OSPF Summarisation

[daniel.bowen]

I am getting conflicting statements regarding the summarising of routes on an ASBR using the summary-address command.

Is the summary-address command used to summarise routes internal to the OSPF domain before advertising them to the outside world, or is it used to advertise external routes before injecting them into the OSPF domain?

thanks in advance for your help

Daniel,

[ralphcarey]

OSPF doesn't advertise anything to the 'outside world', the only way that OSPF learnt routes can get to another routing protocol is by redistribution configured in the other routing protocol process.

The summary-address command is used to summarise addresses redistributed into OSPF from elsehere.

[daniel.bowen]

So for example, if an OSPF domain was connected to an EIGRP domain, the ASBR would summarise the routes given to it by EIGRP and then inject the summarised route into the OSPF domain?

[fsebera]

The OSPF command "Summary-address" is ALSO used to summarize a block of IP addresses from one area into another - and this summarization CAN ONLY take place on an Area Border Router.

I.E.

from a stub area-to-area 0, OR

from a totally stubby area-to-area 0, OR

from an area other than area 0 into area 0

Keep in mind that the summary address does not have to be a contiguous block , but however, the missing addresses CANNOT be used anywhere else in the network except from the original summarized area. Else routing loops will occur.

Hope this helps

[daniel.bowen]

Thank you

[zaziz]

Hi Ralph,

What you are mentioning is accomplished by area x range command and not by

summary-address command.

Zaheer

[ralphcarey]

Hi Zaheer

Maybe my grammar wasn't very clear.

The area x range command is used to summarize addresses into or out of an OSPF area. This can only be done on an ABR and would normally be configured so that the addresses are summarized into area 0 rather than out of area 0.

The summary-address command is used to summarize addresses learnt from a non-OSPF process into an OSPF autonomous system. This can only be done on an ASBR.

Ralph

[zaziz]

summary-address will summarize ONLY external routes and advertise them into

ospf domain. This command can only be applied at ASBR

This new summary will go as an External LSA in all of your OSPF areas.

Hope this helps

Zaheer