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1 Route Redistribution Basics - Route Redistribution - It is preferable to employ a single routing protocol in an internetwork environment, for simplicity and ease of management. Unfortunately, this is
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1 Route Redistribution Basics - Route Redistribution - It is preferable to employ a single routing protocol in an internetwork environment, for simplicity and ease of management. Unfortunately, this is not always possible, making multi-protocol environments common. Route Redistribution allows routes from one routing protocol to be advertised into another routing protocol. The routing protocol receiving these redistributed routes usually marks the routes as external. External routes are usually less preferred than locally-originated routes. At least one redistribution point needs to exist between the two routing domains. This device will actually run both routing protocols. Thus, to perform redistribution in the following example, RouterB would require at least one interface in both the EIGRP and the OSPF routing domains: It is possible to redistribute from one routing protocol to the same routing protocol, such as between two separate OSPF domains (distinguished by unique process ID s). Static routes and connected interfaces can be redistributed into a routing protocol as well. Routes will only be redistributed if they exist in the routing table. Routes that are simply in a topology database (for example, an EIGRP Feasible Successor), will never be redistributed. Routing metrics are a key consideration when performing route redistribution. With the exception of IGRP and EIGRP, each routing protocol utilizes a unique (and thus incompatible) metric. Routes redistributed from the injecting protocol must be manually (or globally) stamped with a metric that is understood by the receiving protocol. (Reference: 2 Redistributing into RIP RIP is a standardized Distance-Vector routing protocol that uses hop-count as its distance metric. Consider the following example: RouterB is our redistribution point between IGRP and RIP. To redistribute all IGRP routes into RIP: RouterB(config)# router rip RouterB(config-router)# network RouterB(config-router)# redistribute igrp 10 metric 2 First, the router rip process was enabled. Next, RIP was configured to advertise the network of /16. Finally, RIP was configured to redistribute all igrp routes from Autonomous System 10, and apply a hopcount metric of 2 to the redistributed routes. If a metric is not specified, RIP will assume a metric of 0, and will not advertise the redistributed routes. Redistributing into IGRP IGRP is a Cisco-proprietary Distance-Vector routing protocol that, by default, uses a composite of bandwidth and delay as its distance metric. IGRP can additionally consider Reliability, Load, and MTU for its metric. Still using the above example, to redistribute all RIP routes into IGRP: RouterB(config)# router igrp 10 RouterB(config-router)# network RouterB(config-router)# redistribute rip metric First, the router igrp process was enabled for Autonomous System 10. Next, IGRP was configured to advertise the network of /8. Finally, IGRP was configured to redistribute all rip routes, and apply a metric of (bandwidth), 1000 (delay), 255 (reliability), 1 (load), and 1500 (MTU) to the redistributed routes. 3 Redistributing into EIGRP EIGRP is a Cisco-proprietary hybrid routing protocol that, by default, uses a composite of bandwidth and delay as its distance metric. EIGRP can additionally consider Reliability, Load, and MTU for its metric. To redistribute all OSPF routes into EIGRP: RouterB(config-router)# network RouterB(config-router)# redistribute ospf 20 metric First, the router eigrp process was enabled for Autonomous System 15. Next, EIGRP was configured to advertise the network of /24. Finally, EIGRP was configured to redistribute all ospf routes from process- ID 20, and apply a metric of (bandwidth), 1000 (delay), 255 (reliability), 1 (load), and 1500 (MTU) to the redistributed routes. It is possible to specify a default-metric for all redistributed routes: RouterB(config-router)# redistribute ospf 20 RouterB(config-router)# default-metric RIP and IGRP also support the default-metric command. Though IGRP/EIGRP use only bandwidth and delay by default to compute the metric, it is still necessary to specify all five metrics when redistributing. If the default-metric or a manual metric is not specified, IGRP/EIGRP will assume a metric of 0, and will not advertise the redistributed routes. Redistribution will occur automatically between IGRP and EIGRP on a router, if both processes are using the same Autonomous System number. EIGRP, by default, will auto-summarize internal routes unless the no autosummary command is used. However, EIGRP will not auto-summarize external routes unless a connected or internal EIGRP route exists in the routing table from the same major network of the external routes. 