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Route OptimizationChapter 10

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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

List of routes or packets to permit or deny

Order of statements in access list is important 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

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 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

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: 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 0.0.0.255Router(config)#route-map POLICY1 permit 10Router(config-route-map)#match ip address 100Router(config-route-map)#set interface s0/0Router(config-route-map)#route-map POLICY1 permit 20Router(config-route-map)#match ip address 110Router(config-route-map)#set interface s0/1Router(config-route-map)#exitRouter(config)#interface s1/1Router(config-if)#ip policy route-map POLICY1Router(config-if)#int f0/0Router(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 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 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 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

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

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 Special type of static route Two ways to configure default route:

Use ip route 0.0.0.0 0.0.0.0 next-hop command

Use ip default-network network command

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 Identify routes whose metrics you

want to change Steps to change metrics

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

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