem.
Summary static routes are an indispensable solution for
managing routing table size.
Floating static route
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A floating static route has a higher administrative distance than
the route learned from a dynamic routing protocol.
For that reason, a floating static route does not display in the
routing table.
The floating static route entry appears in the routing table only if
the dynamic information is lost.
To create a floating static route, add an administrative distance
value to the end of the ip route command:
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Router(config)#ip route 192.168.4.0 255.255.255.0 192.168.9.1 200
The administrative distance specified must be greater than the
AD assigned to the dynamic routing protocol.
The router uses the primary route as long as it is active.
Default route
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The command to create a default route is similar to the
command used to create either an ordinary or a floating static
route.
The network address and subnet mask are both specified as
0.0.0.0, making it a quad zero route.
The command uses either the next-hop address or the exit
interface parameters.
The zeroes indicate to the router that no bits need to match in
order to use this route.
As long as a better match does not exist, the router uses the
default static route.
Distance vector protocol
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A router running a distance vector protocol does not know the
entire path to a destination;
it only knows the distance to the remote network and the
direction, or vector.
Its knowledge comes through information from directly
connected neighbors.
Distance vector protocols calculate the best route based on the
distance from a router to a network.
An example of a metric used is hop count, which is the
number of routers, or hops, between the router and the
destination
Distance vector protocol
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Distance vector protocols usually require less complicated
configurations and management than link-state protocols
Routers using distance vector protocols broadcast or
multicast their entire routing table to their neighbors at
regular intervals (slow)
At any given moment, some routers may not have the most
current information about the network → routing loops
If a router learns more than one route to a destination, it
calculates and advertises the route with the lowest metric
RIP versions 1 and 2 are true distance vector protocols,
whereas EIGRP is actually a distance vector protocol with
advanced capabilities.
RIPng, the newest version of RIP was specifically designed to
support IPv6.
Routing Information Protocol
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RIP was the first IP distance vector routing protocol to be
standardized in a RFC (RFC1058 in 1988)
The first version of RIP is now often called RIPv1 to distinguish
it from the later improved version, RIPv2; and from the IPv6
version, RIPng.
By default RIPv1 broadcasts its routing updates out all active
interfaces every 30 seconds.
RIPv1 is a classful routing protocol. It automatically
summarizes subnets to the classful boundary and does not
send subnet mask information in the update.
Therefore RIPv1 does not support VLSM and CIDR.
RIPv1 is classful
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Router(config)#interface fastEthernet 0/0
Router(config­if)#ip address 172.16.1.1 255.255.255.0
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Router(config)#router rip
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Router(config­router)#network 172.16.1.0
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Router(config­router)#network 172.16.2.0
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Router#show running­config
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router rip
network 172.16.0.0
RIPv2 is classless
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RIPv2 is a classless routing protocol that supports VLSM and
CIDR. A subnet mask field is included in v2 updates, which
allows the use of discontiguous networks.
RIPv2 also has the ability to turn off automatic summarization
of routes.
Both versions of RIP send their entire routing table out all
participating interfaces in updates.
RIP v1 broadcasts these updates to 255.255.255.255. This
requires all devices on a broadcast network like Ethernet to
process the data.
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RIP v2 multicasts its updates to 224.0.0.9.
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Multicasts take up less network bandwidth than broadcasts.
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RIPv2 has an authentication mechanism, RIPv1 does not.
RIP v1 and v2 features
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RIPv2 shares many of the features found in RIPv1, such as:
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Hop-count metric
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15-hop maximum
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TTL equals 16 hops
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Default 30-second update interval
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Route poisoning, poisoned reverse, split horizon, and holddowns
to avoid loops
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Updates using UDP port 520
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Administrative distance of 120
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Message header containing up to 25 routes without authentication
RIP steps
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When a router starts up, each RIP-configured interface sends out
a request message.
RIP-enabled neighbors send a response message with full
routing table
The receiving router evaluates each route entry based on:
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If a route entry is new, the receiving router installs the route in the
routing table.
If the route is already in the table and the entry comes from a different
source, the routing table replaces the existing entry if the new
entry has a better hop count.
If the route is already in the table and the entry comes from the same
source, it replaces the existing entry even if the metric is not better.
The startup router then sends a triggered update out all RIPenabled interfaces containing its own routing table.
Configure RIP Version
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By default, RIPv2 sends and receives only version 2 updates.
