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Can I create two default routes in S4-

We currrently have a 1gb link between S4 and External FW. We want to add a 10gb link and then remove the 1gb. The 10gb link is a different /30 ntwk. Can I add this as a second default route and when traffic starts using it I would remove the 1gb link. Will traffic automatically switch over since the link cost is lower. Trying to migrate without a hard cutover. OSPF environment.
thnks in advance

3 replies

Are you simply trying to upgrade the speed of the link?
Userlevel 3
Yes, you can add different default routes. If cost are the same the router will use both routes. Be aware that L2 cost / priorities are different from L3 cost / metrics.

In your case you should set a lower metric / lower OSPF cost to your new route. As soon a the interface is up (and OSPF adjacency is full) the router will use the new interface explicitly and without interruption.
Userlevel 5
Hi Walt,

Christoph's informed point about metric is a common point of confusion - particularly for those who networked before vlan routers came along. In the old days a 100Mb routed port had a lower ospf priority/metric than a gigabit link. (a poor example since gigabit ports and switch-routers/vlan routing sort of showed up together - but it gets the job done)

So OSPF is not aware of the layer two bandwidth on the S-series. A vlan might contain many ports - ISL's, end stations etc. If you plug in the two links as described OSPF considers both to be equal cost. You would see load share behavior from the router.

There is a way to teach OSPF layer two cost but its really not worth the trouble in this case. Best bang for the buck is Christoph's direction.

In the event there is no OSPF on these links your default routes will load share given equal cost/weight on the route statements. I think its round robin algorithm, flow by flow. So configure the other default route, plug in the big pipe and traffic will automatically begin even distribution (more or less). The established flows on the 1Gb link will continue to use that path - but load will soon even out naturally.

Then cut your one gig. Or pull the route pointing in that direction. seamless transition.

you're done.

Low risk of service impact. You could do this during production - except I never recommend configuring a router - particularly a critical item like your internet egress - during production. Too many people get mad if you happen to make a mistake 🙂

Let us know how it goes.