Transcript Module 7

CCNA 2 v3.1 Module 7
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CCNA 2
Module 7
Distance Vector Routing Protocols
RIP and IGRP
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Distance vector routing updates
• Routing table updates occur periodically or when
topology change
• Routers send their entire routing table to all adjacent
neighbors
• Routing table contains info about routing metrics
Hop count
Bandwidth
Cost
Load
Reliability
Delay
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Distance vector routing loop
• Loops occur when inconsistent routing tables are
not updated due to slow convergence in a
changing network
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Count To Infinity
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Count to infinity
Packets loop continuously around the network
Packet never reach their destination
May occur because
Network is down
Interface not configured correctly
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To overcome count to infinity
Specify a maximum hop count
Split Horizon
Route Poisoning
Hold Down timers
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Maximum hop count
• Used to overcome count to infinity
• Hop count
The number of routers a packet goes through
It increases each time a packet goes through a
router
• RIP maximum hop count is 15
Packet is discarded if the hop count of 16 is
reached
This is because the network 1 is unreachable
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How does the Maximum Hop Count Work?
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Split Horizon
• Solution to count to infinity
• Do not send routing updates to the router
you learned the information about
Reduces incorrect routing information
Reduces routing overhead
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Route poisoning
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• Poison reverse
• When route poisoning is used with
triggered updates it will speed up
convergence time because neighboring
routers do not have to wait 30 seconds
before advertising the poisoned route.
• Failed routes are advertised with infinite
metrics.
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Avoiding routing loops with triggered updates
• Triggered updates
Sent immediately when topology changes
Used in conjunction with route poisoning
Ensure all routers know of failed routes before
any holddown timers can expire
i.e., Don’t wait for the time limit to expire
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Preventing routing loops with holddown timers
• When a router receives an update saying a
route is down, it marks the route as
inaccessible and starts a hold-time timer
• If at any time before the holddown timer
expires an update is received from:
the same neighbour indicating the network is accessible
again
Router marks network as accessible + removes
holddown timer
A different neighbour with a better metric
Router marks network as accessible + removes
holddown timer
A different neighbour with a poorer metric
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RIP Routing Protocol
• Open standard RFC 1058 and Internet Standard
(STD) 56
• Documented by:
Request for Comments (RFC) 1058
Internet Standard (STD) 56.
• RIP Version 1 (RIP v1)
Classful Routing Protocol
• RIP Version 2 (RIP v2)
Classless Routing Protocol
Ability to carry additional packet routing information.
Authentication mechanism to secure table updates.
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• RIP has slow convergence which results in
Routing loops
Count to infinity
• To reduce these RIP uses
Count-to-infinity
Split horizon
Poison reverse
Holddown counters
Triggered updates
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Configuring RIP
• Enter global configuration mode
Router#config t
• Specify RIP as the routing protocol
Router(config)#router rip
• Specify directly connected networks
Router(config-router)#network 10.10.10.0
• A router running RIP can be configured to
Send triggered updates independently the scheduled
updates (every 30 seconds)
Router(config-if)#ip rip triggered
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RIP Configuration Issues
• Count-to-infinity
Does not have to be configured with RIP
• To disable Split Horizon
Router(config-if)#no ip split-horizon
• Default holddown for RIP is 180 seconds
• The default RIP update 30 seconds
Can be configured for longer intervals
To conserve bandwidth
Can be configured for for shorter intervals
To decrease convergence time
• Command
Router(config)#router rip
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• To prevent RIP updates leaving an interface
Router(config-router)#passive-interface Fa0/0
• To configure RIP to exchange routing information
in a non-broadcast network (Frame Relay)
RIP needs to be told of other neighboring RIP routers
Router(config-router)#neighbor <ip address>
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Configure Router to Send/Receive Packets
•To configure software to send and receive Rip V1 and V2 packets
•Router(config-router)#version{1|2}
•To configure an interface to send RIP V1 packets
•Router(config-if)#ip rip send version 1
•To configure an interface to send RIP V2 packets
•Router(config-if)#ip rip send version 2
•To configure an interface to send RIP V1 and V2 packets
•Router(config-if)#ip rip send version 1 2
•To configure and interface to receive RIP V1 packets
•Router(config-if)#ip rip receive version 1
•To configure and interface to receive RIP V2 packets
•Router(config-if)#ip rip receive version 2
•To configure and interface to receive RIP V1 and V2 packets
•Router(config-if)#ip rip receive version 1 2
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Verifying
RIP configuration
• show ip protocols
Shows which routing protocols are carrying IP traffic on the
router
Confirms
RIP routing is configured
The correct interfaces are sending and receiving RIP
updates
The router is advertising the correct networks
• show ip route
Displays the routing table
Verify that routes received by RIP neighbours are installed in
the routing table
• Additional commands to check RIP configuration
show interface interface
show ip interface interface
show running-config
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Troubleshooting RIP update issues
• debug ip rip
Displays RIP routing updates as they are sent
and received
Problems such as discontiguous subnets or
duplicate networks can be diagnosed with this
command
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Discontiguous subnet
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Duplicate Subnets
Duplicate Subnets
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Other commands to troubleshoot RIP:
• show ip rip database
• show ip protocols {summary}
• show ip route
• debug ip rip {events}
• show ip interface brief
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Preventing routing updates through an interface
• network command
If it is issued for a given network using RIP or IGRP
