default gw in ospf

[dave dave]

hi! I've a layer 3 switch with the following config and there's a 29xx router connected to this L3 switch. Both the routers and L3 switch are in the same area which is 0. My question is that, how do i populate the gateway of last resort/default route to all the routers within the same area? I tried the config below, but it doesn't seems to populate the default gateway to other routers. any idea how i can achive this? or i've to manually configure the default route in every routers?

Another question is that how do i manipulate the metric so that i can give preference to one line and not another (both the destinatoin are the same but using different gateway, the traffic shd only go through the backup link with lower preference if the prefered link is down). Can i use the value (254, 252) to control the preference? Is 252 the prefered metric over 254? Thanks.

router ospf 1

log-adjacency-changes

redistribute connected subnets

redistribute static subnets

network 10.10.8.0 0.0.7.255 area 0

!

no ip http server

no ip http secure-server

!

ip route 0.0.0.0 0.0.0.0 10.10.15.101

ip route 10.0.0.0 255.0.0.0 10.10.15.102 254

ip route 10.0.0.0 255.0.0.0 10.10.15.103 252

[Peter Paluch]

Hello Dave,

In Cisco OSPF implementation, it is not possible to redistribute a default route into OSPF. A security check built into the IOS prevents this. The correct way to inject a default route into OSPF is to configure:

router ospf 1

default-information originate

If you do not need to have the 10.0.0.0/8 networks redistributed into OSPF, you may remove the redistribute static subnets command from your OSPF configuration.

Regarding the static routes to the 10.0.0.0/8 network: currently, you have configured the administrative distance (not metric) of the routes so that the route via 10.10.15.103 is the preferred route, and the route via 10.10.15.102 is a backup route. The higher the administrative distance, the less preferred the route is. So - yes, 252 is better than 254.

Best regards,

Peter

[dave dave]

hi! Thanks it works, but then how do i restrict this default route from propagating to the hq and other sites that might have their own default gateway out to the internet? All the sites are in area 0.

or i shd just add in the default route manually in the routers? which is the better option and easiest to manage?

Thanks.

[Peter Paluch]

Dave,

How is the default route currently known on HQ and other sites, i.e. where does it come from? If it is already configured statically, then OSPF will not override that default route on Cisco routers, as static routes are always preferrred to OSPF routes.

Best regards,

Peter

[dave dave]

Hi!

I think most of the sites r using individual default route. As all the sites are using area 0, and to prevent other sites from getting the "default route" fr me. Do you think it's more advisable for me to use individual statically configured default route in all my 4 routers across our region as there share the same gateway to go out to the internet? What's your advise on this? And the normal practice for this? Thanks

[Jon Marshall]

Dave

You seem to be giving conflicting information. One minute you want to propogate this default-route throughout area 0 because they share the same internet gateway and the next you ask how to limit the propogation of the route because each site might have it's own internet connection.

So not sure which it is ?

Standard practice if you have one connection to internet is to inject the default-route from the site with that connection and have it propogated to all other sites

Jon

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Would using an E1 type default route work for you?  (I.e. hosts will chose their own best default)

[Jon Marshall]

Well it might if we could ascertain whether there is a need for multiple default-routes or whether there is actually only one internet connection

[dave dave]

hi! Peter, if one of the site in the U.S is also receiving default route through the default information originate command from on of their router and in the Asia Pac, i'm also using this command to propagate to the sites in Asia PAc, will the default route from my sites propagate to some of the sites in the U.S? What's the rule that will detemine the router to take the default route from U.S or AsiaPAc? thx

[dave dave]

hi! for the route preference question, by setting the AD to 252 (or do not set anything - default 1) will the secondary route kick in when the first route failed? or it's better just to set the backup/secondary link to 254 and leave the main link to learn by itself through ospf (without adding in a default route)?

ip route 10.0.0.0 255.0.0.0 10.10.15.102 254

ip route 10.0.0.0 255.0.0.0 10.10.15.103 252

or is it better to do this? Will the backup link kick in in this config below? or the main link of AD 1 will always be in the routing table even if the link failed or no longer exist?

ip route 10.0.0.0 255.0.0.0 10.10.15.102 254

ip route 10.0.0.0 255.0.0.0 10.10.15.103

thanks.

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

See:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9f0.shtml

[dave dave]

Hi sorry for the confusion, in the region of Asia PAC, we've few sites which share the same internet gw, but in the u.s. We hv multiple sites with multiple Internet gw. For my region, which is the Asia PAC if I put in that command will my default route be propagated to the u.s?there r all in area 0 thx

[dave dave]

hi! Anyone, got an answer on this? I want to prevent the default route (using

default-information originate)

from being propagated to our U.S office, but i want it to be propagated to the Asia Pac sites.

Thanks.

[lgijssel]

When all sites are in area0 it will not be possible to filter or restrict propagation of the default route.

You are deemed to entering statics on every router that's applicable.

BTW: This looks like one of the many improperly designed ospf implementations.

Having such a geographically vast territory (and presumably lots of routers too) covered with just area0 means you are sending lots of LSA's over the WAN, all consuming bandwidth, even more so when they are delivered too late or not at all. This topology is a warranty for longest convergence time.

If you are not end-responsible for this setup, you may consider to apply for the job of the person who is.

regards,

Leo

[dave dave]

hi! in the case of using different area. What's the easiest way to selectively control the default route or extenal route from being propagated?