Distance Vector Routing Protocols
CCNA 2 v3 – Module 7
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Distance Vector Routing – Updates
Updates occur periodically or when the topology changes.
Updates proceed systematically from router to router.
Router sends its entire routing table to each neighbor.
Distance Vector Routing – Loops
Routing loops can occur when inconsistent routing tables are not
updated due to slow convergence in a changing network.
2.
4.
C still unaware of failure,
advertises route via B
5.
D updates its routing table to include route from C,
and forwards this incorrect information to A.
E sends update to A
X
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1.
Network fails
A sends update to B and D
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Maximum Hop Count
Invalid updates can continue to loop until some other process stops them.
Each router the packet passes through increases the hop count by 1.
Packets continuously looping around the network count to infinity.
The routing protocol permits the routing loop to continue until the metric
exceeds its maximum allowed value.
In RIP, if the hop count exceeds the maximum of 15 hops the packet is
discarded and the network is considered unreachable.
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Split Horizon
Split Horizon is another mechanism used to avoid routing loops.
Information about routes is prevented from being advertised out the
router interface through which the information was received.
1.
Router A advertises route to Network A
A
B
2.
3.
Router B updates its routing table
Router B does not include Network A in update to A
Route Poisoning
Poison Reverse updates are used to overcome large routing loops by
sending explicit information when a subnet or network is not accessible.
Sets the hop count to one more than the maximum.
When used with triggered updates it will speed up convergence time.
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Triggered Updates and Holddown Timers
RIP updates occur every 30 secs, but a triggered update is sent immediately.
Router detects topology change, immediately sends update to adjacent
routers – doesn’t wait for the update timer to expire.
Wave of updates propagates throughout the network.
Ensure all routers know of failed routes before holddown timers expire.
Route down!
Pass it on…
Route down!
Send triggered
update
Interface down!
Send triggered
update now
x
The count to infinity problem can be avoided by using holddown timers.
When the router marks a route inaccessible it starts a holddown timer.
Update received:
network inaccessible,
start holddown timer
Update received: network
accessible again
Ignore update,
Wait until holddown
timer expires
Is
Yes Network
update from same
accessible,
neighbor?
remove
holddown
No
timer
Different
No
Yes
neighbor, better
5
metric?
RIP Routing Process
Two versions of RIP:
1. RIP v1: Classful routing protocol
 Does not include subnet masks in updates
2. RIP v2: Classless routing protocol
 Carry additional packet routing information.
 Authentication mechanism to secure table updates.
 Supports variable length subnet masking (VLSM).
30 secs.
RIP updates occur every ________
15.
The maximum number of hops in a path is ____
RIP implements split horizon and holddown mechanisms.
Configuration Example:
GAD(config)# router rip
GAD(config-router)# network 192.168.13.0
GAD(config-router)# network 192.168.14.0
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IP Classless
If one part of a major network is known, but the subnet toward which the packet
is destined within that major network is unknown, the packet is dropped.
I know some 10.0.0.0/24 subnets,
but not 10.2.2.0/24
Default route
The router only uses the default route if the major network destination
does not exist in the routing table.
To forward these packets to the best supernet route possible:
Router(config)# ip classless
Configuring ip classless on the router allows it to ignore classful
boundaries of the networks in its routing table and route to the default route.
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RIP Configuration Issues
RIP routers rely on neighbors for network information - Routing By Rumour.
Convergence is when all routers in the Internetwork have the same routing
information. Slow convergence of DV protocols results in inconsistencies.
RIPs performance can be tuned to improve convergence time:
To disable split horizon: GAD(config-if)# no ip split-horizon
The default holddown is 180 secs. Decrease it to speed up convergence. Set
the timer just longer than the longest possible update time for the network:
GAD(config-router)# timers basic 30 180 180 240
Update
Invalid
Holdtime
Flush
The default RIP update interval is 30 secs. Longer intervals can conserve
bandwidth, shorter intervals may decrease convergence time:
GAD(config-router)# update-timer 40
To disable sending routing updates on specified interfaces:
GAD(config-router)# passive-interface f0/0
To configure the router to send and receive packets from only version 2:
GAD(config-router)# version 2
To control how packets received from an interface are processed:
GAD(config-if)# ip rip receive version 1 2
Verifying RIP Configuration
Dublin# show ip protocols
RIP routing is configured
Interfaces sending and
receiving RIP updates
Router is advertising the
correct networks
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Verifying RIP Configuration
Dublin# show ip route
Route received from
RIP neighbor is in the
routing table
Additional commands to check RIP configuration:
Command
Definition
show interface
Statistics for all interfaces configured on router
show ip interface
show running-config
Interface's IP information and status
Current NESCOT
configuration
in RAM
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Troubleshooting RIP
Typical RIP configuration errors:
 incorrect network statement
 discontiguous subnets
 split horizons
To analyse RIP update issues:
Pretoria# debug ip rip
Other commands to troubleshoot RIP:
Command
show ip rip database
Definition
Summary of entries in RIP routing database
show ip protocols {summary} Data for each routing protocol active on router
show ip route
View the routing table
debug ip rip {events}
show ip interface brief
Check routing updates are being sent
Summary of interface status and parameters
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RIP Load Balancing
Load balancing allows a router to simultaneously use multiple paths to a
destination. RIP can load balance over 6 equal-cost paths, (default 4 paths).
