Query of ospf network advertisment

[Steev112]

                   Dear All,

If i created interface vlan on switch ex: 192.168.10.1/24 but when i advertise this subnet under ospf i use /16 as showing in below

router ospf 2

network 192.168.10.0 0.0.255.255 area 0

my query is what is disadvantage or the problem to keep it like that without correcr the wildcard.

Thanks

[mgalazka]

A couple things to consider -- not really advantages/disadvantages, but talking points I suppose.

1. When you input "network 192.168.10.0 0.0.255.255 area 0" into config terminal, it will "fix" this before actually inserting into running config.  Meaning that your running config will instead contain: network 192.168.0.0 0.0.255.255 area 0

2. By defining your network statement as you did, more potential interfaces will have OSPF enabled on them. This may be what you want from a route advertisement perspective.  However, if you don't have passive-interface on by default, then any routed interface you bring up that falls into 192.168.0.0/16 will be potentially eligable to form an OSPF neighborship (of course other neighbor relationship rules still apply).

3. Another point to be made -- if you for instance decide later on to define specific network statements for other longer prefix'd subnets, or even for the 192.168.10.0/24 network -- OSPF will order the network statements for decision-making from most-specific to least-specific (this should be reflected in the running-config).  So if you were to add a "network 192.168.1.0 0.0.0.255 area 0" later on -- that new network statement would be the one that is used by OSPF with respecf to your SVI above, not the original one with the wildcard mask of 0.0.255.255. 

Anyway, just a couple things to consider.

Matt

[Richard Burts]

Steven's question seems to reflect a common assumption that the mask used in the network statement affects what is advertised. And Matt correctly identifies the effect of the mask as broadening the scope of networks that potentially match the network statement.

So if some interface does match the longer mask and starts to run OSPF where that was not intended then this is a negative of not correcting the mask. Otherwise no negative impact from using the longer mask in the network statement.

Rick

Sent from Cisco Technical Support iPad App

[Kyle McKay]

Matt has made some excellent points.

In a nutshell  you can configure your network statements whichever way you'd like. If you configure them in a broad fashion as you have explained, you are taking more risk but at the same time you are simplifying your overall configuration.

If you were to configure the network statement in a very strict fashion (which is what I do) you will take less risk but incur more complexity.

My personal preference is on the latter.. Configure with Intent, as they say.

router ospf 100

network 192.168.10.1 0.0.0.0 area 0

With this configuration, you are specifying the exact interface on which to activate the OSPF process and are thereby reducing the potential for issues.

[Steev112]

Dear All,

Thanks for your explaination , now it is clear, thanks agian