4- en_ROUTE_v7_Ch04.pdf

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Chapter 4:

Manipulating Routing

Updates

Implementing Cisco IP Routing (ROUTE)

Elaborated by: Ing. Ariel Germán

For: ITLA

Based on: Foundation Learning Guide

CCNP ROUTE 300-101

Diane Teare, Bob Vachon, Rick Graziani

2015

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Chapter 4

2 Chapter 4 Topics



Using Multiple IP Routing Protocols on a Network



Implementing Route Redistribution



Controlling Routing Update Traffic

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Using Multiple IP Routing

Protocols on a Network

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Chapter 4

4 

Upon completing this section, you will be able to do the

following:

• Describe the need for using more than one protocol in a network

• Describe how routing protocols interact

• Describe solutions for operating in a multiple routing protocol

environment

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Why Run Multiple Routing Protocols?



It is desirable to run a single routing protocol throughout an entire

IP internetwork.



However, in some circumstances we need to use more than one:

• When migrating from an older Interior Gateway Protocol (IGP) to a new

IGP.

• Mergers between companies.

• In mixed-router vendor environments.

• When the use of a new protocol is desired, but the old routing protocol is

needed.

• When some departments do not want to upgrade their routers to support

a new routing protocol.

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Chapter 4

6 Running Multiple Routing Protocols



When running multiple routing protocols, a router may learn

of a route from different routing sources.



If a router learns of a specific destination from two different

routing domains, the route with the lowest administrative

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Running Multiple Routing Protocols



If a router receives these routing updates (from three different sources),

which one(s) will be installed in the routing table?

• EIGRP (internal): 192.168.32.0/26

• RIP: 192.168.32.0/24

• OSPF: 192.168.32.0/19

If a packet arrives on a router interface destined for 192.168.32.1, which route would

the router choose?

If a packet arrives on a router interface destined for 192.168.32.100, which route would

the router choose?

10.1.1.1

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Chapter 4

8 Administrative Distance (AD)



Is used to rate a routing protocol’s believability (also called its

trustworthiness).



This criterion is the first thing a router uses to determine which

routing protocol to believe if more than one protocol provides

route information for the same destination.



The path with the lowest administrative distance is installed in the

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Multiple Routing Protocols Solutions



Careful routing protocol design and traffic optimization

solutions should be implemented when supporting complex

multiprotocol networks.



These solutions include the following:

• Summarization (Already covered in previous chapters)

• Redistribution between routing protocols

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Chapter 4

10 Implementing Route

Redistribution

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

Upon completing this section, you will be able to do the

following:

• Describe the need for route redistribution

• Identify some considerations for route redistribution

• Describe how to configure and verify route redistribution

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Chapter 4

12 Defining Route Redistribution



Is defined as the capability of boundary routers connecting

different routing domains to exchange and advertise routing

information between them.



Redistribution shares routing information about routes that

the router has learned with other routing protocols.

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Planning to Redistribute Routes



Network administrators must manage the migration from

one routing protocol to another, or to multiple protocols,

carefully and thoughtfully, or routing loops can occur.



To have a scalable solution and limit the amount of routing

update traffic, the redistribution process must selectively

insert the routes that are learned.



When a router redistributes routes, it only propagates routes

that are in the routing table.



Therefore, a router can redistribute dynamically learned

routes, static routes, and direct connected routes.

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Chapter 4

14 Redistributing Routes



Redistribution is always performed outbound.



This means that the router doing redistribution does not change

its own routing table.



Only downstream routers receiving the redistributed routes could

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Seed Metrics



Because redistributed routes are learned from other sources a boundary

router must be capable of translating the metric of the received route

from the source routing protocol into the receiving routing protocol.



The seed or default metric is defined during redistribution configuration.



After the seed metric for a redistributed route is established, the metric

increments normally within the autonomous system. (Except OE2).



The seed metric can be configured using either of the following:

• The default-metric router configuration command (for all redistributed routes).

• The redistribute router configuration command using either the metric option or a

route map.



To help prevent suboptimal routing and routing loops, always set the

initial seed metric to a value that is larger than the largest metric within

the receiving autonomous system.

