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paulkilcoyne
Beginner

dual WAN failover & OSPF "ring"

To whom it concerns,

I have the following topology;

Two WAN circuits terminated on two separate Cisco routers R1 & R2.

Physically the LAN consists of 6 switches, switch 1 connects to switch 2 connects to switch 3 connects to switch 4 connects to switch5 and finally switch 5 connects to switch 6 i.e. a physical ring.

Logically the lan switches connect to adjacent switches via OSFP.

R1 connects to switch 1 physically and they form a OSPF neighbourship.

R2 connects to switch 6 physically and they form a OSPF neighbourship.

R1 OSPF config:

router ospf 999
 router-id 10.20.30.254
 area 0 authentication message-digest
 redistribute connected subnets
 network 10.20.30.0 0.0.0.255 area 0
 network 10.26.64.0 0.0.0.255 area 0
 default-information originate always

R1 OSPF config:


router ospf 999
 router-id 10.20.30.253
 area 0 authentication message-digest
 redistribute connected subnets
network 10.20.30.0 0.0.0.255 area 0
 network 10.26.64.0 0.0.0.255 area 0
 default-information originate always

Assuming one of the ISPs circuits goes down ( eBGP down ) will traffic from the LAN re-route via the other ISP?

If not, what will happen? Is there a better way to have fail-over without implementing iBGP?

Thanks,

Paul

3 REPLIES 3
chrihussey
Collaborator

With both routers having the "default-information originate always" you may run into issues. If an ISP fails, the router will still be advertising the default route.

If the ISPs are sending you a default route via BGP it would probably be cleaner and more reliable to redistribute the default route into OSPF (just the default route), and then just use "default information originate" minus the "always".

Hi and thank you for the reply.

the ISPs are not sending a default route here. The closet thing to that is:

*>  10.0.0.0         10.x.x.14                          0 19855 65529 65099 65170 ?

Is there a work around here to achieve dual-home failover?

Is it possible with just OSPF?

Thanks again,

PK

You could ask the ISPs to send you the default route. That would be the easiest.

Aside from that, you could try conditional default route injection. Never done it myself but seems pretty simple.  Here's a link:

http://packetlife.net/blog/2008/dec/12/ospf-conditional-default-route-injection/

I'd try and use one of the routes advertised from the ISP first as opposed to the physical link. I say that because the link could be up and the BGP session could be down and you'd still advertise the default route.

Hope this helps.