Configuring Route Redistribution


There are many variables to deal with when discussing the process of configuring route redistribution. The level of complexity and the command structures depend on the routing protocols being distributed. You should first find the router or routers that are on the edge of the different autonomous systems or routing domains. These are the routers that you will configure for route redistribution. Next, you must determine what the core (or backbone) protocol will be. This is typically EIGRP or OSPF in today's internetworking environments because they are classless routing protocols. Now, you should decide which routing protocol is at the edge. This could also be the short- term migration protocol or a static route. Begin with the core , such as OSPF, and enter into router configuration mode for the routing protocol that is to receive the redistributed routes; for example:

 RouterA(config)# router ospf 100 

The final action for configuring redistribution is to configure the boundary router to redistribute routes. These routes can be from any dynamic routing protocol supported by Cisco or a static route.

The redistribute Command

The redistribute command configures the redistribution, or translation, of routing protocols used within one routing domain into the routing protocols used within another routing domain. In other words, the redistribute command actually engineers the injection of routes learned by any one routing source into a dynamic routing protocol. The precise syntax depends on whether you are redistributing into OSPF or EIGRP, the two routing protocols into which you are concerned with redistributing for the BSCI exam. We first look at the process of redistributing routes into an OSPF system. This is the complete syntax for redistributing updates into OSPF:

[View full width]
 
[View full width]
RouterA(config-router)# redistribute protocol [ process-id ] [ metric metric-value ] [ metric-type type-value ][ route-map map-tag ][ subnets ] [ tag tag-value ]

Here is a sample of this command at work:

 RouterA(config)# router ospf 100 RouterA(config-router)# redistribute igrp 1 metric 30 subnets 

With these commands, all subnets contained within the IGRP routing process are injected into OSPF as external Type 2 routes with a metric (cost) of 30, rather than the default cost of 20. The parameter combination of protocol [ process-id ] signifies the protocol that is being redistributed and, if necessary, the process ID of the instance of that protocol (an EIGRP process in this case). The protocol value could be any of the following: connected , bgp , eigrp , egp , igrp , isis , iso-igrp , mobile , odr (on-demand routing), static , or rip . The optional parameter metric metric-value designates the metric or cost used for the redistributed route. The default metric value for routes that are redistributed into OSPF is 20. The metric value is applied as a seed value to all those routes being injected into the receiving routing protocol, in this example, OSPF. Routes with lower cost values are used before routes with higher cost values. The optional parameter metric-type type-value determines whether the redistributed route should be classified as an external Type 1 or external Type 2 OSPF route. The default type value is 2. This is critical. With Type 2 external routes, the cost metric does not aggregate and with Type 1 it does aggregate. The default is Type 2. Use Type 1 if there is more than one path to the external (edge) network because you want metric calculations to actually reflect actual path cost. The route-map map-tag parameter pair is the optional identifier of the route map that is queried for filtering policies. This subject is addressed in the "Resolving Path Selection Issues in Redistributed Networks" section later in this chapter.

The keyword subnets is another optional OSPF parameter that designates that subnetted network routes, in addition to non- subnetted network routes, should be redistributed. If the subnets keyword is not used, only the routes that are not subnetted are redistributed. This command is critical if you want any subnets of the other routing process to be qualified for redistribution. Without the subnets keyword, one may find that no routes are injected into the receiving routing process.

The tag tag-value parameter is an optional 32-bit decimal value assigned to every external route. This is not used by OSPF but rather between ASBRs. The following example configures redistribution of EIGRP routes from AS 500 into OSPF, using a default cost of 60 and assigning the E1 external type code.

 RouterA(config-router)#  redistribute  eigrp 500  metric  60  metric  -  type  1  subnets  

The metric 60 parameter specifies the metric of the core protocol, which in this case is OSPF. Therefore, the metric value 60 denotes the cost that is assigned to EIGRP redistributed routes. EIGRP uses a composite metric based on bandwidth and delay (and optionally reliability, load, and MTU). The optional metric-type type-value parameter specifies the OSPF external link type. OSPF external Type 2 routes, which are given the E2 origin code in the routing table, use only external cost as their metric. If no metric type is assigned, then the router considers the redistributed routes as E2 external routes by default. The optional subnets keyword specifies that OSPF will redistribute routes from subnets of the major network that is shared by the EIGRP routing domain and the OSPF routing domain. The subnets keyword should be used for classful routing protocols such as RIP and IGRP. EIGRP is special in that it is a converted classful routing protocol and still possesses many of the characteristics of IGRP. If you redistribute a classless protocol into another classless protocol, the subnets keyword is not an option.

