4- en_ROUTE_v7_Ch04.pdf

Implementing Cisco IP Routing (ROUTE)

Chapter 4: Manipulating Routing Updates

Elaborated by: Ing. Ariel Germán For: ITLA Based on: Foundation Learning Guide CCNP ROUTE 300-101 Diane Teare, Bob Vachon, Rick Graziani 2015 ROUTE v6 Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Chapter 4 Topics  Using Multiple IP Routing Protocols on a Network

 Implementing Route Redistribution  Controlling Routing Update Traffic  Summary

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Using Multiple IP Routing Protocols on a Network

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 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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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 distance would get installed in routing table.

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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? 10.1.1.1

If a packet arrives on a router interface destined for 192.168.32.100, which route would the router choose? 10.1.1.2 Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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 routing table. Routes with higher AD are rejected.

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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 • Route filtering

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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 • Identify the different types of route redistribution

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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 add them to their respective routing tables.

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Default Seed Metrics

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Configuring and Verifying Basic Redistribution in IPv4 and IPv6

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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 delaymetric reliability-metric effective-bandwidth-metric mtu-bytes] [route-map map-tag]

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Redistributing OSPFv2 Routes into the EIGRP Routing Domain

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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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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. • Use the keyword include-connected

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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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Redistributing EIGRP Routes into the OSPFv2 Routing Domain

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Redistributing EIGRP Routes into the OSPFv2 Routing Domain

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Redistributing EIGRP Routes into the OSPFv2 Routing Domain

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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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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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.

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Metric = 7 / AD = 120

Metric = 6

Metric = 4

Metric = 6

Metric = 3 ?

Redistribution Problems

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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.  Use default routes to avoid having to do two-way redistribution. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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 boundary routers and on internal routers. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Controlling Routing Update Traffic

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 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

• Describe how to modify administrative distance

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Why Filter Routes?

 Avoid redistributing external routes.  Routing tables not too big.  Redistribution issues.

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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.  All other routes should not be permitted. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Distribute List and ACL Example

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Distribute List and ACL Example

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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.

• Extended access lists can be cumbersome to configure.

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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 that the subnet mask must be in a specified range.

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Configuring Prefix Lists

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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.  All other routes should not be permitted. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Distribute List and Prefix List Example

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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 ge 16 le 24. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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  R1 learns only about a default route (if one exists). Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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] • Resets the hit count shown on prefix list entries.

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Manipulating Redistribution Using ACLs, Prefix Lists, and Distribute Lists

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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 10.10.24.0/24 routes into the EIGRP routing domain.

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Redistributing OSPFv2 Routes into the EIGRP Routing Domain Using an ACL and Distribute List

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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 172.16.0.0/16 to /24 into the OSPF routing domain.

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Redistributing EIGRP Routes into the OSPF Routing Domain Using a Prefix List and Distribute List

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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 • Route maps are the primary tools for implementing BGP policy.

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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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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 sequencenumber parameter), the whole route map is deleted. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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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

match tag

Matches tag of a route

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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

Indicates where to output packets that pass a match clause of a route set ip default next-hop 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

set weight

Specifies the BGP weight value

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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 • deny: indicates that the matched route is not to be redistributed.

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Using Route Maps with Redistribution

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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.  Change administrative distance for certain routes to enable optimal routing. Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Mutual Redistribution without Route Filtering

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Mutual Redistribution without Route Filtering

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Mutual Redistribution with Route Maps

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Mutual Redistribution with Route Maps

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Mutual Redistribution with Route Maps

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Change Administrative Distance to Enable Optimal Routing

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Change Administrative Distance to Enable Optimal Routing

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Change Administrative Distance to Enable Optimal Routing

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Change Administrative Distance to Enable Optimal Routing

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Manipulating Redistribution Using Route Tagging

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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) • Inconsistent convergence time

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Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Summary

Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Summary

Check the book!

Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Additional resources  http://www.cisco.com/c/en/us/support/docs/ip/enhancedinterior-gateway-routing-protocol-eigrp/8651-21.html  http://www.ciscopress.com/articles/article.asp?p=2273507

 http://network-101.blogspot.com/2011/09/changing-ad-inospf.html

Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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Chapter 4 © 2007 – 2010, Cisco Systems, Inc. All rights reserved.

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