Redistribute OSPF default with higher metric

aweise · ‎06-01-2020

I have two edge routers on my enterprise network that are both receiving a default route via BGP with my ISP. I want to be able to redistribute the default routes into my internal network, but preferring one over the other. I'm doing OSPF on these routers with the neighbor being a Palo Alto firewall. The topology is like this at both locations:

Edge router (ISR4431) --- Palo Alto 3050 FW --- Nexus core switches --- Internal WAN router (ASR 1002-X)

Site A is the primary data center and Site B is the DR site (which we use as secondary for internet traffic). In my lab, I tried configuring the Site A router to tell the neighbor that the default was an external type-2 using the "default-information originate metric-type 1". However, this didn't seem to propagate to the internal routers.

Instead, on Site B can I do the following:

default-information originate metric 10

So that the metric from Site B will be higher and tell the internal neighbors just that, thus preferring everything through Site A.

The way things are working now is that all core devices in Site B will use Site B's perimeter router as the default, all the way through to the Internal WAN router. Likewise, the devices in Site A will use Site A's perimeter router as the default.

I would prefer to see the default advertised so that everything prefers Site A, but I'm not sure how this can be done in this environment.

Giuseppe Larosa · ‎06-01-2020

Hello @aweise ,

you can use a route-map to decide when the default route is generated in OSPF in each router.

In doing this you can set the metric type to O E1 for router in site A so that because O E1 is preferred over O E2 (the default type) until site A generates a default route it will be preferred.

Alternatively you can generate both default routes of type O E1 but site B can use an higher seed metric like 1000 so that in normal conditions it is not used by any device.

I have used this second approach in the past with good results.

In my case I was checking for the existance of a default route learned in eBGP by checking the next-hop of the default route.

Hope to help

Giuseppe

aweise · ‎06-02-2020

Thank you, Giuseppe. The perimeter routers both hear a default route via eBGP, so the default route exists.

I was going to do something like the following:

route-map DefaultToOSPF permit 10

set metric-type type-1

!

router ospf 1

default-information originate route-map DefaultToOSPF

I will attempt this during our next maintenance window.

Giuseppe Larosa · ‎06-02-2020

Hello @aweise ,

you can also set the seed metric so I would use the following:

RA:

route-map DefaultToOSPF permit 10

set metric-type type-1

set metric 50

!

RB:

route-map DefaultToOSPF permit 10

set metric-type type-1

set metric 1000

!.

In this way RA of site A will be preferred in normal conditions if RA stops to send its own default route in OSPF, RB's default route will be used.

Hope to help

Giuseppe