Understand the Redistribution of OSPF Routes into BGP

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Updated:November 2, 2022
Document ID:5242

Bias-Free Language

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    Introduction

    This document describes the behavior of Open Shortest Path First (OSPF) to Border Gateway Protocol (BGP) redistribution on Cisco routers.

    Prerequisites

    Requirements

    Cisco recommends that you have knowledge of the OSPF route types before you use this document.

    Components Used

    This document is not restricted to specific software and hardware versions.

    The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

    Conventions

    For more information on document conventions, see the Cisco Technical Tips Conventions.

    Background Information

    This Tech Note explains the behavior of the OSPF to BGP redistribution on Cisco routers. The behavior of OSPF to BGP redistribution is outlined inRFC 1403. There are several types of OSPF routes:

    • Intra-Area — In a multiarea OSPF network, routes that originate within an area are known by the routers in the same area as Intra-Area routes. These routes are flagged as O in the show ip route command output.

    • Inter-Area — When a route crosses an OSPF Area Border Router (ABR), the route is known as an OSPF Inter-Area route. These routes are flagged as O IA in the show ip route  command output.

      Both Intra and Inter-Area routes are also called OSPF Internal routes, as they are generated by OSPF itself when an interface is covered with the OSPF network  command.

    • External Type-2 or External Type-1 — Routes that were redistributed into OSPF, such as Connected, Static, or other Routing Protocol, are known as External Type-2 or External Type-1. These routes are flagged as O E2 or O E1 in the show ip route  command output.

    • NSSA External Type-2 or NSSA External Type 1 — When an area is configured as a Not-So-Stub Area (NSSA), and routes are redistributed into OSPF, the routes are known as NSSA External Type-2 or NSSA External Type-1. These routes are flagged as O N2 or O N1 in the show ip route  command output.

    The explanation of the differences between External and NSSA Type 2 or 1 is beyond the scope of this document — refer to the OSPF Design Guide for more information.

    The default behavior is not to redistribute any routes from OSPF into BGP. Redistribution must be configured. You can use the  route-map  command to filter routes during OSPF to BGP redistribution. To complete the redistribution, specific keywords like internal, external, and nssa-external  are required to redistribute respective routes.

    Network Setup

    There are four redistribution cases of OSPF routes into BGP discussed below. The network diagram applies to the first three cases. The diagram and set up for the fourth case can be found in the Redistribution of OSPF NSSA-External Routes into BGP  section.

    Redistribution OSPF to BGP Topology ARedistribution OSPF to BGP Topology A

    Redistribution of Only OSPF Internal (Intra and Inter-Area) Routes into BGP

    If you configure the redistribution of OSPF into BGP without keywords, only OSPF intra-area and inter-area routes are redistributed into BGP, by default. You can use the internal keyword along with the redistribute command under router bgp  to redistribute OSPF Intra and Inter-area routes.

    This configuration is a new configuration of Router B that redistributes only the Intra-area route (10.108.2.0/24) and Inter-area route (10.108.1.0/24) into BGP and only OSPF Internal (Intra-area and Inter-area) routes are redistributed into BGP:

    RTB 
    hostname RTB
!
interface GigabitEthernet0/0
 ip address 10.3.3.1 255.255.255.0
 duplex auto
 speed auto
 media-type rj45
!
interface GigabitEthernet0/1
 ip address 10.2.2.3 255.255.255.0
 duplex auto
 speed auto
 media-type rj45  
    ! 
    router ospf 1
 network 10.2.2.0 0.0.0.255 area 1  
    ! 
    router bgp 100 
    redistribute ospf 1 

    !-- This redistributes only OSPF intra-area and inter-area routes into BGP.      

