Download show ip protocols - Febby Dian Anggraini

CCNA 2 v3.1 Module 7
Distance Vector Routing Protocols
© 2004, Cisco Systems, Inc. All rights reserved.
1
Objectives
Distance Vector Updates
Problem: Routing Loops
• Routing loops can occur when inconsistent routing
tables are not updated due to slow convergence in a
changing network.
Problem: Counting to Infinity
3.
A sends update to B and D
2.
4.
E sends update to A
C still unaware of failure,
advertises route via B
X
5.
D updates its routing table to include route from C,
and forwards this incorrect information to A.
1.
Network fails
Solution: Defining a Maximum for Infinity
• The routing protocol permits the routing update to propagate (loop) 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.
Solution: 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.
Solution: 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 for that network to infinity (one more than the maximum).
• Advertise it with
Solution: Triggered Updates
• Router detects topology change, immediately sends update to adjacent routers –
doesn’t wait for the update timer to expire.
• Wave of updates (single route not routing tables) propagates throughout the network.
• Ensure all routers know of failed routes before holddown timers expire.
RIP Triggered Updates
• Triggered updates can be configured for RIP
• int s0
• ip rip triggered
• Periodic broadcasts are suppressed on the interface
where triggered updates are enabled
Solution: Holddown Timers
The count to infinity problem can be avoided by using holddown timers.
When the router marks a route inaccessible it starts a holddown timer.
if a route with a worse metric is received for a route that is marked as inaccessible
ignore it until holddown timer expires (see next slide)
Holddown Timers
Update received:
network inaccessible,
start holddown timer
Is
update from same
neighbor?
Update received: network
accessible again
Yes
No
Ignore update,
Wait until holddown
timer expires
No
Different
neighbor,
better metric?
Yes
Network
accessible,
remove
holddown
timer
Key Characteristics of RIP
RIP
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.
RIP Commands
• We will now cover the following in more detail
• Using router rip to enable and start RIP
• Use the network commands to seed the routing
table
• Monitoring IP packet flow using the show ip
protocol command
• Displaying the routing table using the show ip
route command
Configuring RIP
Using the ip classless Command
I know some 10.0.0.0/24 subnets,
but not 10.2.2.0/24
So packet is dropped
IP Classless
• A router by default assumes that all subnets of a
directly connected network should be present in the
routing table.
• If a packet is received with an unknown destination
address within an unknown subnet of a directly
attached network, the router assumes that the subnet
does not exist. So the router will drop the packet
even if there is a default route.
• Configuring ip classless on the router resolves this
problem by allowing the router to ignore the classful
boundaries of the networks in its routing table and
simply route to the default route.
RIP Configuration Issues
• To reduce routing loops and counting to infinity,
RIP uses the following:
Defining infinity
Split horizon
Route poisoning & poison reverse
Triggered updates
Holddown timers
Preventing Routing Updates through an Interface
The show ip protocols Command
The show ip route Command
Typical RIP configuration errors:
•
incorrect network statement
•
discontiguous subnets (bad planning)
– can result in wrong route being advertised
•
split horizons
Troubleshooting RIP Update Issues
• debug ip rip
• show ip rip database
– Displays summary address entries in the RIP routing
database
• show ip protocols {summary}
• show ip route
– shows routing table
• debug ip rip {events}
– send and receive info
• show ip interface brief
– summarized information on protocols per interface
Debug Router RIP
Load Balancing with RIP
Here each path is considered
equal by RIP metric (2 hops)
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 _______________
Administrative Distance
Integrating Static Route with RIP
• Static routes are user-defined routes
• A router running RIP can receive a default route via an update from
another router running RIP.
• Another option is for the router to generate the default route itself.
• The administrator can override a static route with dynamic routing
information by adjusting the administrative distance values.
– The default administrative distance for RIP is 120
–By default the administrative distance of a static route is 1
–and that of a directly connected link is 0 (most desirable).
• Static routes are not normally distributed but distribution can be
enabled
Router(config) router rip
Router(config-router)# redistribute static
Floating Static Route
• A backup route that takes over if the original route
fails
• A route that has an AD of 130 will not be used until
the RIP route with an AD of 120 is not available.
IGRP
• 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 as it has multiple metric which can handle
different bandwidth and delay characteristics
• Scalable to large networks (max TTL 255)
IGRP Features
IGRP Commands
• Configure using the
–router igrp and network commands
• The show ip protocol command
– For monitoring IP packet flow
– Displays parameters including metric values K1 to K5.
K1= bandwidth, K3= delay.
• The show ip interfaces command
• The show ip route command
–Shows the actual metric values for each specific route
• The debug ip igrp command
IGRP Metrics
• Bandwidth – The lowest bandwidth value in the path
• Delay – The cumulative interface delay along the path
• Reliability – The reliability on the link towards the
destination as determined by the exchange of keepalives
• Load – The load on a link towards the destination based
on bits per second
• MTU – Maximum Transmission Unit for the path
IGRP Routes: Interior, System, & Exterior
• Interior
– 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 outside AS.
– Used to identify default gateway.
– Different routers may choose different routes as the gateway of
last resort.
• System
–Routes to networks within the AS.
–Derived from directly connected interfaces and information from
other IGRP-speaking devices.
–Do not include subnet information.
IGRP Stability Features
• 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.
Show IP Protocols
Show IP Protocols
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
poorer routes is ignored (Default: 3 x U + 10)
Configuring IGRP
Migrating RIP to IGRP
Migrating RIP to 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
Next
update
>
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
Verifying IGRP Configuration
• Some commands for checking IGRP configuration
are as follows:
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
Troubleshooting IGRP
• The following commands are useful when
troubleshooting IGRP:
show ip protocols {summary}
show ip route
debug ip igrp events IGRP protocol
events
debug ip igrp transactions IGRP protocol
transactions
ping
traceroute
Summary