4 Redistributing into OSPF OSPF is a standardized Link-State routing protocol that uses cost (based on bandwidth) as its link-state metric. An OSPF router performing redistribution automatically becomes an ASBR. To redistribute all EIGRP routes into OSPF: RouterB(config)# router ospf 20 RouterB(config-router)# network area 0 RouterB(config-router)# redistribute eigrp 15 RouterB(config-router)# default-metric 30 First, the router ospf process was enabled with a process-id of 20. Next, OSPF was configured to place any interfaces in the network of /16 into area 0. Then, OSPF will redistribute all eigrp routes from AS 15. Finally, a default-metric of 30 was applied to all redistributed routes. If the default-metric or a manual metric is not specified for the redistributed routes, a default metric of 20 will be applied to routes of all routing protocols except for BGP. Redistributed BGP routes will have a default metric of 1 applied by OSPF. By default, OSPF will only redistribute classful routes into the OSPF domain. To configure OSPF to accept subnetted networks during redistribution, the subnets parameter must be used: RouterB(config)# router ospf 20 RouterB(config-router)# redistribute eigrp 15 subnets Routes redistributed into OSPF are marked external. OSPF identifies two types of external routes, Type-1 (which is preferred) and Type-2 (which is default). To change the type of redistributed routes: RouterB(config)# router ospf 20 RouterB(config-router)# redistribute eigrp 15 subnets metric-type 1 5 Redistributing Static and Connected Routes Redistributing static routes into a routing protocol is straightforward: RouterB(config-router)# redistribute static Redistributing networks on connected interfaces into a routing protocol is equally straightforward: RouterB(config-router)# redistribute connected The above commands redistribute all connected networks into EIGRP. Route-maps can be used to provide more granular control: RouterB(config)# route-map CONNECTED permit 10 RouterB(config-route-map)# match interface fa0/0, fa0/1, s0/0, s0/1 RouterB(config-router)# redistribute connected route-map CONNECTED Connected networks can be indirectly redistributed into a routing protocol. Recall that routes will only be redistributed if they exist in the routing table, and consider again the following example: If RouterB is configured as follows: RouterB(config-router)# network RouterB will advertise the /24 network to RouterA, but it will not have an EIGRP route in its routing table for that network, as the network is directly connected. Despite this, when redistributing EIGRP into OSPF, the /24 is still injected into OSPF. The network command under the EIGRP process will indirectly redistribute this network into OSPF. 6 Pitfalls of Route Redistribution Administrative Distance Route redistribution introduces unique problems when there are multiple points of redistribution. Consider the following diagram: The first issue is caused by Administrative Distance (AD), which determines which routing protocol is trusted the most. By default, OSPF routes have an AD of 110, whereas RIP routes have an AD of 120. Lowest AD is preferred, thus making the OSPF routes the most trusted. Assume mutual redistribution has been performed on RouterC and RouterD. The following networks will be injected from RIP into OSPF: /24, /24, /24, /24, and /24. RouterC will eventually receive OSPF routes to the above networks from RouterD, in addition to the RIP routes already in its table. Likewise, RouterD will receive OSPF routes to these networks from RouterC. Because OSPF s AD is lower than RIP s, both RouterC and RouterD will prefer the sub-optimal path through OSPF to reach the non-connected networks. Thus, RouterC will choose the OSPF route for all the 10.x.x.x/24 networks except for /24, as it is already directly connected. This actually creates a routing loop. RouterC will prefer the OSPF path through RouterA to reach the 10.x.x.x networks (except for /24), and RouterA will likely consider RouterC its shortest path to reach those same networks. Traffic will be continuously looped between these two routers. Even if RouterC managed to send the traffic through RouterA and RouterB to RouterD, the preferred path to the 10.x.x.x networks for RouterD is still through OSPF. Thus, the routing loop is inevitable. 