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Chapter 4

16

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Configuring and Verifying Basic Redistribution

in IPv4 and IPv6

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Chapter 4

18 Redistributing OSPFv2 Routes into the EIGRP

Routing Domain



To redistribute routes from one routing domain into another routing

domain, use the redistribute router configuration command.



It is important to note that routes are redistributed into a routing

protocol, so the redistribute command is configured under the routing

process that is to receive the redistributed routes.



Because different routing protocols use different metrics, the

redistribute command parameters vary between routing protocols.



To configure redistribution into EIGRP, the following command syntax is

used:

• Router(config-router)# redistribute protocol process-id [metric bandwidth-metric

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Redistributing OSPFv2 Routes into the EIGRP

Routing Domain

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Chapter 4

20 Redistributing OSPFv2 Routes into the EIGRP

Routing Domain

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Redistributing OSPFv3 Routes into the EIGRP

for IPv6 Routing Domain

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Chapter 4

22 Redistributing OSPFv3 Routes into the EIGRP

for IPv6 Routing Domain

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Redistributing OSPFv3 Routes into the EIGRP

for IPv6 Routing Domain



Unlike in IPv4, in IPV6 EIGRP does not automatically

include connected routes in the redistribution.

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Chapter 4

24 Redistributing OSPFv3 Routes into the EIGRP

for IPv6 Routing Domain

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Redistributing EIGRP Routes into the OSPFv2

Routing Domain



To configure redistribution into OSPF, use the following

command syntax:



The subnets keyword is necessary for classless networks

to be advertised.



Without this keyword, only routes that are in the routing

table with the default classful mask will be redistributed.

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Chapter 4

26 Redistributing EIGRP Routes into the OSPFv2

Routing Domain

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Redistributing EIGRP Routes into the OSPFv2

Routing Domain

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Chapter 4

28 Redistributing EIGRP Routes into the OSPFv2

Routing Domain



External link-state advertisements (LSAs) appear in the routing

table and are marked as external type 1 (E1) or external type 2

(E2) routes.

• E1: Type O E1 external routes calculate the cost by adding the external

cost to the internal cost of each link that the packet crosses.

• Use this type when there are multiple ASBRs advertising an external route to

the same autonomous system.

• E2 (default): The external cost of O E2 routes is fixed and does not

change across OSPF domain.

• Use this type if only one ASBR is advertising an external route to the

autonomous system.



If an OSPF router receives both type E1 and type E2 routes for

the same destination, the type E1 route is always preferred over

type E2 regardless of the actual calculated cost.

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Redistributing EIGRP Routes into the OSPFv2

Routing Domain

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Chapter 4

30 Redistributing EIGRP for IPv6 Routes into the

OSPFv3 Routing Domain

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Redistributing EIGRP for IPv6 Routes into the

OSPFv3 Routing Domain

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Chapter 4

32 Types of Redistribution Techniques



One-Point Redistribution:

• One way

• Two way



Multipoint Redistribution:

• One way

• Two way

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One-Point Redistribution (One way)



This method only redistributes the networks learned from one routing

protocol into the other routing protocol.



R1 performs one-way redistribution because it only redistributes AS1

routes into the AS2 routing domain.



AS2 routes are not being redistributed in AS1.



Typically, AS1 routers would require the use of a default route or one or

more static routes to reach AS2 routes.

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Chapter 4

34 One-Point Redistribution (Two way)



This method redistributes routes between the two routing processes in

both directions.



R1 provides two-way redistribution because it redistributes AS1 routes

into AS2 and AS2 routes into AS1.



One-way or two-way redistribution at one point is always safe.



One-point redistribution represents the only exit and entrance from one

routing protocol to another.

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Multipoint Redistribution (One way)



This method consists of two or more boundary routers only

redistributing networks learned from one routing protocol into the other

routing protocol.



The boundary routers R3 and R4 are both redistributing AS1 routes into

the AS2 routing domain.



Again, AS1 routers would require the use of a default route or one or

more static routes to reach AS2 routes.

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Chapter 4

36 Multipoint Redistribution (Two way)



Also referred to as mutual redistribution, this method consists of two or

more boundary routers redistributing routes in both directions.



Multipoint redistribution is likely to introduce potential routing loops.



