discard routes internal and external on NSSA ABR ?

manoj9372 · ‎03-18-2014

link between r1 and r2 is in area 0.(10.1.1.0/30)

link between r2 and r3 is in area1.(10.1.1.5/30)

link between r3 and switch(i.e f0/1) is running rip.(20.1.1.1)

area 1 is nssa.

all routers have a loopback of x.x.x.x/32,
'x' is the router numbers.

r1's loopback is in area 0,
r2 and r3's loopback in area 1.

redistribute rip subnets is configured under r3's ospf process.

case 1:

1)i configured this command on NSSA ABR.(i.e r2)

area 1 range 3.0.0.0 255.0.0.0

and i checked on r1's routing table i found classful
route for 3.0.0.0/8

again i configured this command on NSSA ABR.

2)area 1 range 3.0.0.0 255.0.0.0 not-advertise

i checked r1's routing table no route for 3.0.0.0 network.

fine.

again i configured this command on NSSA ABR.

3)no discard route-internal

but i didn't find the 3.0.0.0/8 in r1's routing table and in r2's routing table.

question:

1)did this command "no discard-route internal" prevents the installation of summary route in the
routing table or just remove the summary route pointing to null0,so that we can route with
less matching routes such as default routes ?

case 2:

1)i configured this command on NSSA ABR.(i.e r2)

summary-address 20.0.0.0 255.0.0.0 not-advertise

and i checked on r1's routing table i found classful
route for 20.0.0.0/8

2)again i configured this command on NSSA ABR.

summary-address 20.0.0.0 255.0.0.0 not-advertise

and i checked on r1's routing table i didn't found any routes for 20.0.0.0/8 on r1.

3) again i configured this command on NSSA ABR.

no discard route-external

but i didn't find the 20.0.0.0/8 in r1's routing table and in r2's routing table.

again my question is same as the first case,

questions:
1)did this command "no discard-route external" prevents the installation of summary route in the
routing table or just remove the summary route pointing to null0,so that we can route with
less matching routes such as default routes ?

2)shouldn't i use both discard routes external and internal on this NSSA ABR(i.e r2) ?

what i tought regarding these question is when i enter not-advertise in both the cases,
i discarded the routes from routing table using discard route external and internal command.

i expected both the prefixes 3.0.0.0/8 and 20.0.0.0/8 will be in r1's routing table,but it doesn't

I had attached the configs i used on the routers:

Rolf Fischer · ‎03-24-2014

Hi,

what i tought regarding these question is when i enter not-advertise in both the cases,
i discarded the routes from routing table using discard route external and internal command.
i expected both the prefixes 3.0.0.0/8 and 20.0.0.0/8 will be in r1's routing table,but it doesn't

with the not-advertise keyword, the advertisement of all networks matching the network/mask defined in the area range or summary-address command will be suppressed. In other words: With the not-advertise keyword, those commands act like filters and do not generate summaries. That's why you didn't see the prefixes in R1's routing table.

Disabling the auto-generation of discard-routes is only locally significant.

Btw: Suppressing the 3.0.0.0/8 range (not-advertise) will also remove the external 20.1.1.1/24 prefix from R1's routing table, because the Forwarding Address of the Type-5 LSA (3.3.3.3) is no longer reachable. As soon as you configure a summary for this external network, R2 sets the FA to 0.0.0.0 and the external summary will be installed in R1's routing table.

HTH

Rolf