Mailing List Archive: Routing for LSA 7 in NSSA not working as expected

Hi all,

I am testing a setup with 3 routers in an NSSA:

router A, router B, router C, connected in triangle. Router A
redistributes the connected 10.10.0.0/24 network.
On router B, I see:
10.10.0.0/24 via 192.168.250.10 dev eth1 proto zebra metric 30

On router C however, I see:
10.10.0.0/24 proto zebra metric 40
nexthop via 192.168.251.11 dev eth1 weight 1
nexthop via 10.200.0.10 dev eth2 weight 1

I do not want the next hop 192.168.251.11 and I do not understand why
it shows up with the same metric as it is one extra hop.

If I make the area a normal OSPF area instead of an NSSA, all works
fine. Please find the configurations below, can someone shed some
light on this please?

Thanks in advance!
Erwin

IP configuration:
Router A:
eth1 = 192.168.250.10/24
eth2 = 10.200.0.10/24
eth3 = 10.10.0.10/24

Router B:
eth0 = 10.0.2.15/24
eth1 = 192.168.250.11/24
eth2 = 192.168.251.11/24

Router C:
eth1 = 192.168.250.12/24
eth2 = 10.200.0.12/24
eth3 = 10.12.0.12/24

Router A:
ip prefix-list into-ospf seq 5 permit 10.10.0.0/16 le 32
ip prefix-list into-ospf seq 10 deny 0.0.0.0/0 le 32
route-map connected-to-ospf permit 5
match ip address prefix-list into-ospf
router ospf
network 192.168.0.0/16 area 1
network 10.200.0.0/24 area 1
router-id 10.200.0.10
redistribute connected route-map connected-to-ospf metric-type 1
log-adjacency-changes
area 1 nssa

Router B:
router ospf
network 192.168.0.0/16 area 1
network 10.200.0.0/24 area 1
network 10.0.2.0/23 area 0
router-id 192.168.251.11
area 1 nssa

Router C:
router ospf
network 192.168.0.0/16 area 1
network 10.200.0.0/24 area 1
router-id 10.200.0.12
redistribute connected route-map connected-to-ospf metric-type 1
area 1 nssa

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Hi all,

Small update: it appears that the forwarding address in the LSA 7 is the issue:
Router A sends the LSA7 with forwarding address 192.168.250.10. Router
C receives it and calculates the route falsely as [cost to get to
route A] + [cost to get to 192.168.250.0/24] + [cost of external
route] = 10 + 10 + 20 = 40 and this is exactly the same as [cost to
get to route C] + [cost to get to 192.168.250.0/24] + [cost of
external route], hence the two routes with equal cost. Of course, in
the rouge via A the cost to get to 192.168.250.0/24 should not be
counted as we just needed to get to router A.

It seems like a bug to me, but how can I fix it? Is there a valid workaround?

Kind regards,
Erwin

On Mon, Feb 6, 2017 at 1:31 PM, Erwin Van de Velde
<erwin.vandevelde@gmail.com> wrote:
> Hi all,
>
> I am testing a setup with 3 routers in an NSSA:
>
> router A, router B, router C, connected in triangle. Router A
> redistributes the connected 10.10.0.0/24 network.
> On router B, I see:
> 10.10.0.0/24 via 192.168.250.10 dev eth1 proto zebra metric 30
>
> On router C however, I see:
> 10.10.0.0/24 proto zebra metric 40
> nexthop via 192.168.251.11 dev eth1 weight 1
> nexthop via 10.200.0.10 dev eth2 weight 1
>
> I do not want the next hop 192.168.251.11 and I do not understand why
> it shows up with the same metric as it is one extra hop.
>
> If I make the area a normal OSPF area instead of an NSSA, all works
> fine. Please find the configurations below, can someone shed some
> light on this please?
>
> Thanks in advance!
> Erwin
>
>
> IP configuration:
> Router A:
> eth1 = 192.168.250.10/24
> eth2 = 10.200.0.10/24
> eth3 = 10.10.0.10/24
>
> Router B:
> eth0 = 10.0.2.15/24
> eth1 = 192.168.250.11/24
> eth2 = 192.168.251.11/24
>
> Router C:
> eth1 = 192.168.250.12/24
> eth2 = 10.200.0.12/24
> eth3 = 10.12.0.12/24
>
> Router A:
> ip prefix-list into-ospf seq 5 permit 10.10.0.0/16 le 32
> ip prefix-list into-ospf seq 10 deny 0.0.0.0/0 le 32
> route-map connected-to-ospf permit 5
> match ip address prefix-list into-ospf
> router ospf
> network 192.168.0.0/16 area 1
> network 10.200.0.0/24 area 1
> router-id 10.200.0.10
> redistribute connected route-map connected-to-ospf metric-type 1
> log-adjacency-changes
> area 1 nssa
>
>
> Router B:
> router ospf
> network 192.168.0.0/16 area 1
> network 10.200.0.0/24 area 1
> network 10.0.2.0/23 area 0
> router-id 192.168.251.11
> area 1 nssa
>
>
> Router C:
> router ospf
> network 192.168.0.0/16 area 1
> network 10.200.0.0/24 area 1
> router-id 10.200.0.12
> redistribute connected route-map connected-to-ospf metric-type 1
> area 1 nssa

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