Multipoint one-way redistribution is problematic, and multipoint two-way

redistribution is highly dangerous.

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Redistribution Problems



Problems that can occur during multipoint two-way

redistribution include the following:

• Suboptimal routing. (Only part of the total cost is considered in routing

decisions.)

• Self-sustained routing loops upon route loss.

If metric is lost, R1

might choose R2 to

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Chapter 4

38 Redistribution Problems

M

etric

=

6 Metric = 6

Metric

=

7 /

AD

=

120 M

et

ric

=

3 ?

Metric = 4

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Preventing Routing Loops in a Redistribution

Environment



The safest way to perform redistribution is to redistribute routes in

only one direction, on only one boundary router within the

network.



Only redistribute internal routes from one autonomous system to

another (and vice versa).



Tag routes in redistribution points and filter based on these tags

when configuring redistribution in the other direction.



Propagate metrics from one autonomous system to another

autonomous system properly.

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Chapter 4

40 Verifying Redistribution Operation



Know your network topology, particularly where redundant routes

exist.



Study the routing tables on a variety of routers in the internetwork

using the show ip route [ip-address] EXEC command.



Examine the topology table of each configured routing protocol to

ensure that all appropriate prefixes are being learned.



Perform a trace using the traceroute [ip-address] EXEC

command on some of the routes that go across the autonomous

systems to verify that the shortest path is being used for routing.



If you encounter routing problems, use the traceroute and

debug commands to observe the routing update traffic on the

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Controlling

Routing

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Chapter 4

42 

Upon completing this section, you will be able to do the

following:

• Describe the general mechanics and need for route filtering

• Identify how to use and configure distribute lists

• Identify how to use and configure prefix lists

• Identify how to use and configure route maps

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Why Filter Routes?



Avoid redistributing external routes.



Routing tables not too big.

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Chapter 4

44 Route Filtering Methods



Routing updates compete with user data for bandwidth and router

resources.



Information about networks must be sent where it is needed and filtered

from where it is not needed.



This can involve controlling routing update traffic using static and default

routes, and passive interfaces.



However, more advanced route filtering mechanisms are available:

• Distribute lists: A distribute list allows an access control lists (ACLs) to be applied to

routing updates.

• Prefix lists: A prefix list is an alternative to ACLs designed to filter routes. It can be

used with distribute lists, route maps, and other commands.

• Route maps: Route maps are complex access lists that allow conditions to be tested

against a packet or route, and then actions taken to modify attributes of the packet or

route.

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Using Distribute Lists



Classic ACLs do not affect traffic that is originated by the

router.



When you link an ACL to a distribute list, routing updates

can be controlled no matter what their source is.



ACLs are configured in the global configuration mode and

are then associated with a distribute list under the routing

protocol.



The ACL should permit the networks that should be

advertised or redistributed and deny the networks that

should be filtered.

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Chapter 4

46 Configuring Distribute Lists



A distribute list filter can be applied for received, sent or

redistributed routes.



The distribute-list out command filters updates going out of the

interface or routing protocol specified in the command, into the

routing process under which it is configured.



The distribute-list in command filters updates going into the

interface specified in the command, into the routing process

under which it is configured.

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Distribute List and ACL Example



In this example, R3 must redistribute EIGRP routes into the

OSPF domain with a metric of 40.



However, the administrator only wants to allow the 10.10.11.0/24

and 10.10.12.0/24 routes to be propagated.

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Chapter 4

48

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Chapter 4

50 Using Prefix Lists



Traditionally, route filtering was accomplished using ACLs

with the distribute-list command.



However, using ACLs as route filters for distribute lists has

several drawbacks, including the following:

• A subnet mask cannot be easily matched.

• Access lists are evaluated sequentially for every IP prefix in the

routing update.

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Prefix List Characteristics



The advantages of using prefix lists include the following:

• Friendlier command-line interface.

• Faster processing: A router transforms a prefix list into a tree

structure, with each branch of the tree serving as a test.

• Support for incremental modifications: Sequence numbers are

assigned to ip prefix-list statements, making it easier to edit.

• Greater flexibility: Routers match networks in a routing update

against the prefix list using as many bits as indicated.