    neighbor 10.3.3.2 remote-as 200 
    ! 
    end 

    RTB#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C        10.2.2.0/24 is directly connected, GigabitEthernet0/1
L        10.2.2.3/32 is directly connected, GigabitEthernet0/1
C        10.3.3.0/24 is directly connected, GigabitEthernet0/0
L        10.3.3.1/32 is directly connected, GigabitEthernet0/0
O IA     10.108.1.0/24 [110/3] via 10.2.2.2, 00:08:38, GigabitEthernet0/1
O        10.108.2.0/24 [110/2] via 10.2.2.2, 00:39:13, GigabitEthernet0/1
O E2  192.168.1.0/24 [110/20] via 10.2.2.2, 00:07:39, GigabitEthernet0/1
O E1  192.168.2.0/24 [110/23] via 10.2.2.2, 00:07:38, GigabitEthernet0/1
RTB#

    Router B redistributes only OSPF Internal routes:

    RTB#show ip bgp
BGP table version is 12, local router ID is 10.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   10.2.2.0/24      0.0.0.0                  0         32768 ?
 *>   10.108.1.0/24    10.2.2.2                 3         32768 ?
 *>   10.108.2.0/24    10.2.2.2                 2         32768 ?
RTB#

    Router C learns these routes from BGP:

    RTC#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
B        10.2.2.0/24 [20/0] via 10.3.3.1, 00:07:07
C        10.3.3.0/24 is directly connected, GigabitEthernet0/0
L        10.3.3.2/32 is directly connected, GigabitEthernet0/0
B        10.108.1.0/24 [20/3] via 10.3.3.1, 00:07:07
B        10.108.2.0/24 [20/2] via 10.3.3.1, 00:07:07
RTC#
RTC#show ip bgp
BGP table version is 8, local router ID is 10.3.3.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   10.2.2.0/24      10.3.3.1                 0             0 100 ?
 *>   10.108.1.0/24    10.3.3.1                 3             0 100 ?
 *>   10.108.2.0/24    10.3.3.1                 2             0 100 ?
RTC#

    Redistribution of Only OSPF External (Type 1 and 2) Routes into BGP

    Use the external  keyword along with the redistribute  command under router bgp  to redistribute OSPF external routes into BGP. With the external  keyword, you have three choices:

    • Redistribute both External Type-1 and Type-2 (Default)

    • Redistribute Type-1

    • Redistribute Type-2


    Enter the commands in the configuration mode as described here:

    RTB(config-router)#router bgp 100
RTB(config-router)#redistribute ospf 1 match external

    In this configuration of Router B, redistribute only OSPF External routes, but both Type-1 and Type-2:

    RTB 
    hostname RTB
!
interface GigabitEthernet0/0
 ip address 10.3.3.1 255.255.255.0
 duplex auto
 speed auto
 media-type rj45
!
interface GigabitEthernet0/1
 ip address 10.2.2.3 255.255.255.0
 duplex auto
 speed auto
 media-type rj45  
! 
router ospf 1
 network 10.2.2.0 0.0.0.255 area 1  
! 
router bgp 100   
     redistribute ospf 1 match external 1 external 2 

    !--- This redistributes ONLY OSPF External routes, but both type-1 and type-2.  

    neighbor 10.3.3.2 remote-as 200 
    ! 
    end

    Note: The configuration shows match external 1 external 2  and the command entered was redistribute ospf 1 match external. This is normal because OSPF automatically appends external 1 external 2  in the configuration. It matches both OSPF external 1 and external 2 routes and it redistributes both routes into BGP.

    Router B redistributes only the OSPF external routes:

    RTB#show ip bgp
BGP table version is 25, local router ID is 10.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.1.0      10.2.2.2                20         32768 ?
 *>   192.168.2.0      10.2.2.2                23         32768 ?
RTB#

    Router C learns about these two OSPF external routes from BGP:

    RTC#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        10.3.3.0/24 is directly connected, GigabitEthernet0/0
L        10.3.3.2/32 is directly connected, GigabitEthernet0/0
B     192.168.1.0/24 [20/20] via 10.3.3.1, 00:02:16
B     192.168.2.0/24 [20/23] via 10.3.3.1, 00:02:16

    RTC#show ip bgp
BGP table version is 21, local router ID is 10.3.3.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.1.0      10.3.3.1                20             0 100 ?
 *>   192.168.2.0      10.3.3.1                23             0 100 ?
RTC#

