Route Optimization
Chapter 10
1
Route Filters
Use access list to filter out unwanted
routes
 Identifies packets or addresses to be
filtered
 Prevents certain routes from being
advertised
 Controls routing updates
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Filtering
Access Groups
 Distribute List
 Route Maps
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Access Lists

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List of routes or packets to permit or deny
Order of statements in access list is
important
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If packet matches access list, router goes to
next statement
If packet does not match access list, router
denies or permits packet
If packet does not match anything on access
list, implicit denial causes router to deny packet
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Access Groups

Access Groups are applying the
access lists you learned about in 240
Router(config)#access list 100 deny ip 192.168.24.0
0.0.0.255 192.168.50.0 0.0.0.255
Router(config)#access list 100 permit ip any any
Router(config)#int f0/0
Router (config-if)#ip access-group 100 in
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How Route Filters Work

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Route filters use access lists to accept or
reject routes
Filter both routes advertised and incoming
routes accepted from other routers
Router examines interface to see if routing
filter is applied

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If so, examines access list to see if route should
be dropped in update
If route matches statement in list, processes it
according to deny or permit keyword
If route does not match statement, route is
dropped by implicit denial
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Configuring Route Filters

Steps to configure route filter include
Creating access list to match routes
 Determine which interface(s) to apply route
filter to and whether it will apply to incoming
or outgoing route updates
 Apply route filter with distribute-list
command
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Distribute List
Blocks the advertising of a route by
using a Access List
 Applied to the Router not the
interface
 Can block out a certain interface or all

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Router(config)#access list 1 deny 192.168.50.0 0.0.0.255
Router(config)#access list 1 permit any
Router(config)#access list 2 deny 192.168.88.0 0.0.0.255
Router(config)#access list 2 permit any
Router(config)#router eigrp 100
Router(config-router)#distribute-list 1 out s1
Router(config-router)#distribute-list 2 out
Router(config)#access list 100 permit ip any any
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Using Route Maps to
Implement Routing Policy

Configure policy-based routing

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Routes packets differently based on properties of
packets
Introduced in Cisco 11.0
Use to mark packet with precedence or TOS value
Provides different Quality of Service (QOS) to
different types of traffic
Can use values in queuing packets
Allows service providers to route packets from
different sources through different paths
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How Route Maps Work

Route maps make policies based on
attributes of a packet:

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Source address of packet
Protocol
Application
Packet size
Route map has series of permit and deny
statements

Unlike access lists, route maps are processed in
order specified by sequence number
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Processing Route Maps

Each permit or deny statement works
like if/then statement
If packet matches the match statement,
router applies set command to packet
 If packet doesn’t match any statement in
route map, it is denied

 May
add statement at end to match all
packets calling for some default action
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Policy-Based Routing
Router(config)#access-list 100 permit ip 192.168.200.0 0.0.0.255
192.168.50.0 0.0.0.255
Router(config)#access-list 100 permit ip 192.168.200.0 0.0.0.255
192.168.100.0 0.0.0.255
Router(config)#access-list 110 permit ip 192.168.50.0 0.0.0.255
Router(config)#access-list 110 permit ip 192.168.100.0 0.0.0.255
Router(config)#route-map POLICY1 permit 10
Router(config-route-map)#match ip address 100
Router(config-route-map)#set interface s0/0
Router(config-route-map)#route-map POLICY1 permit 20
Router(config-route-map)#match ip address 110
Router(config-route-map)#set interface s0/1
Router(config-route-map)#exit
Router(config)#interface s1/1
Router(config-if)#ip policy route-map POLICY1
Router(config-if)#int f0/0
Router(config-if)#ip policy route-map POLICY1
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Benefits and Disadvantages
of Route Redistribution

Redistribute routes for a variety of
reasons
When two organizations merge, to
redistribute routes between Autonomous
Systems
 When migrating from one interior
gateway protocol to another
 When you must use multiple routing
protocols on parts of network
 To use different routing protocols on
different sections of hierarchical network
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Redistributing Routes
• Allows one routing protocol to
exchange information with different
routing protocol
• Border router takes routes learned
from one source of routing
information and injects them into
second

Alternative to using work-intensive static
routing
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Two Connected Autonomous
Systems
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Potential Redistribution
Problems
Several potential problems
 Routing loops
 Poor path selection
 Inconsistent convergence times
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Protocol Considerations
• Routing protocols must support same
routed protocol stack to redistribute
• Protocol determines how you redistribute
routes
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Automatically redistributes between EIGRP and
IGRP in same Autonomous System
EIGRP metrics equal IGRP metrics multiplied
by 256
Automatically redistributes between IPX and
AppleTalk
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Configuring Route
Redistribution

Steps to configure basic route
redistribution
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Identify border routers that will redistribute
routes
Decide which protocols will inject routes into
other routing protocol
Enter routing configuration mode for protocol
that will learn routes
Configure route distribution between two
routing protocols
May need to redistribute one instance of
routing protocol into another
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Setting Default Metrics
• Two ways to set a default or seed metric
• Use the default-metric command with
arguments

