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Route Redistribution in EIGRP

Route Redistribution in EIGRP

Route Redistribution in EIGRP: Redistribution is a technique using which the boundary routers connecting different routing domains can exchange and advertise routing information. In this article, we will learn how to redistribute any routing protocol into EIGRP. Below syntax is followed. The command has to be configured in router configuration mode for the Route Redistribution into EIGRP.

Route Redistribution in EIGRP - Syntax

redistribute protocol [process-id] [metric metric-value] [match route-type] [route-map name]

Description:

  • protocol: For example the protocol you want to redistribute into EIGRP (i.e. OSPF, RIP, or BGP etc.)
  • process-id: This field is specifically used while redistributing the protocols OSPF and BGP into EIGRP. The value of this field is AS number in case of BGP and  process-id in case of OSPF. This field is not required for RIP and ISIS.
  • metric: This is an optional field  that specifies the EIGRP Seed Metric, in the fixed order of bandwidth, delay, reliability, load, and maximum transmission unit (MTU), for the route going to be redistributed. When redistributing other protocol routes into EIGRP (other than ISIS), if this metric value is not specified and  also if no value is specified using the 'default-metric' router configuration command, the default metric is 0. The routes will not be redistributed because like RIP, EIGRP also considers the 0 metric as infinity. In EIGRP, the metric is calculated based only on bandwidth and delay by default.
  • match/route-type: This is an optional field and specifically used while redistributing OSPF into EIGRP. Three types of routes exist in OSPF namely Internal (that are internal to an OSPF domain), External type-1, External type 2, or NSSA-External.
  • route-map: This is again an optional field and used when we have to do any route filtering while redistribution.

Some more details of EIGRP Metric fields:
  • Bandwidth -  The route’s minimum bandwidth in kilobits per second (kbps).
  • Delay Route - Delay in tens of microseconds.
  • Reliability - The likelihood of successful packet transmission, expressed as a number from  0 to 255, where 255 means that the route is 100 percent reliable.
  • Load - The route’s effective loading, expressed as a number from 1 to 255, where 255 means that the route is 100 percent loaded.
  • MTU -  Maximum transmission unit. The maximum packet size in bytes along the route; an integer greater than or equal to 1.

Imp Point: When redistributing routes from other routing protocols into EIRGP, the routes are denoted by D EX and the Administrative Distance (AD) of the redistributed routes 170 compared to internal EIGRP routes that are denoted by D and have AD value of 90. Because of their higher AD, EIGRP external routes are less preferred than EIGRP internal routes.


Route redistribution in EIGRP Guidelines

  • It is recommended to always use a route-map for filtering the routes while route redistribution into EIGRP. This is to ensure that EIGRP redistributes only the routes that an network admin intend to redistribute into EIGRP.
     
  • You should configure default metric to redistribute routes from another routing protocols into EIGRP. If you do not specify the default metric while redistribution into EIGRP or RIP, the routes will not be redistributed because EIGRP considers default seed metric as Infinity when routes are redistributed into EIGRP without default metrric. The ways to configure default metric in EIGRP are explained later in this article.
 
 

Redistribute OSPF into EIGRP

Redistribute OSPF into EIGRP
 
Let us understand how to Redistribute OSPF into EIGRP with the above lab simulation. We will redistribute OSPF prefix 10.10.0.1/32 which is Lo0 IP of router R1 into EIGRP. 
 
In below configuration we are redistributing OSPF routes into EIGRP on router R2. We are declaring the metric in redistribution command only.
 
R2(config)#router eigrp 10
R2(config-router)#redistribute ospf 1 metric 10000  100  255  1  1500


Or if you don't want to specify metric specifically while redistributing OSPF but a general reference value for all protocols, we can explicitly use 'default-metric' router configuration command to specify the metric.
 
R2(config)#router eigrp 10
R2(config-router)#redistribute ospf 1
R2(config-router)#default-metric 10000  100  255  1  1500


Another way to specify the metric is using the route map. This method is explained in route-filtering section below (step2).

Route Filtering in EIGRP

We can also perform Route Filtering in EIGRP while redistribution. EIGRP supports route filtering using ACLs, prefix-lists and route-maps. Let us now take 2 Loopbacks IP on R1 using the same topology shared above. Lo0 - 10.10.0.1/32 and Lo1 - 172.10.0.1/32. Let us filter the Lo0 and allow Lo1 while redistribution of OSPF routes into EIGRP on router R2 .

Mark the traffic using an ACL or Prefix-list.

ip access-list standard CLASSIFY
permit 10.10.0.1



Create a route-map.
 
route-map FILTER deny 10
match ip address CLASSIFY
 
route-map FILTER permit 20
set metric  10000  100  255  1  1500



Apply route-map for route-filtering.

R2(config)#router eigrp 10
redistribute ospf 1 route-map FILTER


Using above route-filtering configuration we wan see that prefix 10.10.0.1/32 will be blocked during redistribution of OSPF routes into EIGRP and prefix 172.10.0.1/32 will be allowed with the metric set in rule 20 of the route-map FILTER. 
 
Imp Point: Kindly also note that you have to configure the the route-map 'FILTER'. An empty route-map or wrong route-map name configured will not allow redistribution of routes.


 
 
I hope you have found this article informative and useful and now have a fair understanding of Route Redistribution in EIGRP, the guidelines to follow while redistribution of other protocol routes in EIGRP, ways to set metric the metric while redistribution in EIGRP and finally how to filter the routes during redistribution. For any of the related queries or feedback, kindly write to us at networkurge@gmail.com.

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