    Redistribution of Only OSPF External Type 1 or Type 2 Routes into BGP

    Enter this command under the router bgp 100  command on Router B to redistribute only OSPF External 1 routes:

    RTB(config)#router bgp 100 
    RTB(config-router)#redistribute ospf 1 match external 1

    With the previous configuration Router B (RTB) BGP table shows that it is only able to redistribute external 1 routes into BGP and all other OSPF routes are not redistributed into BGP:

    RTB#show ip bgp
BGP table version is 28, local router ID is 10.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.2.0      10.2.2.2                23         32768 ?
RTB#

    In the same way, enter this command under router bgp 100  on Router B to redistribute only OSPF external 2 routes:

    RTB(config)#router bgp 100 
    RTB(config-router)#redistribute ospf 1 match external 2

    Redistribution of OSPF Internal and External Routes into BGP

    In this case, all OSPF routes are redistributed into BGP with the use of both the internal  and external  keywords within the command redistribute ospf <process> match internal external, as shown in this Router B configuration:

    RTB 
    hostname RTB
!
interface GigabitEthernet0/0
 ip address 10.3.3.1 255.255.255.0
 duplex auto
 speed auto
 media-type rj45
!
interface GigabitEthernet0/1
 ip address 10.2.2.3 255.255.255.0
 duplex auto
 speed auto
 media-type rj45  
! 
router ospf 1
 network 10.2.2.0 0.0.0.255 area 1  
! 
router bgp 100
     redistribute ospf 1 match internal external 1 external 2 

    !--- This redistributes all OSPF routes into BGP.  

    neighbor 10.3.3.2 remote-as 200 
    ! 
    end


    Again, external  is replaced with external 1 external 2  in the configuration. This is normal unless you specify which specific external routes you want to redistribute into BGP. After the configuration change is completed, Router B redistributes all OSPF routes and Router C starts to learn all routes from BGP:

    RTB#show ip bgp
BGP table version is 6, local router ID is 10.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   10.2.2.0/24      0.0.0.0                  0         32768 ?
 *>   10.108.1.0/24    10.2.2.2                 3         32768 ?
 *>   10.108.2.0/24    10.2.2.2                 2         32768 ?
 *>   192.168.1.0      10.2.2.2                20         32768 ?
 *>   192.168.2.0      10.2.2.2                23         32768 ?
RTB#  RTC#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
B        10.2.2.0/24 [20/0] via 10.3.3.1, 00:03:27
C        10.3.3.0/24 is directly connected, GigabitEthernet0/0
L        10.3.3.2/32 is directly connected, GigabitEthernet0/0
B        10.108.1.0/24 [20/3] via 10.3.3.1, 00:03:27
B        10.108.2.0/24 [20/2] via 10.3.3.1, 00:03:27
B     192.168.1.0/24 [20/20] via 10.3.3.1, 00:03:27
B     192.168.2.0/24 [20/23] via 10.3.3.1, 00:03:27
RTC#

    Redistribution of OSPF NSSA-External Routes into BGP

    This is a special case in which only NSSA routes are redistributed into BGP. This case is very similar to the case described in the  Redistribution of only OSPF External (Type 1 and 2) Routes into BGP  section. The only difference is that OSPF now matches NSSA-external routes instead of just external routes. The routing table of Router B shows these OSPF NSSA-external routes:

    RTB#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C        10.2.2.0/24 is directly connected, GigabitEthernet0/1
L        10.2.2.3/32 is directly connected, GigabitEthernet0/1
C        10.3.3.0/24 is directly connected, GigabitEthernet0/0
L        10.3.3.1/32 is directly connected, GigabitEthernet0/0
O IA     10.108.1.0/24 [110/3] via 10.2.2.2, 00:05:00, GigabitEthernet0/1
O        10.108.2.0/24 [110/2] via 10.2.2.2, 00:05:00, GigabitEthernet0/1
O N2  192.168.1.0/24 [110/20] via 10.2.2.1, 00:10:14, GigabitEthernet0/1
O N1  192.168.2.0/24 [110/22] via 10.2.2.2, 00:03:43, GigabitEthernet0/1
RTB#

