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OSPF not advertising a "secondary" network on an interface?

Answered Question
Aug 15th, 2012
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Hi there,


Bit of an odd one. I've got a /30 network I've configured over a transit tail circuit between our L3 switch and a network providers equipment, which requires media conversion from 1G fibre to 10MB copper. As part of this I've also configured another /30 over this interface as a secondary network to manage the media converter.


Basically, the problem is that when I try and advertise the secondary network (the media converter /30) into OSPF, it's not being propagated - I understand that OSPF will only form adjacencies with the primary IP's on an interface, but I'm not looking to do this - I just want to advertise a "secondary" network assigned to the interface - possible?



Config:-


!

!

interface GigabitEthernet0/25

description LARGE_GLOBAL_CARRIER_CIRCUIT_NAME_HERE

no switchport

ip address 172.21.67.18 255.255.255.252 secondary

ip address PUBLIC_IP 255.255.255.252

load-interval 30

end

!

!

router ospf 1

router-id 172.21.67.1

log-adjacency-changes

passive-interface default

no passive-interface Vlan200

network xxx.xxx.xxx.xxx 0.0.0.255 area 0

network xxx.xxx.xxx.xxx 0.0.0.255 area 0

network xxx.xxx.xxx.xxx 0.0.0.255 area 0

network 172.21.67.16 0.0.0.3 area 0

!



Any ideas?


Many thanks in advance,


Damian

Correct Answer by Peter Paluch about 5 years 3 days ago

Hi John,


To get around this, you could create a loopback with this secondary  interface address on it and change the network type to point-to-point to  keep it from advertising a stub network.


I am afraid this will cause the device to complain about IP address overlap - two different interfaces having overlapping subnets, even if one of these subnets is a secondary network.


The only way I see currently to do this is to perform redistribute connected subnets, presumably with a route-map to allow only selected networks to be redistributed. This will cause the networks to be learned as external, though.


Best regards,

Peter

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John Blakley Wed, 08/15/2012 - 03:37
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Damian,


At first glance, I'm going to assume that it's because of the passive-interface default command in the ospf process. Try adding "no passive-interface g0/25" and see if that helps.


HTH,

John


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damianbell Wed, 08/15/2012 - 03:43
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Hi John,


Thanks for the pointer, however I'm not looking to try and form an OSPF adjacency over that interface - I'm just trying to advertise the /30 network located on it. As seen in the config, I only use VLAN 200 to form my OSPF adjacency with a neighbor router...


Maybe code revision? I'm running 12.2(46)SE, platform is 3650E.


I wouldn't think there should be a problem with advertising a secondary interface?



Damian

John Blakley Wed, 08/15/2012 - 04:04
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Damian,


I understand now...OSPF will only advertise a secondary address when you are advertising the primary address as well into the same area. For example, I don't know about your public address above, but let's assume that you're not advertising that interface into ospf. If that's the case, you won't be able to advertise the secondary on that interface either. It also has to be in the same area that you're advertising your primary interface in as well. To get around this, you could create a loopback with this secondary interface address on it and change the network type to point-to-point to keep it from advertising a stub network.


HTH,

John

Correct Answer
Peter Paluch Wed, 08/15/2012 - 05:18
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  • Cisco Employee,

Hi John,


To get around this, you could create a loopback with this secondary  interface address on it and change the network type to point-to-point to  keep it from advertising a stub network.


I am afraid this will cause the device to complain about IP address overlap - two different interfaces having overlapping subnets, even if one of these subnets is a secondary network.


The only way I see currently to do this is to perform redistribute connected subnets, presumably with a route-map to allow only selected networks to be redistributed. This will cause the networks to be learned as external, though.


Best regards,

Peter

damianbell Wed, 08/15/2012 - 05:29
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Hi John,


To get around this, you could create a loopback with this secondary  interface address on it and change the network type to point-to-point to  keep it from advertising a stub network.


I am afraid this will cause the device to complain about IP address overlap - two different interfaces having overlapping subnets, even if one of these subnets is a secondary network.


The only way I see currently to do this is to perform redistribute connected subnets, presumably with a route-map to allow only selected networks to be redistributed. This will cause the networks to be learned as external, though.


Best regards,

Peter




Many thanks for the confirmation Peter - this was the conclusion I drew as well, unfortunately. A rather convoluted way of doing this (I don't particularly want to advertise the primary /30 on the interface as it's actually a BGP transit link) so this will be the approach I take. Many thanks for yours (and of course John Blakleys') help.


Appreciated!


Damian

adnane dakna Wed, 08/15/2012 - 03:48
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effectively , try removing "passive interface default " will resolve the problem

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