If a network must use both versions of RIP, the network
administrator configures RIPv2 to send and receive both
versions 1 and 2.
By default, RIPv1 sends version 1 updates, but receives both
versions 1 and 2.
Commands:
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ip rip send version <1 | 2 | 1 2>
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ip rip receive version <1 | 2 | 1 2>
Exercise
Configure RIP
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By default, RIPv2 will summarize each network to be advertised
to its classful boundary as the graphic shows.
RIPv2 updates can be configured to be authenticated.
RIPv2 propagates a default route to its neighbor routers as part
of its routing updates.
To accomplish this, create the default route and then add
redistribute static to the RIPv2 configuration.
Configure RIP Authentication
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key chain kal
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!--- Name a key chain. A key chain may contain more than one key for added security.
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!--- It need not be identical on the remote router.
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key 1
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!--- This is the Identification number of an authentication key on a key chain.
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!--- It need not be identical on the remote router.
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key-string 234
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!--- The actual password or key-string. It needs to be identical to the key-string on the remote
router.
interface Serial0
ip address 141.108.0.10 255.255.255.252
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ip rip authentication key-chain kal
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!--- Enables authentication on the interface and configures. The key chain that will be used.
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router rip
version 2
Disable auomatic summarization
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Unlike RIPv1, with RIPv2 the automatic summarization feature
can be disabled.
When disabled, RIPv2 will report all subnets with subnet mask
information.
This is done to ensure a more accurate routing table.
To accomplish this, add the no auto-summary command to the
RIPv2 configuration.
Router(config­router)#no auto­summary
Disable routing updates
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Another issue to consider is the broadcast nature of RIP
updates.
RIP immediately begins to send advertisements out all interfaces
that belong to that network.
These updates may not be needed on all portions of a network.
For example, an Ethernet LAN interface passes these updates to
every device on its network segment, which produces
unnecessary traffic and makes the network less secure.
The passive­interface command, issued in interface
mode, disables routing updates on specified interfaces.
Router(config­router)#passive­interface interface­type interface­number
passive-interface command
Routing Loops
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A network running RIP needs time to converge. Some routers
may contain incorrect routes in their routing tables until all
routers have updated and have the same view of the network.
Erroneous network information may cause routing updates and
traffic to loop endlessly as they count to infinity.
In the RIP routing protocol, infinity occurs when the hop
count is 16.
Routing loops negatively affect network performance. RIP
contains several features designed to combat this impact.
These features are often used in combination:
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Poisoned reverse
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Split horizon
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Holddown timer
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Triggered updates
Poisoned reverse
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Is a network update that explicitly indicates that a network is
unreachable
Poisoned reverse sets the metric for a route to 16, making it
unreachable.
Because RIP defines infinity as 16 hops, any network further
away than 15 hops is unreachable.
If a network is down, a router changes the metric for that route
to 16 so that all other routers see it as unreachable.
This feature prevents the routing protocol from sending
information via poisoned routes.
Split horizon
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Split horizon prevents the formation of loops.
When multiple routers advertise the same network routes to
each other, routing loops may form.
Split horizon dictates that a router receiving routing
information on an interface cannot send an update about
that same network back out the same interface.
Holddown timer
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The holddown timer stabilizes routes.
The holddown timer refuses to accept route updates with a
higher metric to the same destination network for a period after
a route goes down.
If, during the holddown period, the original route comes back up
or the router receives route information with a lower metric, the
router installs the route in the routing table and immediately
begins to use it.
The default holddown time is 180 seconds, six times the
regular update period. The default can be changed.
However, any holddown period increases the convergence
time and has a negative impact on network performance.
Triggered update
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When a route fails, RIP does not wait for the next periodic
update.
Instead, RIP sends an immediate update, called a triggered
update.
It advertises the failed route by increasing the metric to 16,
effectively poisoning the route.
This update places the route in holddown status while RIP
attempts to locate an alternate route with a better metric.
Verifying RIP
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The show ip protocols and show ip route commands
are important for verification and troubleshooting on any routing
protocol.
The following commands specifically verify and troubleshoot
RIP:
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show ip rip database: Lists all the routes known by RIP
debug ip rip or debug ip rip {events}: Displays RIP
routing updates as sent and received in real time
The output of this debug command displays the source address
and interface of each update, as well as the version and the
metric.
End of lesson