Distance Vector Routing Protocols

1
Distance Vector Routing Protocols

• CCNA 2 v3 Module 7

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.

3
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.
4
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 B updates its routing table
2. 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.
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 doesnt 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.
Interface down! Send triggered update now
Route down! Send triggered update
Route down! Pass it on
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
6
RIP Routing Process

• 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.
GAD(config)
router rip
network 192.168.13.0
GAD(config-router)
GAD(config-router)
network 192.168.14.0
7
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.
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
no ip split-horizon
timers basic 30 180 180 240
update-timer 40
passive-interface f0/0
version 2
ip rip receive version 1 2
9
Verifying RIP Configuration
Dublin
show ip protocols
10
Verifying RIP Configuration
Dublin
show ip route
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
11
Troubleshooting RIP

• Typical RIP configuration errors
• incorrect network statement
• discontiguous subnets
• split horizons

debug ip rip
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
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.
13
Redistributing Static Routes into RIP

1. Static routes are important for destinations not
   included in dynamic routing processes. They are
   also useful for specifying a ____________.
2. Each dynamic routing protocol has a default
   ________________________
3. A static route can be defined as less desirable
   than a dynamically learned route if its ____ is
   higher than the dynamic routes.    
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

default route
administrative distance (AD).
AD
Router(config)
router rip
Router(config-router)
redistribute static

1. _____________________ are routes with an AD set
   greater than the AD of the dynamic routing
   protocol in use.
2. Static routes are removed from the routing table
   when their corresponding interface ___________ or
   when the next hop is ________________.
3. Static routes can be removed using the
   ______________ global configuration command.

Floating Static routes
goes down
no longer valid
no ip route
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
the cumulative interface delay along the path
of the link based on exchange of keepalives
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.
15
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.
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
show ip protocols
Router
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 lt output omitted gt
17
Configuring IGRP
Consider this network on which RIP is already
running
192.168.3.0/24
A
B
192.168.1.0/24
192.168.2.0/24
A show ip route lt output omitted gt C
192.168.1.0/24 is directly connected,
FastEthernet0/0 C 192.168.2.0/24 is directly
connected, Serial0/0 R 192.168.3.0/24 120/1 via
192.168.2.2, 000029, 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
AD and metric
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