7 Pitfalls of Route Redistribution Administrative Distance (continued) There are two methods to correct this particular routing loop. The first method involves filtering incoming routes using a distribution-list, preventing RouterC and RouterD from accepting any routes that originated in RIP from their OSPF neighbors. RouterC s configuration would be as follows: RouterC(config)# access-list 10 deny RouterC(config)# access-list 10 deny RouterC(config)# access-list 10 deny RouterC(config)# access-list 10 deny RouterC(config)# access-list 10 permit any RouterC(config)# router ospf 20 RouterC(config-router)# distribute-list 10 in fastethernet0/0 An access-list was created that is denying the RIP networks in question, and permitting all other networks. Under the OSPF process, a distribute-list is created for routes coming inbound off of the fastethernet0/0 interface. The access-list and distribute-list numbers must match. RouterD s configuration would be similar. This prevents each router from building OSPF routes for the networks that originated in RIP, and thus eliminates the possibility of a loop. However, redundancy is also destroyed if RouterC s fa0/1 interface were to fail, it could not choose the alternate path through OSPF. 8 Pitfalls of Route Redistribution Administrative Distance (continued) The second method involves using the distance command to adjust the AD of specific routes. This can accomplished two ways: Lowering the AD of the local RIP-learned routes Raising the AD of the external OSPF-learned routes To force the RIP routes to be preferred, RouterC s configuration would be as follows: RouterC(config)# access-list 10 permit RouterC(config)# access-list 10 permit RouterC(config)# access-list 10 permit RouterC(config)# access-list 10 permit RouterC(config)# access-list 10 deny any RouterC(config)# router rip RouterC(config-router)# distance An access-list was created that is permitting the RIP networks in question, and denying all other networks. Under the RIP process, an administrative distance of 70 is applied to updates from routers on the network, for the specific networks matching access-list 10. RouterD s configuration would be similar. Thus, the RIP-originated networks will now have a lower AD than the redistributed routes from OSPF. The loop has again been eliminated. Another solution would be to raise the AD of the external OSPF routes. OSPF provides a simple mechanism to accomplish this: RouterC(config)# router ospf 20 RouterC(config-router)# distance ospf external 240 9 Pitfalls of Route Redistribution Route Feedback A routing loop is only one annoying issue resulting from the above design. Route feedback is another problem that must be addressed. OSPF routes redistributed into RIP on RouterC will eventually reach RouterD, and then be redistributed again back into OSPF. This is a basic example of route feedback. Depending on the metrics used, this could potentially cause RouterB to prefer the route through RouterD (and through the RIP domain), to reach the /24 network. This is an obvious example of suboptimal routing. Thus, routes that originated in a routing domain should not to be re-injected into that domain. Distribution-lists and the distance command can be utilized to accomplish this, but route tags may provide a more robust solution. Tagging routes provides a mechanism to both identify and filter those routes further along in the routing domain. A route retains its tag as it passes from router to router. Thus, if a route is tagged when redistributed into RIP on RouterC, that same route can be selectively filtered once it is advertised to RouterD. Route tags are applied using route-maps. Route-maps provide a sequential list of commands, each having a permit or deny result: RouterC(config)# route-map OSPF2RIP deny 5 RouterC(config-route-map)# match tag 33 RouterC(config-route-map)# route-map OSPF2RIP permit 15 RouterC(config-route-map)# set tag 44 Route-maps are covered in great detail in a separate guide. 10 Pitfalls of Route Redistribution Route Feedback (continued) The full configuration necessary on RouterC would be as follows: RouterC(config)# route-map OSPF2RIP deny 5 RouterC(config-route-map)# match tag 33 RouterC(config-route-map)# route-map OSPF2RIP permit 15 RouterC(config-route-map)# set tag 44 RouterC(config)# router rip RouterC(config)# redistribute ospf 20 route-map OSPF2RIP RouterC(config)# route-map RIP2OSPF deny 5 RouterC(config-route-map)# match tag 44 RouterC(config-route-map)# route-map RIP2OSPF permit 15 RouterC(config-route-map)# set tag 33 RouterC(config)# router ospf 20 RouterC(config)# redistribute rip route-map RIP2OSPF Thus, OSPF routes being redistributed into RIP are set with a tag of 44. When RIP is redistributed back into OSPF, any route with a tag that matches 44 is denied. Similarly, RIP routes being redistributed into OSPF are set with a tag of 33. When OSPF is redistributed back into RIP, any route with a tag that matches 33 is denied. The net result: routes originating from a routing domain will not redistributed back into that domain.
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