Router(config-router)# maximum-paths 5
RIP performs what is referred to as “round robin” load balancing:
per-packet basis.
 If process switching is enabled, paths alternate on a ___________
per-destination basis.
 If fast switching is enabled, paths alternate on a _______________
Here each path from GAD to
BHM is considered equal by
RIP metric (2 hops)
Equal cost routes can be found by using #show ip route.
Each route is represented by a routing descriptor block.
An asterisk (*) next to one of the entries corresponds to the active route.
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Redistributing Static Routes into RIP
1. Static routes are important for destinations not included in dynamic routing
default route
processes. They are also useful for specifying a ____________.
administrative distance (AD).
2. Each dynamic routing protocol has a default ________________________
3. A static route can be defined as less desirable than a dynamically learned
AD is higher than the dynamic route’s.
route if its ____
4. If a static route points to an interface that is not part of the RIP process (as
defined with a network command) RIP will not advertise the route unless
configured to:
Router(config)# router rip
Router(config-router)# redistribute static
Floating Static routes are routes with an AD set greater than the AD of
5. _____________________
the dynamic routing protocol in use.
6. Static routes are removed from the routing table when their corresponding
goes down or when the next hop is ________________.
no longer valid
interface ___________
no ip route global
7. Static routes can be removed using the ______________
configuration command.
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IGRP Features
Interior Gateway Routing Protocol is a DV protocol proprietary to Cisco.
IGRP sends routing updates at 90 second intervals, advertising networks for
a particular AS.
Key design characteristics of IGRP are a follows:
 The versatility to automatically handle indefinite, complex topologies
 The flexibility to handle different bandwidth and delay characteristics
 Scalable to large networks
IGRP can be configured to use a combination of variables for its metric:
 Bandwidth – based on lowest bandwidth value in the path Default metric
 Delay – the cumulative interface delay along the path
components
 Reliability – of the link based on exchange of keepalives
 Load – amount of traffic on a link based on bits per second
 MTU – Maximum Transmission Unit value of the path
These parameters are considered only if enabled via configuration
show ip protocols – displays parameters including metric values
K1 to K5. K1= bandwidth, K3= delay.
show ip route – displays metric values in brackets for each route.
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IGRP Routes
Match the IGRP route type to its definition:
Type
Definition
Interior
Routes to networks outside AS.
Used to identify default gateway.
Different routers may choose different
routes as the gateway of last resort.
System
Routes between subnets of a network
attached to a router interface.
If the network is not subnetted, IGRP does
not advertise them.
Exterior
Routes to networks within the AS. Derived
from directly connected interfaces and
information from other IGRP-speaking
devices. Do not include subnet information.
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IGRP Stability Features
Like RIP, IGRP has a number of features designed to enhance its stability:



Holddowns
Split horizons
Poison reverse updates
With IGRP, poison reverse updates are sent only if a route metric has
increased by a factor of 1.1 or greater.
IGRP default timer values
Router# show ip protocols
Routing protocol is “IGRP 101”
Sending updates every 90 seconds, next due in 51 seconds
Invalid after 270 seconds, holddown 280 seconds, flushed
after 630 seconds
< output omitted >
How long to wait in the absence of specific updates
before declaring a route invalid (Default: 3 x U)
Time before a route is flushed from
the routing table (Default: 7 x U)
How frequently routing update
messages should be sent
Amount of time for which information about
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Configuring IGRP
Consider this network on which RIP is already running:
192.168.1.0/24
A# show ip route
< output omitted
C 192.168.1.0/24
C 192.168.2.0/24
R 192.168.3.0/24
A
192.168.2.0/24
B
192.168.3.0/24
>
is directly connected, FastEthernet0/0
is directly connected, Serial0/0
[120/1] via 192.168.2.2, 00:00:29, Serial0/0
IGRP is then configured on both routers, example:
A(config)# router igrp 101
A(config-router)# network 192.168.1.0
A(config-router)# network 192.168.2.0
Once A has received an IGRP update from B:
A# show ip route
< output omitted
C 192.168.1.0/24
C 192.168.2.0/24
I 192.168.3.0/24
>
is directly connected, FastEthernet0/0
is directly connected, Serial0/0
[100/80135] via 192.168.2.2, 00:00:69, Serial0/0
AD and metric
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Verifying and Troubleshooting IGRP
Command
Related Use
show ip route
Check routing table for any ‘I’ routes
show ip protocols
To check if IGRP is enabled
show running-config
Verify the router is configured for IGRP networks
show running-config begin …
show interface
Filter the tail of the output from full command
show running-config int …
Filter interface specific information from full command
Overview of IGRP activity, updates sent and received
debug ip igrp events
debug ip igrp transactions
ping
traceroute
Verify an interface is properly configured
IGRP activity including route update details
End to end connectivity test at layer 3
Hop by hop path determination utility
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