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Bandwidth - minimum in Kbps
Delay - in tens of microseconds
Reliability - with number from 1-255 where 255
means 100% reliable
Loading - with number from 1-255 where 255
means 100% loaded
MTU - in bytes
• Use metric-value keyword with
redistribute command
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RIP Redistribution
Router(config)#router rip
Router(config-router)#redistribute igrp 100
metric 1
Router(config)#router rip
Router(config-router)#default-metric 1
Router(config-router)#redistribute igrp 100
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IGRP Distribution
Router(config)#router igrp 100
Router(config-router)#redistribute rip metric 100 100 200 1
1500
Router(config)#router igrp 100
Router(config-router)#default-metric 100 100 200 1 1500
Router(config-router)#redistribute rip
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OSPF Distribution
Router(config)# router ospf 1
Router(config-router)# redistribute eigrp 100 metric 100
subnets
Allows Classless routers to
be redistributed
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Static, Default, and
Connected Routes

Situations where static or default
routes are better than dynamic routes
Stub networks with only one outgoing
connection
 Internet connections
 Back-up links
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Static routes must be redistributed for
other routers in Autonomous System to
use them
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Redistributing Static Routes
• To configure a static route, use ip
route command
Route tag lets you match a static route
in route maps
 Permanent keyword makes router keep
route in its routing table even if
associated interface goes down

• Static routes can be configured to go
through an interface instead of next
hop
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Redistributing Connected
Routes

Use redistribute connected
command to redistribute any
connected network
By default, all connected networks are
redistributed
 Can use route filter to remove network
that should not be redistributed

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Use redistribute static
command to redistribute between
routing protocols
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Redistributed Static and
Connected Routes
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Redistributing Default
Routes

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Special type of static route
Two ways to configure default route:
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Use ip route 0.0.0.0 0.0.0.0 next-hop
command
Use ip default-network network
command

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Can help router choose default gateway or gateway
of last resort
Protocol determines how commands are
used
Weigh benefits of static and dynamic
routes against disadvantages
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Redistributing into Classful
Routing Protocols

Potential problems when
redistributing between classless
routing protocols with VLSMs those
that do not support VLSMs

Classless routing protocols may inject
routes that are not subnetted along
classful boundaries
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Two Strategies to Solve
Problem
Propagate default route to OSPF
domain through RIPv1 domain
 Summarize or filter routes injected
from OSPF domain to make RIPv1
routers learn only about routes with
classful netmasks
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Clean Up Routing Tables
Before Redistribution

Prevent potential problems by
simplifying topology of each
Autonomous System before
redistribution
Reduce amount of routing information
that each protocol redistributes
 Reduce size of routing table
 Use route filters and route maps
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Changing Metrics with Route
Maps

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Identify routes whose metrics you want to
change
Steps to change metrics
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Write access list identifying routes to be
matched
Define route map identifying routes with match
statement
Use set command to set metric for routes
Decide how to handle all other routes
Use redistribution command to activate
redistribution, set default metric, and activate
the route map
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One-Way Redistribution Through a
Single Border Router
• Two Autonomous Systems
redistributing routes at a single router
Packets traveling from one AS to other
go through border router
 For RIP domain, redistribute default
route and use ip classless command
 For EIGRP domain, use static route to
allow EIGRP AS to learn about routes in
RIP AS

• Disadvantage is single point of failure
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Redistributing from RIP into EIGRP
with a Default Route
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One-Way Redistribution Through
Multiple Border Routers
Connect two Autonomous Systems at
multiple places to avoid single point
of failure
 Set seed metric for RIP router
redistribution into EIGRP
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Possible Problems with
Multiple Border Routers

Possible problem since both EIGRP
and RIP propagate default routes
All packets sent on Internet may enter
routing loop if primary route to Internet
is down
 Solution is to filter default route so that
EIGRP Autonomous System does not
learn about it
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EIGRP Autonomous System
Connected to OSPF AS
Each Autonomous System has default
route to the Internet
 Neither Autonomous System can use
a default route to get to each other
 Each Autonomous System must learn
routes from the other
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Redistributing OSPF into
EIGRP
Set seed metrics for routes
redistributed into each routing
protocol
 Use match keyword to match only
certain types of routes
 Use subnets keyword to have
protocol inject information about
subnets
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Redistributing EIGRP into
OSPF
Redistribution does not allow control
of types of routes injected into OSPF
 Use a route filter to eliminate
problems

Identify networks to be filtered and
make an access list
 Use distribute-list out command to apply
access list
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Monitoring and Troubleshooting
Redistribution

Two ways to verify that route
redistribution is configured
Use show running-config command
 Use show ip protocols command
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Use show ip route command to
examine routing tables
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Troubleshooting
Redistribution Problems
• Examine topology of each involved
Autonomous System
• Examine routing tables of any border
routers
• Examine routing tables of routers inside
each Autonomous System
• Use ping and traceroute commands to
check routes crossing the boundary
between Autonomous Systems
• Use debug commands on routers that
seem to have a problem
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