Redistribute OSPF default with higher metric
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06-01-2020 08:29 AM
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.
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06-01-2020 08:35 AM
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
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06-02-2020 06:57 AM
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.
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06-02-2020 07:58 AM
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
