reducing routing tables using stub areas

Answered Question
Sep 2nd, 2010

Hello,

I would like to reduce the number of default routes on all of the routers on our corporate network which is around 60 sites.  The routing tables are too big and i am running OSPF.  Is it possible to create stub routers on the routers that are off the core network (Layer 3 switches) and then they will just have a default route out of their router to the layer 3.

At the moment if i do a show ip route the table is huge because it has learned every route on the network.  I just want it to know a default route to the next hop which should be the core.  Their are about 6 core router sites on the network.

Any advice is welcome.

I have this problem too.
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Correct Answer by Jon Marshall about 6 years 4 months ago

ohareka70 wrote:

I was thinking of putting this onto the router that hangs off the core (area 0 layer 3 switch) and also putting the same config on the other routers that have a serial link to the first router.

conf t
router ospf 1
area 1 stub no-summary

so i should just be left with connected routes and a default route back into the network thereby reducing the size of the network tables.

What do you think?

Kevin

Kevin

Yes that will work. Just remember that all routers in area 1 will need the same config ie. they msut be configured as stub routers also.

Jon

Correct Answer by Jon Marshall about 6 years 4 months ago

ohareka70 wrote:

Hello,

I would like to reduce the number of default routes on all of the routers on our corporate network which is around 60 sites.  The routing tables are too big and i am running OSPF.  Is it possible to create stub routers on the routers that are off the core network (Layer 3 switches) and then they will just have a default route out of their router to the layer 3.

At the moment if i do a show ip route the table is huge because it has learned every route on the network.  I just want it to know a default route to the next hop which should be the core.  Their are about 6 core router sites on the network.

Any advice is welcome.

Kevin

If they are in different areas then yes you can use stub or totally stubby area configuration on those routers which would dramatically reduce the routing table size.

Be aware that there are certain restrictions for stub areas though ie.

1) you cannot create a virtual link across a stub area

2) a stub area can contain an ASBR - if you have an area with an ASBR you want to make a stub area then make it an NSSA.

As i say all the above assumes the routers you want to  make stub routers are in different areas from your core routers.

Jon

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Correct Answer
Jon Marshall Thu, 09/02/2010 - 14:19

ohareka70 wrote:

Hello,

I would like to reduce the number of default routes on all of the routers on our corporate network which is around 60 sites.  The routing tables are too big and i am running OSPF.  Is it possible to create stub routers on the routers that are off the core network (Layer 3 switches) and then they will just have a default route out of their router to the layer 3.

At the moment if i do a show ip route the table is huge because it has learned every route on the network.  I just want it to know a default route to the next hop which should be the core.  Their are about 6 core router sites on the network.

Any advice is welcome.

Kevin

If they are in different areas then yes you can use stub or totally stubby area configuration on those routers which would dramatically reduce the routing table size.

Be aware that there are certain restrictions for stub areas though ie.

1) you cannot create a virtual link across a stub area

2) a stub area can contain an ASBR - if you have an area with an ASBR you want to make a stub area then make it an NSSA.

As i say all the above assumes the routers you want to  make stub routers are in different areas from your core routers.

Jon

ohareka70 Mon, 09/06/2010 - 09:20

I was thinking of putting this onto the router that hangs off the core (area 0 layer 3 switch) and also putting the same config on the other routers that have a serial link to the first router.

conf t
router ospf 1
area 1 stub no-summary

so i should just be left with connected routes and a default route back into the network thereby reducing the size of the network tables.

What do you think?

Kevin

Correct Answer
Jon Marshall Mon, 09/06/2010 - 09:47

ohareka70 wrote:

I was thinking of putting this onto the router that hangs off the core (area 0 layer 3 switch) and also putting the same config on the other routers that have a serial link to the first router.

conf t
router ospf 1
area 1 stub no-summary

so i should just be left with connected routes and a default route back into the network thereby reducing the size of the network tables.

What do you think?

Kevin

Kevin

Yes that will work. Just remember that all routers in area 1 will need the same config ie. they msut be configured as stub routers also.

Jon

Paolo Bevilacqua Thu, 09/02/2010 - 16:50

Huge like what, 200, 300 or how many routes ?

Consider routers and switches handles routinely hundred of thousands.

The size of routing table doesn't matter much.

Of course in stub locations there is no meaning having it, but even if you do, nothing bad happens.

What matter is if you are happy with the network setup in its overall aspect of performances, stability,  administration overhead.

ohareka70 Mon, 09/06/2010 - 09:24

Its about 500 routes in each table.  Its interesting that you think the routing table doesnt matter much - maybe you are right about this.  I take it i dont put a stub area on the core sites?

Kevin

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