• A prefix list can specify the exact size of the subnet mask, or it can indicate

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Chapter 4

52

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Distribute List and Prefix List Example



In this example, R3 must redistribute EIGRP routes into the

OSPF domain with a metric of 40.



However, the administrator only wants to allow the 10.10.11.0/24

and 10.10.12.0/24 routes to be propagated.

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Chapter 4

54

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Prefix List Examples



ip prefix-list TEST permit 172.0.0.0/8 le 24

• R1 learns about 172.16.0.0/16, 172.16.10.0/24, and 172.16.11.0/24.

• These are the routes that match the first 8 bits of 172.0.0.0 and have a prefix length

between 8 and 24.



ip prefix-list TEST permit 172.0.0.0/8 le 16

• R1 learns only about 172.16.0.0/16.

• This is the only route that matches the first 8 bits of 172.0.0.0 and has a prefix length

between 8 and 16.

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Chapter 4

56 Prefix List Examples



ip prefix-list TEST permit 172.0.0.0/8 ge 17

• R1 learns only about 172.16.10.0/24 and 172.16.11.0/24.

• Router A ignores the /8 parameter and treats the command as if it has the parameters

ge 17 le 32.



ip prefix-list TEST permit 172.0.0.0/8 ge 16 le 24

• R1 learns about 172.16.0.0/16, 172.16.10.0/24, and 172.16.11.0/24.

• Router A ignores the /8 parameter and treats the command as if it has the parameters

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Prefix List Examples



ip prefix-list TEST permit 172.0.0.0/8 ge 17 le 23



R1 does not learn about any network.



ip prefix-list TEST permit 0.0.0.0/0 le 32



R1 learns about all EIGRP Routes.



ip prefix-list TEST permit 0.0.0.0/0

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Chapter 4

58 Verifying Prefix Lists



Use the show ip prefix-list ? command to see all the show commands

available for prefix lists.



show ip prefix-list [detail | summary]

• Displays information on all prefix lists. Specifying the detail keyword includes the

description and the hit count.



show ip prefix-list prefix-list-name [network/length]

• Displays the policy associated with a specific network/ length in a prefix list.



clear ip prefix-list prefix-list-name [network/length]

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Manipulating Redistribution Using ACLs, Prefix

Lists, and Distribute Lists

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Chapter 4

60 Redistributing OSPFv2 Routes into the EIGRP

Routing Domain Using an ACL and Distribute List



R1 will be configured to specifically filter and not redistribute

the 10.10.21.0/24, 10.10.22.0/24, 10.10.23.0/24, and

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Redistributing OSPFv2 Routes into the EIGRP

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Chapter 4

62 Redistributing EIGRP Routes into the OSPF

Routing Domain Using a Prefix List and Distribute

List



R1 will be configured to specifically filter and only

redistribute all matching prefixes in the range of

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Redistributing EIGRP Routes into the OSPF

Routing Domain Using a Prefix List and Distribute

List

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Chapter 4

64 Using Route Maps



Route maps provide another technique to manipulate and

control routing protocol updates.



Route maps may be used for a variety of purposes.



All the IP routing protocols can use route maps for

redistribution filtering.

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Understanding Route Maps



Route maps are complex access lists



They allow some conditions to be tested against the packet or

route in question using match commands.



If the conditions match, some actions can be taken to modify

attributes of the packet or route.

• These actions are specified by set commands.



A collection of route-map statements that have the same route

map name is considered one route map.



One major difference between route maps and access lists is that

route maps can use the set commands to modify the packet or

route.

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Chapter 4

66 Route Map Applications



Route filtering during redistribution

• Although distribute lists can be used for this purpose, route maps offer

the added benefit of manipulating routing metrics through the use of

set commands.



Policy-based routing (PBR)

• When a match occurs, a set command can be used to define the

interface or next-hop address to which the packet should be sent.



BGP

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Configuring Route Maps



Step 1. Define the route map using the route-map global

configuration command.



Step 2. Define the matching conditions using the match

command and optionally the action to be taken when each

condition is matched using the set command.



Step 3. Apply the route map.



To define a route map, use the route-map map-tag [permit

| deny] [sequence-number] global configuration command.