    This network diagram is used for this case:

    Redistribution OSPF to BGP Topology BRedistribution OSPF to BGP Topology B

    The network diagram shows that Router B receives both OSPF N1 and N2 routes. The default behavior is to redistribute both N1 and N2 routes if only the nssa-external keyword is used. This configuration of Router B allows us to redistribute OSPF N2 (192.168.1.0/24) and OSPF N1 (192.168.2.0/24) routes into BGP:

    RTB 
    hostname RTB
!
interface GigabitEthernet0/0
 ip address 10.3.3.1 255.255.255.0
 duplex auto
 speed auto
 media-type rj45
!
interface GigabitEthernet0/1
 ip address 10.2.2.3 255.255.255.0
 duplex auto
 speed auto
 media-type rj45  
! 
router ospf 1
     area 1 nssa  network 10.2.2.0 0.0.0.255 area 1  
    ! 
    router bgp 100     
     redistribute ospf 1 match nssa-external 1 nssa-external 2

!--- This redistributes only OSPF NSSA-external routes Type-1 and Type-2 into BGP.

 neighbor 10.3.3.2 remote-as 200
!
end

    Note: Like the OSPF external configuration, the previous configuration displays match nssa-external 1 nssa-external 2  and the command entered was redistribute ospf 1 match nssa-external.This is normal because OSPF automatically appends nssa-external 1 nssa-external 2 in the configuration. It matches both OSPF N1 and OSPF N2 routes and redistributes both routes into BGP.

    After the configuration change on Router B, it redistributes OSPF NSSA-external routes, and Router C learns OSPF NSSA-external routes from BGP:

    RTB#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C        10.2.2.0/24 is directly connected, GigabitEthernet0/1
L        10.2.2.3/32 is directly connected, GigabitEthernet0/1
C        10.3.3.0/24 is directly connected, GigabitEthernet0/0
L        10.3.3.1/32 is directly connected, GigabitEthernet0/0
O IA     10.108.1.0/24 [110/3] via 10.2.2.2, 00:09:40, GigabitEthernet0/1
O        10.108.2.0/24 [110/2] via 10.2.2.2, 00:09:40, GigabitEthernet0/1
O N2  192.168.1.0/24 [110/20] via 10.2.2.1, 00:14:54, GigabitEthernet0/1
O N1  192.168.2.0/24 [110/22] via 10.2.2.2, 00:08:23, GigabitEthernet0/1
RTB#
RTB#show ip bgp
BGP table version is 17, local router ID is 10.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.1.0      10.2.2.1                20         32768 ?
 *>   192.168.2.0      10.2.2.2                22         32768 ?
RTB#  RTC#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        10.3.3.0/24 is directly connected, GigabitEthernet0/0
L        10.3.3.2/32 is directly connected, GigabitEthernet0/0
B     192.168.1.0/24 [20/20] via 10.3.3.1, 00:01:29
B     192.168.2.0/24 [20/22] via 10.3.3.1, 00:01:29
RTC# 
RTC#show ip bgp
BGP table version is 41, local router ID is 10.3.3.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.1.0      10.3.3.1                20             0 100 ?
 *>   192.168.2.0      10.3.3.1                22             0 100 ?
RTC#

    In the same way as with OSPF External routes, to redistribute only OSPF N1 routes, enter this command under router BGP 100 on Router B:

    RTB(config)#router bgp 100 
    RTB(config-router)#redistribute ospf 1 match nssa-external 1 

    !--- This redistributes only OSPF NSSA-external Type-1 routes into BGP.

    To redistribute only OSPF N2 routes, enter this command under router BGP 100 on Router B:

    RTB(config)#router bgp 100  
    RTB(config-router)#    redistribute ospf 1 match nssa-external 2 

    !--- This redistributes only OSPF NSSA-external Type-2 routes into BGP.

