OSPF to BGP redistribution

[roger perkin]

I am trying to re-create in my lab the basic setup of the work network to test some failover.

The basic setup is two core switches connected to an MPLS cloud with a site to site backup link between the cores.

OSPF is running between all devices and the OSPF gets redistributed into BGP across the MPLS

I have setup a very basic lab to gain a better understanding of this but cannot get the OSPF routes to be redistributed across the cloud between the 2 BPG routers.

The redistribution is not working as sh ip route on the routers does not show any BGP routes, I am sure I am missing a very simple BGP concept here!

Config of R1

interface FastEthernet0/0
ip address 10.251.251.194 255.255.255.240
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 172.16.1.1 255.255.255.0
duplex auto
speed auto
!
router ospf 1
log-adjacency-changes
network 10.251.251.194 0.0.0.0 area 0
!
router bgp 1
no synchronization
bgp log-neighbor-changes
network 172.16.1.0
redistribute ospf 1
neighbor 172.16.1.2 remote-as 1
no auto-summary

Config R2

interface FastEthernet0/0
ip address 10.250.251.194 255.255.255.240
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 172.16.1.2 255.255.255.0
shutdown
duplex auto
speed auto
!
router ospf 1
log-adjacency-changes
network 10.250.251.194 0.0.0.0 area 0
!
router bgp 1
no synchronization
bgp log-neighbor-changes
network 172.16.1.0
redistribute ospf 1
neighbor 172.16.1.1 remote-as 1
no auto-summary

R1#sh ip route
Codes: 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

Gateway of last resort is not set

     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/3] via 10.251.251.193, 00:01:58, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 10.251.251.193, 00:01:58, FastEthernet0/0
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.1.0 is directly connected, FastEthernet1/0
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.10.11.0/24 [110/2] via 10.251.251.193, 00:01:58, FastEthernet0/0
C       10.251.251.192/28 is directly connected, FastEthernet0/0
O       10.250.251.192/28
           [110/3] via 10.251.251.193, 00:01:59, FastEthernet0/0
R1#

R2#sh ip route
Codes: 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

Gateway of last resort is not set

     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/2] via 10.250.251.193, 00:02:10, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/3] via 10.250.251.193, 00:02:10, FastEthernet0/0
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.1.0 is directly connected, FastEthernet1/0
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.10.11.0/24 [110/2] via 10.250.251.193, 00:02:10, FastEthernet0/0
O       10.251.251.192/28
           [110/3] via 10.250.251.193, 00:02:11, FastEthernet0/0
C       10.250.251.192/28 is directly connected, FastEthernet0/0
R2#

[milan.kulik]

Hi,

is you BGP session Up?

What does

sh ip bgp sum

sh ip bgp

show on your routers?

HTH,

Milan

[roger perkin]

BGP seems to be up

R1#sh ip bgp sum
BGP router identifier 172.16.1.1, local AS number 1
BGP table version is 14, main routing table version 14
5 network entries using 505 bytes of memory
10 path entries using 480 bytes of memory
6 BGP path attribute entries using 360 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1345 total bytes of memory
BGP activity 9/4 prefixes, 14/4 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.1.2      4     1     131     131       14    0    0 02:04:56        5
R1#sh ip bgp
BGP table version is 14, local router ID is 172.16.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
* i3.3.3.3/32       10.250.251.193           2    100      0 ?
*>                  10.251.251.193           3         32768 ?
* i4.4.4.4/32       10.250.251.193           3    100      0 ?
*>                  10.251.251.193           2         32768 ?
* i10.10.11.0/24    10.250.251.193           2    100      0 ?
*>                  10.251.251.193           2         32768 ?
* i10.250.251.192/28
                    172.16.1.2               0    100      0 ?
*>                  10.251.251.193           3         32768 ?
* i10.251.251.192/28
                    10.250.251.193           3    100      0 ?
*>                  0.0.0.0                  0         32768 ?
R1#

R2#sh ip bgp sum
BGP router identifier 10.250.251.194, local AS number 1
BGP table version is 8, main routing table version 8
5 network entries using 505 bytes of memory
10 path entries using 480 bytes of memory
6 BGP path attribute entries using 360 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1345 total bytes of memory
BGP activity 6/1 prefixes, 11/1 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.1.1      4     1     131     131        8    0    0 02:04:51        5
R2#sh ip bgp
BGP table version is 8, local router ID is 10.250.251.194
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
* i3.3.3.3/32       10.251.251.193           3    100      0 ?
*>                  10.250.251.193           2         32768 ?
* i4.4.4.4/32       10.251.251.193           2    100      0 ?
*>                  10.250.251.193           3         32768 ?
* i10.10.11.0/24    10.251.251.193           2    100      0 ?
*>                  10.250.251.193           2         32768 ?
* i10.250.251.192/28
                    10.251.251.193           3    100      0 ?
*>                  0.0.0.0                  0         32768 ?
* i10.251.251.192/28
                    172.16.1.1               0    100      0 ?
*>                  10.250.251.193           3         32768 ?
R2#

[roger perkin]

I have now idendifited what is probably normal behaviour but need to understand why?

If I shut down int f1/0 on R4 - this breaks the OSPF between R4 & R3

sh ip route on R1 then shows a BGP route to 3.3.3.3 via 10.250.251.193

So it appears to be working and the redistribution is working however when OSPF is running between R4 & R3 that is the preffered route to get to networks between R1 & R2

Thanks

Roger

[Jon Marshall]

Please ignore, looks like Milan has provided the answer.

Jon

[milan.kulik]

Hi,

that might be another issue:

OSPF having a better Administrative Distance (110) than iBGP (200).

HTH,

Milan

[milan.kulik · Accepted solution]

Hi,

looking to the outputs you provided, IMHO:

The OSPF redistribution to BGP is working.

But taking the 10.250.251.192/28 as an example, what had happened in the past?

I'm guessing:

Your router R1 had received this prefix originally via OSPF (through your MPLS backbone probably).

It redistributed that to his BGP table.

That's why you can see

R1#sh ip bgp
...
* i10.250.251.192/28 172.16.1.2               0    100      0 ?
*>                  10.251.251.193           3         32768 ?

Then you established the iBGP peering between your R1 and R2 routers.

R1 received the same prefix from R2, so you can see

   R1#sh ip bgp
...
* i10.250.251.192/28 172.16.1.2               0    100      0 ?
*>                  10.251.251.193           3         32768 ?

But as the original BGP prefix had  weight=32768 (created by this router), it remained the best within the BGP table.

I'd recommend:

a) use next-hop-self for iBGP session between your R1 and R2 routers

b) configure a route-map setting weight=0 when redistributing OSPF to BGP on those routers,

c) configure a route-map setting better weight or local preference for BGP prefixes incoming from R2 on R1 and vice versa

d) when a)-c) done, resetting OSPF neighbourship

HTH,

Milan

[roger perkin]

Thanks Milan,

The OSPF was the first thing I brought up. I will try all the things you suggested.

I really need to learn the behaviour of this setup

Roger