The following redistribute command looks slightly different from the redistribute command shown previously because you are redistributing OSPF into EIGRP:

[View full width]
 
[View full width]
RouterA(config-router)# redistribute protocol [ process-id ] [ match { internal external 1 external 2}] [metric metric -value ] [route-map map-tag ]

The parameter protocol designates the protocol that is being redistributed. This value could be any of the following: connected , bgp , eigrp , egp , igrp , isis , iso-igrp , mobile , odr (on-demand routing), static , or rip . The [ process ID ] in this case is the EIGRP, BGP, EGP, or IGRP autonomous system number for the instance of that protocol. You use the match keyword to determine which types of OSPF routes will be distributed into other routing domains. This is vital because it enables you to block external routes (which have already been redistributed) from being redistributed into the receiving routing process. Again, the goal is to help prevent routing loops and optimize your scalable internetwork. The match keyword can have one of the following values:

  • internal ” Routes that are internal to the designated autonomous system

  • external 1 ” Routes that are external to the autonomous system and injected into OSPF as Type 1 external

  • external 2 ” Routes that are external to the autonomous system and injected into OSPF as Type 2 external

The metric metric-value parameter is an option used to designate the seed metric for the redistributed route. The route-map map-tag parameter pair is the optional identifier of the route map that is queried for filtering policies.

The default-metric Command

One of the ways that you can influence how routes are redistributed is by modifying the default metric of the route or routes being redistributed. There are a couple of ways to do this. You can specifically designate it with the default-metric command or you can use the metric parameter in the redistribute command shown previously. If you use the metric parameters in the redistribute command, you have more granularity if you set a different default metric for every protocol into which you are redistributing. When using the default-metric command, the specified default is applicable to all the protocols into which you redistribute. You must use the default-metric command separately rather than as part of the redistribute command.

The command syntax for redistributing default metrics into IGRP and EIGRP is as follows :

 RouterA(config-router)#  default-metric   bandwidth delay reliability loading mtu  

Table 10.3 is a review of the metrics for IGRP and EIGRP, which also serve as parameters for the default-metric command for IGRP and EIGRP. It is important to note here that EIGRP multiplies what IGRP would calculate by a factor of 256, because EIGRP uses a 32-bit metric field, versus the 24-bit field for IGRP.

Table 10.3. The default-metric Command to Be Used with IGRP and EIGRP

Command Parameter

Description

bandwidth

The minimum bandwidth value of the route in Kbps.

delay

The route delay value in tens of milliseconds .

reliability

A numerical value between 1 and 255, where 255 represents a route that is 100% reliable.

loading

A numerical value between 1 and 255, where 255 represents a route that is 100% loaded.

mtu

The maximum transmission unit (MTU) is the maximum packet size on the route in bytes.

In the following example, you want RouterA to redistribute RIPv2 and EIGRP routes. The network 192.168.100.1 is redistributed into the RIPv2 network with a metric of 3 hops and EIGRP routing in AS 2 is learning routes from RIPv2. Listing 10.2 shows how you would configure redistribution with the default-metric command.

Listing 10.2 Sample Configuration of Redistribution Using the default-metric Command
 RouterA(config)# router rip RouterA(config-router) network 172.16.0.0 RouterA(config-router) redistribute eigrp 2 RouterA(config-router) default-metric 3 ! RouterA(config)# router eigrp 2 RouterA(config-router) network 192.168.100.0 RouterA(config-router) redistribute rip RouterA(config-router) default-metric 64 2500 255 1 1500 

In Listing 10.2, the router configuration command redistribute eigrp 2 allows redistribution of routes that are learned from EIGRP AS 2 into the RIPv2 network. The command default-metric 3 designates that the EIGRP learned routes are to be injected as three hops away. The router configuration command redistribute rip allows for the redistribution of RIP-learned routes into the EIGRP AS 2. The command default-metric 64 2500 255 1 1500 indicates that the bandwidth metric is 64Kbps, the delay metric is to be 2500 tens of microseconds, the reliability metric is 100%, the loading metric is less than 1%, and the MTU is 1500 bytes.

Use the following, and slightly different, command syntax for configuring the seed metric when redistributing into the OSPF, RIP, EGP, and BGP routing protocols:

 RouterA(config-router)#  default-metric   number  

Do not confuse this discussion with the ip default-gateway command. This command is used on servers or routers that have IP routing disabled. They are functioning as just another host on the internetwork.




Cisco BSCI Exam Cram 2 (Exam Cram 642-801)
CCNP BSCI Exam Cram 2 (Exam Cram 642-801)
ISBN: 0789730170
EAN: 2147483647
Year: 2003
Pages: 170

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net