    Note: Route-Maps can also be used to redistribute OSPF Type 1/2 into BGP. Refer to  Redistribute OSPF E2 Routes in BGP  for more information.

    Modify the Redistribution Option in OSPF

    It is important to understand how successive configuration change alters your configuration. A new command with the match option does not overwrite the previous one but is added to it. The next example explains how the configuration command sequence can have an impact on redistribution:

    R4#configure terminal
R4(config)#router bgp 100
R4(config-router)#redistribute ospf 1 match internal
R4(config-router)#^Z

!--- Initially, you redistribute internal OSPF routes into BGP 100.

R4#show run | include redistribute ospf
 redistribute ospf 1 match internal
R4#configure terminal
R4(config)#router bgp 100
R4(config-router)#redistribute ospf 1 match external
R4(config-router)#^Z

!--- With this second command, you tell BGP to also redistribute external OSPF routes.

R4#show run | include redistribute ospf
 redistribute ospf 1 match internal external 1 external 2
R4#
R4#configure terminal
R4(config)#router bgp 100
R4(config-router)#no redistribute ospf 1 match external 2
R4(config-router)#^Z

!--- With this no command, you only disable the redistribution of external type 2 into BGP. 
    !--- All other types of routes previously configured remain.
    
R4#show run | include redistribute ospf
 redistribute ospf 1 match internal external 1
 
!--- As you can see, internal and external type 1 remain.

R4#configure terminal
R4(config)#router bgp 100
R4(config-router)#no redistribute ospf 1 match internal external 1
R4(config-router)#^Z 

!--- Now, with this no command, which includes all configured keywords, it is important to note that you 
    !--- still do not disable the redistribution fully. you only removed the keyword. After this, 
    !--- the IOS still acts as default—redistributing internal routes only.
    
R4#show run | include redistribute ospf
 redistribute ospf 1
R4#configure terminal
R4(config)#router bgp 100
R4(config-router)#no redistribute ospf 1 

!--- Always use the previous command in order to completely disable redistribution.

R4(config-router)# ^Z
R4#show run | include redistribute ospf
R4#


    Unable to Redistribute iBGP Learnt Routes into an IGP such as EIGRP and OSPF

    Route Redistribution is used to propagate routes learned with the use of one protocol, into another routing protocol. When BGP is redistributed into an IGP, only eBGP learned routes get redistributed. The internal Border Gateway Protocol (iBGP) learned routes known on the router are not introduced into the IGP in order to prevent the creation of routing loops.

    By default, the iBGP redistribution into IGP is disabled. Issue the bgp redistribute-internal  command in order to enable redistribution of iBGP routes into IGP. Precautions need to be taken to redistribute specific routes with the use of route maps into IGP.

    A sample configuration for redistribution of iBGP routes into OSPF is shown here:

    Router(config)#router bgp 65345 
Router(config-router)#bgp redistribute-internal
!
Router(config)#router ospf 100
Router(config-router)#redistribute bgp 65345 subnets

    Note: The redistribution of iBGP routes into an Interior Gateway Protocol can cause routing loops within the Autonomous System (AS). This is not recommended. Route filters need to be set in order to control the information, which is imported into the IGP.

    Redistribute OSPF Default Routes into BGP

    In order to redistribute default routes into BGP, use the network  statement and default-information originate. In this example, the OSPF default routes are redistributed into BGP. This is done with the creation of a route map and the distribution of the default network, which is permitted by the standard ACL.

    !
route-map map_default_only permit 10
 match ip address acl_default_only
!
ip access-list standard acl_default_only
 permit 0.0.0.0
!
router bgp 64601
 network 0.0.0.0
 redistribute ospf 1 route-map map_default_only
 default-information originate
!

    !--- Distributes the default route in bgp

    After the configuration, clear the bgp sessions with the clear ip bgp *  command.

    Related Information

    Revision History

    Revision Publish Date Comments
    1.0
    11-Oct-2001
    Initial Release
    TAC Authored

    Contributed by Cisco Engineers

    • Julio Jimenez
      Cisco Project Manager

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