Chapter 10 – Route Redistribution

The need for Route Redistribution

  • What is the main problem with redistribution?
    • The metrics are different.
  • From where does the IOS takes the routes for redistribution and why?
    • IOS always using the routing table for redistribution.
  • Where and what command do you implement for ie.: take OSPF routes into EIGRP? After this where will be these new routes taken?
    • (router EIGRP): redistribute ospf  
      The OSPF routes willl be put into EIGRP topology table
  • What default metrics are going to use each routing protocols when redistributing another protocol?
    • RIP: infinite
    • EIGRP: infinite
    • OSPF: 20 (in case of BGP 1)
    • BGP: metric set to IGP metric value
    • RIP and EIGRP will not redistribute any routes until a metric has been statically configured. 

Redistribution into EIGRP

Default route:
– There is no need for default-information originate, to advertise default route. If the router has in its routing table EIGRP can advertise it without further configuration.


  • How do you redistribute into EIGRP?
    • redistribute  
      default-metric 1544 10 255 5 1500
    • R2#sho int Se2/0
        MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
    • redistribute <proc/as>
    • Bandwidth: Use the interface bandwidth (Kbit/sec)
    • Delay: interface delay / 10
    • Reliability: 1 =not reliable 255=100% reliable
    • Load: 255=100% loaded
    • MTU: use the interface MTU
    • Bagosi Dávid Reggel Limuzint Mosott
  • How do you verify a redistribution into EIGRP?
    • sho ip eigrp topology
  • What methods do you know to set the EIGRP metrics when redistributing?
    • You can set the default metric for all redistribute command:
      (router EIGRP)default-metric
    • Or set the metric for a single redistribute command :(router EIGRP) redistribute ospf 1 metric
    • Or you can set different metrics for each routes which are redistributed in a single redistribution by route-map: redistribute ospf 1 route-map metric 
      This can override the previous 2 options.
    • All redistributed routes will have the same feasible distance since they use the same component metric (configured above).
  • What additional meaning does this command have: redistribute ospf 2 ?
    • It redistributes routes learned from OSPF 2 and also learns the connected routes for interfaces on which OSPF 2 process had been enabled. (It will be an external route on the other EIGRP devices).
  • What is the EIGRP AD (internal and external)? When can be external EIGRP AD seen in routing table?
    • EIGRP internal AD: 90 external: 170
    • External AD can be seen for example when something is redistributed into EIGRP.

Redistribution into OSPF

Default route:
– Assume that the routing table has
– OSPF can advertise default route if “default-information originate” command is used.
– OSPF can advertise default route with “default-information originate always” even if the routing table do not have

What are the default metrics when redistributing into OSPF (BGP, RIP/EIGRP/IS-IS, OSPF)?

    • Redistributing BGP: metric = 1
    • Redistributing other OSPF process: metric = source metric
    • Redistributing from all other source: metric = 20
  • What problem can cause OSPF when using Loopbacks for simulating networks?
    • For example OSPF realises that the /24 Loopback is still a loopback and it’s really /32 because it isn’t a whole network.
      So when it is redistributed it seems like this on the other router:
      To resolve this issue we can use this command in the Loopbacks configuration section:

      int lo 3
       ip ospf network point-to-point

      After this we will see as a /24 network on the other router (where it is redistributed):

  • How do you configure metric when redistributing to OSPF?
    • (ospf)default-metric cost
    • (ospf) redistribute protocol number metric cost
    • use route-map on the redistribute command
  • What is the difference between E1 and E2 metric types?
    • for E1 both external and internal cost matter to the choice of best route
      • E1 is better if you have multiple exit points in the area.
    • for E2 only the external cost matter (except a tie need to be broken)
      • NOTE: When 2 E2 routes have the same metric (usually 20 when redistributing), OSPF will use the Forwarding Metric.  This way E2 will act as an E1 route.
  • What is Type 4 LSA?
    • it lists RID of ASBR and RID of ABR
    • Type 4 LSA lists the ABR’s cost to reach the ASBR
  • If a R1 knows 2 routes to the same E2 destination (with same metric) and the ASBR is within the area, how will R1 choose?
    • It will find the ASBR listed in the Type 5 LSAs
    • Calculates the lowest-cost route to reach any of the ASBRs based on the intra-area LSDB topology.
    • use the next hop based on the best route to reach ASBR
    • e2-intra
  • If a R1 knows 2 routes to the same E2 destination (with same metric), but the ASBR is in another area, how will R1 choose?
    • it will find the ASBR listed in the Type 5 LSA
    • calculates the lowest-cost route to reach any of the ASBRs
      • to do this it will use Type 3 LSA because the ASBR is in another area
      • it finds the ABR with the best metric and use that route
    • the router will add the metric of E2, ASBR and ABR and choose the lowest
  • What is the default redistribute metric type and how can you set the other one?
    • default = E2
    • (ospf) redistribute eigp 1 subnets metric-type 1
  • What if the ASBR is in another area?
    • the router will calculate as in the case of E2
    • adds the best intra-area cost to reach ABR
    • adds the cost from ABR to ASBR (info in LSA Type 4)
    • adds the external cost (LSA Type 5)
  • What happens if you redistribute into a NSSA area?
    • there will be Type 7 LSAs
    • if these Tpye 7 LSAs are propagated into another area they will change to Type 5 LSA

