area x nssa no-summary question

[shannonr]

Hi,

We have a router that on one side connects to the backbone area and has an nssa area on the other side:

CORE -- > Area 0 <-- OUR ROUTER --> Area 123 <-- DOWNSTREAM ROUTER

We have the command "area 123 nssa no-summary" added to the router OSPF config which generates and advertises only the default route to the downstream router which is great.

OUR ROUTER learns the default route from the CORE.

The downstream router has another connection as a backup and what we have found is that if OUR ROUTER loses connectivity the CORE it loses its default route (expected) but still advertises the default route to the downstream router.

This causes an issue where the downstream router wont use its backup path because it is still learning the route from OUR ROUTER which has nowhere to send the traffic.

How do I configure the OSPF NSSA area on OUR ROUTER to only advertise the default route if it has that route in its own routing table?

Note: The command in OSPF is exactly "area 123 nssa no-summary" - there is no "default-information originate always" or anything like that appended to it, or elsewhere in the OSPF config.

OSPF CONFIG:
router ospf 10 vrf CORP

router-id 10.123.123.1

auto-cost reference-bandwidth 100000

capability vrf-lite

area 123 nssa no-summary

area 123 default-cost 100

passive-interface default

no passive-interface Tunnel11 <----to CORE

no passive-interface Vlan530 <----to DOWNSTREAM ROUTER

Thanks!!

[MHM Cisco World]

This defualt route is O IA or E ?

I think it O IA and it by defualt generate when you config totally stub NSSA area. 

to solve issue you need to use NSSA not NSSA totally stub 

MHM

[shannonr]

Hey,

Yes it is O IA - coming from the CORE.

[MHM Cisco World · Accepted solution]

the ABR between Area0 and NSSA Totally stub area always inject default route if it have or it have not. 

take your time and update us 

thanks 

MHM

[shannonr]

Thanks for confirming it is good to understand how that works.

We wanted it as a Totally stub area to only advertise the default route as the downstream router can't handle all the routes coming from the backbone area.
Will look at testing and implementing Paul's suggestion to track the default route and remove/add the NSSA total stub area config based on the tracking.

[paul driver]

Hello

---

@shannonr wrote:We wanted it as a Totally stub area to only advertise the default route as the downstream router can't handle all the routes coming from the backbone area.

---

So based on this information, you will need to tweak the EEM script to tear down the ospf adjacency between to the downstream router, as as it stands that script will ONLY remove the advertisement of the default route and negate it being a no so totally stubby area and become a not-so-stubby area thus will then allow the advertisement of type1,2,3 LSAs into that down stream rtr which may not be what you require.

action 1:3 cli command "no passive-interface Vlan530"
action 2:3 cli command "passive-interface Vlan530"

May i ask, If its only a default route you require then maybe static routing is another option other than OSPF, this default can also be applied to be conditional if the upstream router lost it own connectivity- just a thought?

[David Ruess]

Hello,

Under the OSPF process there should be a route-map (or ip prefix) option to append to the command. In that configuration you should be able to specify which route needs to be in the routing table (or not in the routing table) to advertise the default. This might help solve your issue. Id be able to lab it up later tonight to get specifics as well.

Hope that helps

-David

[paul driver · Accepted solution]

Hello

---

@shannonr wrote:

How do I configure the OSPF NSSA area on OUR ROUTER to only advertise the default route if it has that route in its own routing table?

Ospf will always install a default when you are negating the advertisement of inter-area routes (no summary)
try the following using eem scripting, this should negate the Not-So-Totally Stubby Area default ONLY if the BB rtr does not have its own default in its rib table.

OUR ROUTER

track 10 ip route 0.0.0.0 0.0.0.0 reachability

event manager applet Active-default-route
event track 10 state up
action 1:0 cli command "enable"
action 1:1 cli command "conf t"
action 1:2 cli command "router ospf 10 vrf CORP"
action 1:3 cli command "area 123 nssa no-summary"
action 1:4 cli command "do wr mem"
action 1:5 cli command "end"

event manager applet no-default-route
event track 10 state down
action 2:0 cli command "enable"
action 2:1 cli command "conf t"
action 2:2 cli command "router ospf 10 vrf CORP"
action 2:3 cli command "no area 123 nssa no-summary"
action 2:4 cli command "do wr mem"
action 2:5 cli command "end"

[shannonr]

Hey,

This is a really clever idea - thanks for sharing. This is a live/production site so it may take a bit of time for me to arrange the required window to implement and test this, but I see no reason why it wouldn't work.

Thanks a lot!