1. Distance Vector Routing Protocols
CCNA 2 v3 – Module 7
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CCNA 2 V3 Module 07 DC

2. 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.
E sends update to A
A sends update to B and D
C still unaware of failure, advertises route via B
D updates its routing table to include route from C, and forwards this incorrect information to A. X
Network fails
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3. Distance Vector Routing Protocols
Maximum Hop Count
Distance Vector Routing Protocols
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.
CCNA 2 V3 Module 07 DC

4. Split Horizon Route Poisoning
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. B A
Router A advertises route to Network A
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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5. 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. Is
update from same neighbor?
Update received: network inaccessible, start holddown timer
Update received: network accessible again
Yes
Network accessible,
remove holddown timer No
Different
neighbor, better metric? No
Ignore update,
Wait until holddown timer expires
Yes

6. RIP Routing Process Configuration Example:
RIP v1: Classful routing protocol
Does not include subnet masks in updates
RIP v2: Classless routing protocol
Carry additional packet routing information.
Authentication mechanism to secure table updates.
Supports variable length subnet masking (VLSM).
Two versions of RIP:
RIP updates occur every ________
The maximum number of hops in a path is ____
RIP implements split horizon and holddown mechanisms.
30 secs.
15.
Configuration Example:
GAD(config)#
router rip
GAD(config-router)#
network
GAD(config-router)#
network
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7. I know some 10.0.0.0/24 subnets, but not 10.2.2.0/24
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 /24 subnets, but not /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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8. 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
Holdtime
Flush
Update
Invalid
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

9. Verifying RIP Configuration
RIP routing is configured
Dublin#
show ip protocols
Interfaces sending and receiving RIP updates
Router is advertising the correct networks
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10. 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
Interface's IP information and status
show running-config
Statistics for all interfaces configured on router
show ip interface
Current configuration in RAM
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11. Troubleshooting RIP Typical RIP configuration errors:
incorrect network statement
discontiguous subnets
split horizons
Pretoria#
To analyse RIP update issues:
debug ip rip
Other commands to troubleshoot RIP:
Summary of interface status and parameters
debug ip rip {events}
View the routing table
show ip protocols {summary}
Summary of entries in RIP routing database
Definition
Command
show ip rip database
Data for each routing protocol active on router
show ip route
Check routing updates are being sent
show ip interface brief
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12. 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:
If process switching is enabled, paths alternate on a ___________ basis.
If fast switching is enabled, paths alternate on a _______________ basis.
per-packet
per-destination
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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13. Redistributing Static Routes into RIP
Static routes are important for destinations not included in dynamic routing processes. They are also useful for specifying a ____________.
Each dynamic routing protocol has a default ________________________
A static route can be defined as less desirable than a dynamically learned route if its ____ is higher than the dynamic route’s.    
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:
default route
administrative distance (AD). AD
Router(config)#
router rip
Router(config-router)#
redistribute static
_____________________ are routes with an AD set greater than the AD of the dynamic routing protocol in use.
Static routes are removed from the routing table when their corresponding interface ___________ or when the next hop is ________________.
Static routes can be removed using the ______________ global configuration command.
Floating Static routes
goes down
no longer valid
no ip route
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14. 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
Delay
Reliability
Load
MTU
– based on lowest bandwidth value in the path
Default metric components
– the cumulative interface delay along the path
– of the link based on exchange of keepalives
These parameters are considered only if enabled via configuration
– amount of traffic on a link based on bits per second
– Maximum Transmission Unit value of the path
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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15. IGRP Routes Match the IGRP route type to its definition: Type
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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16. 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 frequently routing update messages should be sent
Time before a route is flushed from the routing table (Default: 7 x U)
How long to wait in the absence of specific updates before declaring a route invalid (Default: 3 x U)
Amount of time for which information about poorer routes is ignored (Default: 3 x U + 10)
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17. Configuring IGRP
Consider this network on which RIP is already running:
/24
/24 A B
/24
A# show ip route
< output omitted >
C /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
R /24 [120/1] via , 00:00:29, Serial0/0
IGRP is then configured on both routers, example:
A(config)#
router igrp 101
A(config-router)#
network
A(config-router)#
network
Once A has received an IGRP update from B:
A# show ip route
< output omitted >
C /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
I /24 [100/80135] via , 00:00:69, Serial0/0
AD and metric
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18. Verifying and Troubleshooting IGRP
Command
Related Use
show ip route
To check if IGRP is enabled
show running-config
Filter the tail of the output from full command
show interface
Filter interface specific information from full command
debug ip igrp events
IGRP activity including route update details
ping
Hop by hop path determination utility
Check routing table for any ‘I’ routes
show ip protocols
Verify the router is configured for IGRP networks
show running-config begin …
Verify an interface is properly configured
show running-config int …
Overview of IGRP activity, updates sent and received
debug ip igrp transactions
End to end connectivity test at layer 3
traceroute
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