Redistribution into BGP

Default route:
– BGP will NOT advertise default route without the “default-information originate” command, even if it redistributes


Redistribution with Route Maps

  • What is a Rotue Map able to?
    • identifies routes and able to change them
    • makes filtering choices which routes are redistributed and which are not
    • set the metric
    • set the type of external route (OSPF ie)
    • set a route tag (unitless integer) which can be later used
  • How can you filter the redistributed routes with the redistribute command?
    • You can use the redistribute ospf 1 match { internal | external 1 | external 2 | nssa-external }
    • or you can use the redistribute ospf 1 route-map
  • With what tools can you filter in the route-map command’s match section?
    • ACL
    • prefix list
  • How do you filter the following route ?
    • In the route maps match section: match ip address ACL_10
      In ACL_10: permit ip any host
  • What does the permit and deny mean in the route-map? How does it work?
    • route-map TEST permit 10
      Each sequence of a route map can be permited or denied. The permit means that the route (selected by the match clause) will be redistributed (additionally many values can be set in the SET section of the sequence). The denied routes will be filtered.
  • What can be set by route-map?
    • The route metric: set metric
    • An EIGRP route metric (more values): set metric
    • The type of route (OSPF): set metric-type { type-1 | type-2}
    • A uniteless tag: set tag tag-value
  •  Does route-map has implicit deny? How do ACLs work with route-maps? What should we not use in ACLs? 
    • ACL is used for selecting a specific route. So denying anything in ACL has no point. Routes go through on route-map one by one and they can match with routes selected by ACL ( if ACL denies something that is like an empty line for route-map, useless ).
    • There is no implicit deny in route-maps but if  a route has no match then it won’t be redistributed. 
    • When we using route-maps to set something on some routes then it is important to use a last sequence which permits all other non-matched routes to be redistributed:
      route-map TEST permit 100
      (match part is empty -> matches everything which haven’t been matched yet)
  • How do you set the metric of a single route (ie:
    • redistribute ospf 1 route-map SET-METRIC
       route-map SET-METRIC permit 10
         match ip address 100
         set metric 100 444 255 1 1500
       ip access-list standard 100
         permit host
  • How do you set the external route type?
    • set metric type {type-1 | type-2}
  • With what methods can you prevent Domain Loops?
    • setting metrics
    • setting AD
  • How does OSPF relate to internal routes and external routes (prioritization)?
    • OSPF will choose the “internal” routes before even considering metrics:
      internal routes > E1 routes > E2 routes
  • Which route will the router choose if it has multiple choices (out of domain routes)?
    • The one with the lower AD. If the routes have same AD then it will use the metric as tie breaker.
  • What will defeat the domain loop when redistributing between OSPF and EIGRP?
    • The AD: EIGRP internal 90 < OSPF 110 < EIGRP external 170
  • What will defeat the domain loop when redistributing between EIGRP and RIP?
    • The AD: EIGRP internal 90 < RIP 120 < EIGRP external 170
  • What will defeat the domain loop when redistributing between RIP and OSPF?
    • RIP can be set with only one AD but OSPF understands external routes, intra-area routes and inter-area routes.
  • How do you change the AD value of RIP, EIGRP but especially of OSPF?
    • RIP: distance
    • EIGRP: distance eigrp { internal-ad | external-ad }
    • OSPF: distance ospf { external-ad | intra-area-ad | inter-area-ad }
  • When start the AD become problematic?
    • With redistribution between more than 2 routing protocols.domain-loop-with-3-rp
  • How can you solve the problem on the previous topology?
    • There is an option to set an individual AD for a given router (which advertises routing information ie:RD2):
      This can be extended with an ACL. This way only given routes AD will be changed even from the specific router.
    • Another way is to filter the redistributed routes with a route-map, so what’s already redistributed from an area wont be redistributed back:
    • The routes can be tagged by a route-map and later the tagged route can be matched with another route-map. This way we can follow a specific route advertisement.
  • What does Prefix List do and why do we need it?
    • Prefix list filter routes like Distribute List but it is used in a much bigger scale.
      Mostly we use it with BGP. For example when there are 2 huge ISP and they exchange routes they have to summarize the routes or else the routing table would overflow. To be sure that the other ISP don’t advertises too specific routes we can use Prefix Lists:
      ip prefix-list TEST permit le 20
      The previous prefix lists mean: we allow routes with the /16 subnet but maximum with /20 prefix. or is not allowed but is allowed.





Comments are closed.

Blog at

Up ↑