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Route Optimization. Chapter 10. 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. Filtering. Access Groups Distribute List Route Maps. Access Lists.

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route filters
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
  • Access Groups
  • Distribute List
  • Route Maps
access lists
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
access groups
Access Groups
  • Access Groups are applying the access lists you learned about in 240

Router(config)#access list 100 deny ip

Router(config)#access list 100 permit ip any any

Router(config)#int f0/0

Router (config-if)#ip access-group 100 in

how route filters work
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
configuring route filters
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
distribute list
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
Router(config)#access list 1 deny

Router(config)#access list 1 permit any

Router(config)#access list 2 deny

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

using route maps to implement routing policy
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
how route maps work
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
processing route maps
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
policy based routing
Policy-Based Routing

Router(config)#access-list 100 permit ip

Router(config)#access-list 100 permit ip

Router(config)#access-list 110 permit ip

Router(config)#access-list 110 permit ip

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)#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

benefits and disadvantages of route redistribution
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
redistributing routes
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
potential redistribution problems
Potential Redistribution Problems
  • Several potential problems
  • Routing loops
  • Poor path selection
  • Inconsistent convergence times
protocol considerations
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
configuring route redistribution
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
setting default metrics
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
rip redistribution
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

igrp distribution
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

ospf distribution
OSPF Distribution

Router(config)# router ospf 1

Router(config-router)# redistribute eigrp 100 metric 100 subnets

Allows Classless routers to be redistributed

static default and connected routes
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
redistributing static routes
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
redistributing connected routes
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
redistributing default routes
Redistributing Default Routes
  • Special type of static route
  • Two ways to configure default route:
    • Use ip route 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
redistributing into classful routing protocols
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
two strategies to solve problem
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
clean up routing tables before redistribution
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
changing metrics with route maps
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
one way redistribution through a single border router
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
one way redistribution through multiple border routers
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
possible problems with multiple border routers
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
eigrp autonomous system connected to ospf as
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
redistributing ospf into eigrp
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
redistributing eigrp into ospf
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
monitoring and troubleshooting redistribution
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
troubleshooting redistribution problems
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