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Chapter 4

68 Configuring Route Maps



A route map may be made up of multiple route-map statements (with

different sequence numbers).



The statements are processed top-down, similar to an access list. The

first match found for a route is applied.



Route map sequence numbers do not automatically increment. When

the sequence-number parameter of the route-map command is not

used, the following occurs:

• If no other entry is already defined with the supplied route-map map-tag, an entry

is created, with the sequence-number set to 10.

• If only one entry is already defined with the supplied route-map tag, The router

assumes you are editing the one entry that is already defined.

• If more than one entry is already defined with the supplied route-map tag, an

error message is displayed, indicating that the sequence-number is required.

• If the no route-map map-tag command is specified (without the

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Configuring Route Maps



Like an access list, an implicit deny any appears at the end

of a route map. The consequences of this deny depend on

how the route map is being used.



The match condition route map configuration commands

are used to define the conditions to be checked.



The set condition route map configuration commands are

used to define the actions to be followed if there is a match

and the action to be taken is permit.



A route-map statement without any match statements will

be considered matched.

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Chapter 4

70 Configuring Route Maps



A single match statement may contain multiple conditions. Only

one condition in the same match statement must be true for that

match statement to be considered a match. (Logical OR)



route-map statement may contain multiple match statements. All

match statements within a route-map statement must be

considered true for the route-map statement to be considered

matched. (Logical AND).

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Route Map Match and Set Statements

Command

Description

match community

Matches a BGP community

match interface

Matches any routes that have the next hop out of one of the

interfaces specified

match ip address

Matches any routes that have a destination network number

address that is permitted by a standard or extended ACL

match ip next-hop

Matches any routes that have a next-hop router address that is

passed by one of the ACLs specified

match ip route-source

Matches routes that have been advertised by routers and access

servers at the address that is specified by the ACLs

match length

Matches based on the layer 3 length of a packet

match metric

Matches routes with the metric specified

match route-type

Matches routes of the specified type

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Chapter 4

72 Route Map Match and Set Statements

Command

Description

set as-path

Modifies an AS path for BGP routes

set automatic-tag

Computes automatically the tag value

set community

Sets the BGP communities attribute

set default interface

Indicates where to output packets that pass a match clause of a route

map for policy routing and have no explicit route to the destination

set interface

Indicates where to output packets that pass a match clause of a route

map for policy routing

set ip default next-hop

Indicates where to output packets that pass a match clause of a route

map for policy routing and for which the Cisco IOS software has no

explicit route to a destination

set ip next-hop

Indicates where to output packets that pass a match clause of a route

map for policy routing

set level

Indicates where to import routes for IS-IS and OSPF

set local-preference

Specifies a BGP local preference value

set metric

Sets the metric value for a routing protocol

set metric-type

Sets the metric type for the destination routing protocol

set tag

Sets tag value for destination routing protocol

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Configuring Route Redistribution Using Route

Maps



Use route maps when you want detailed control over how

routes are redistributed between routing protocols.



The redistribute command has a route-map keyword with

a map-tag parameter.



It is important to understand what the permit and deny

mean when redistributing.

• permit: indicates that the matched route is to be redistributed

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Chapter 4

74

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Manipulating Redistribution Using Route Maps



R1 and R4 will be configured to support mutual redistribution without any filtering

mechanism.



R1 and R4 will be configured to support mutual redistribution using route maps.

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Chapter 4

76

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Chapter 4

78

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Chapter 4

80

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Change Administrative Distance to Enable Optimal

Routing

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Chapter 4

82 Change Administrative Distance to Enable Optimal

Routing

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Change Administrative Distance to Enable Optimal

Routing

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Chapter 4

84 Change Administrative Distance to Enable Optimal

Routing

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Manipulating Redistribution Using Route

Tagging

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Chapter 4

86 Caveats of Redistribution



Redistribution of routing information adds to the complexity

of a network and increases the potential for routing

confusion, so you should use it only when necessary.



The key issues that arise when you are using redistribution

are as follows:

• Routing loops

• Incompatible routing information (suboptimal routing)

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Chapter 4

88

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Summary

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Chapter 4

90 Additional resources



http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html