Default Route Advertisement from iBGP causing OSPF to need "always"

[GH3]

Hello,

I am currently doing some labs, and I wanted to understand why this configuration requires the always command:

As shown, R1 and R2 are iBGP neighbors, and R2 receives its default route from R1 via the neighbor x.x.x.x default-originate command. R2 then advertises this default route into OSPF via the default-information originate command, but requires always to be appended.

Without the "always" option, the Edge RTR will not install the default route advertised from R2 (Received from R1)

Alternatively, if R1 and R2 are in different AS's, the always option is not required.

Is this a result of AD values or another function?

Thank you.

[paul driver]

Hello

The reason being is that ospf edge rtr has only one way to exit  and thats via R2 so in this instance applying the ospf always keyword just makes that default permanent in the rib table of the edge rtr. ( as long as ospf is active)

[GH3]

Why would this be different than if R2 and R1 were eBGP neighbors rather than iBGP?

Without the always command, Edge, will not install the default route advertised from R2 (which is learned from R1).

I added this information to the post.

[paul driver]

Hello

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@GH3 wrote:

Why would this be different than if R2 and R1 were eBGP neighbors rather than iBGP?

Without the always command, Edge, will not install the default route advertised from R2 (which is learned from R1).

---

As long as R2 has a default in its own routing table, being received by R1 or with its own static route then the default-information originate (without always keyword) should provide a default route to the edge rtr, if r2 didn’t have any default route in its own route table then you would need to add the "always" keyword so that the edge rtr is able to receive a default.

[GH3]

Thank you, Paul

R2's routing table looks like this:

Gateway of last resort is 20.20.20.30 to network 0.0.0.0




B* 0.0.0.0/0 [200/0] via 20.20.20.30, 00:40:07
2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

Does the default route received from iBGP not count as a default route in its own routing table? If not, why would eBGP act differently?

[paul driver · Accepted solution]

Hello
Thats by design, an ibgp route wont by default be advertise into an internal routing protocol such as ospf, unless you specially state it to do so.

try the following when ibgp peering
On the R2 

router bgp xx
bgp redistribute-internal

[GH3]

Thank you, Paul

That did the trick. I assume it is to prevent routing loops.

Thanks again!

[paul driver]

Hello

It is but also bgp can carry a lot of prefixes which could max out smaller igp rtrs.