router immediately begin sending advertisements out all
interfaces within the specified network address range
Router(config)#router RIP
Router(config-router)#network 192.101.21.0
• passive interface command
Stops the router from sending updates through a particular
interface to a neighbor
Router continues to listen and use routing updates from that
neighbor
Prevent other systems from learning about routes
dynamically
Router(config)#router RIP
Router(config-router)#passive-interface fa0/0
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Load balancing with RIP
• Allows a router to take advantage of
multiple best paths to a given destination
Calculated by dynamic routing protocol
Entered statically by administrator
• RIP performs “round robin” load
balancing
over 6 equal-cost paths, with 4 paths being
default
takes turns forwarding packets over the
parallel paths
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• Interface pointer cycles through the
interfaces and routes in a deterministic
fashion such as 1-2-3-4-1-2-3-4-1
• RIP metric is hop count
no regard is given to the speed of the links
56 Kbps path will be given the same
preference as the 155 Mbps path
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• Equal cost routes can be found by using the
“show ip route “
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• When a router learns multiple routes to a
specific network, the route with the lowest
administrative distance is installed in the
routing table
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• When a router has multiple routes to a network
route with lowest admin distance is installed in routing table
• If the router has many routes, learned via the same
routing process with the same admin distance
router chooses the path with the lowest cost or metric
• If the router has multiple paths to a destination with the
same admin distance and cost
load-balancing can occur
• Cisco IOS allows up to 6 equal cost routes in a routing
table
EIGRP allows up to 4 equal cost routes
Static routes always install 6 routes
BGP by default allows only 1 route
IGRP can load balance up to 6 unequal links
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Router(config-router)#maximum-paths [no.]
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Cisco IOS offers two methods of load
balancing:
per-packet:If process switching is enabled,
Router will alternate paths on a per-packet basis
per-destination: If fast switching is enabled
Only one route will be used for that destination
address
All packets bound for a specific host take the same
path
Packets bound for a different host on the same
network may use an alternate route
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static routes with RIP
•Integrating
Static routes
User-defined routes
Force packets to take a specific path
Useful for specifying a “gateway of last resort”; a default
route
• Administrative Distance (AD)
Each dynamic routing protocol has a default AD
A static route can be defined as less desirable than a
dynamically learned route, as long as the AD of the static
route is higher than that of the dynamic route
• floating static route
A route configured on a router to take the place of the RIP
route in the event that the RIP routing process fails
• RIP can advertise static routes using the redistribute
static command
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IGRP features
• Distance vector routing protocol
• Developed by Cisco
• Sends routing updates every 90 second
Sends entire table
• Advertising networks for an autonomous system
• Metrics used are normally bandwidth and delay
Bandwidth
Delay – amount of time to get to the destination
Reliability – reliability of link determined by exchange of
keepalives
Load
MTU – maximum transmission unit of the path
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• IGRP converges faster than RIP
thereby avoiding the routing loops
• IGRP does not share the hop count
limitation of RIP.
• Useful Commands
show interface interface
show running-config
show running-config interface interface
show running-config | begin interface interface
show running-config | begin igrp
show ip protocols
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Metric K1 represents bandwidth by default is set to 1
Metric K3 represents delay and by default is set to 1
Metric K2, K4 and K5 are set to 0.
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show ip route shows the IGRP metric values in brackets.
A link with a higher bandwidth will have a lower metric
A route with a lower cumulative delay will have a lower metric
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•
GRP advertises three types of routes:
IGRP
Routes
Interior
Routes between subnets of a network attached to a router
interface
If the network attached to a router is not subnetted, IGRP does
not advertise interior routes
System
Routes to networks within an autonomous system number
The IOS derives system routes from directly connected
network interfaces
System routes information provided by other IGRP routers.
System routes do not include subnet information
Exterior
Routes to networks outside the autonomous system that are
considered when identifying a gateway of last resort.
Gateway of last resort is used if
a better route is not found
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IGRP Routes
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IGRP stability features
Holddowns
Prevent regular update messages from inappropriately reinstating a route
that may not be up
Split horizons
• It is usually not useful to send information about a route back in the
direction from which it came
• Prevent routing loops
Poison reverse
• Poison reverse updates then are sent to remove the route and place it in
holddown.
• With IGRP, poison reverse updates are sent only if a route metric has
increased by a factor of 1.1 or greater.
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IGRP Timers
• update timer specifies
how frequently routing update messages should be sent
Default is 90 sec
• invalid timer specifies
how long a router should wait in the absence of routingupdate messages about a route before declaring that route
invalid
Default is 3 times the update period (270 sec)
• holddown timer specifies
the amount of time for which information about poorer routes
is ignored
Default is 3 times the update timer period + 10 sec (280 sec)
• flush timer indicates
how much time should pass before a route is flushed from the
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Configuring IGRP
• To set IGRP routing on
Router# config t
Router(config)# router igrp 101
Router(config-router)#network 192.101.2.0
• To turn IGRP routing off
Router(config)# no router igrp 101
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Troubleshooting IGRP
• Problems to look out for
mistyped network statement
discontiguous subnets
Incorrect Autonomous System Number
• Useful commands
show ip protocols
show ip route
debug ip igrp events
debug ip igrp transactions
ping
traceroute
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