Nexus 7K Ignoring Received OSPF Route

[paul.l.kyte]

I have a Catalyst switch that is redistributing some static routes into OSPF. These are received on a Nexus 7K and appear in the database however the 7K does not add them to its routing table, one of the routes is ignored and not added. I haven't got a clue why this is happening, can anyone please help with this?

The routes on the Catalyst are as follows with ID of 172.30.255.22:

ip route 172.24.59.0 255.255.255.0 10.56.7.46

ip route 192.168.168.0 255.255.255.0 10.56.7.62

on the 7K the database shows:

172.24.59.0     172.30.255.22   1374       0x80001a44 0x1a48    0

192.168.168.0   172.30.255.22   1374       0x80001a45 0x6c5b    0

The routing table shows:

sh ip ro 172.24.59.0/24
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]

172.24.59.0/24, ubest/mbest: 1/0
    *via 172.30.253.10, Po7, [110/20], 20w4d, ospf-NCC, type-2

sh ip ro 192.168.168.0/24
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]

Route not found

Help!!!!!!!!!!!

[Giuseppe Larosa]

Hello Paul,

the two routes have different next-hops on the ASBR node.

verify that the forwarding address in the LSA 5 type 2 data structure is reachable via an internal OSPF route.

OSPF does not accept an external route with a forwarding address that is reachable via another external route

Check with

show ip route 10.56.7.62

and look at the contents of the LSA type 5

Hope to help

Giuseppe

[paul.l.kyte]

Giuseppe,

I have attached a log of the output from sh ip ospf data external detail. Could you please look at this and advise.

When looking a the route for 10.56.7.62 I get the below, but I also get this for 10.56.7.46 for the other static route.

Please advise/explain further.

Thanks,

Paul

sh ip ro 10.56.7.62
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]

10.56.0.0/16, ubest/mbest: 1/0
    *via 172.30.253.10, Po7, [110/20], 2d19h, ospf-NCC, type-2

sh ip ro 10.56.7.46
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]

10.56.0.0/16, ubest/mbest: 1/0
    *via 172.30.253.10, Po7, [110/20], 2d19h, ospf-NCC, type-2

[Giuseppe Larosa · Accepted solution]

Hello Paul,

the forwarding addresses are actually both learned via an external route

LS age: 2006

   Options: 0x20 (No TOS-capability, DC)

   LS Type: Type-5 AS-External

   Link State ID: 10.56.0.0 (Network address)

   Advertising Router: 10.104.7.18

   LS Seq Number: 0x80000079

   Checksum: 0x96c3

   Length: 36

   Network Mask: /16

         Metric Type: 2 (Larger than any link state path)

         TOS: 0

         Metric: 20

         Forward Address: 0.0.0.0

         External Route Tag: 0

in this scenario both external routes SHOULD not be installed in the IP routing table of Nexus node!

To fix this you can on the ASBR node use:

router ospf XX

network 10.56.0.0 0.0.0.255 area YY

passive-interface

in this way you will generate an OSPF internal route for the IP next-hops / forwarding addresses

Hope to help

Giuseppe

[paul.l.kyte]

Guiseppe,

Ok, thanks.

I think I understand. I'm going to do some reconfiguring on the ASBR and will let you know how I get on. Please come back and have a look in about a weeks time. I'll post an update.

Thanks,

Paul

[paul.l.kyte]

Giuseppe, all fixed now

I looked at my ASBR (172.30.255.22) and saw that the link to the target network wasn't configured as a passive interface hence the next hop address of 10.56.7.62 was being advertised into OSPF. This link should be passive because there isn't an OSPF neighbour on that link.

I changed it to be passive and the database on the downstream 7K changed to have 0.0.0.0 as the forwarding address, everything then worked. The 7K placed a route into its routing table and this was passed downstream into the rest of the network.

Many thanks for your help. I've learn't something new.

Paul