Default-information in IGP

I was looking at this interesting question about default-information originate , which got me thinking. Is default-information originate basically the same thing as a redistribute statement with a prefix-list matching 0.0.0.0/0?

Trying to find an answer to this, I thought it was a good idea to remind myself how the default is originated in the IGP’s.

Behaviours of default-information originate

First. let’s explore the ways default-information originate works in all the protocols. For this I use a small setup of three routers. One router is external to the routing protocol, as an external source for the default route. One router will be the one doing the originating or redistributing, and the third will be our window within the domain.

default_originate

RIPv2

On both routers in our domain, we activate RIP

Now we add a default-information originate on int_source and see the behaviour at reciever:

Apparently, RIP does not care if the default route exists or not. Can we make RIP care? turns out we can:

EIGRP

How about EIGRP? That is simple. EIGRP will only redistribute (or summarise) a default route, but it will not originate it.

OSPF

So, how does OSPF react?

And on int_source, we add default-information originate . We now have no default route on int_source, so we shouldn’t see a default on reciever:

Adding the magic keyword always to the origination command, gives us a default route:

Can we add a route-map, and have it originate from a different route? Let’s try:

Hah, this is basically a default-information originate always without using the always keyword.

And, the last permutation is of course, add it from a different source:

The default route we are getting from RIP is enough to trigger the origination of a default route. Notice that the show command on int_source says “Redistributing via rip”, there is no mention of OSPF.

OSPf from iBGP

Now, the whole part that started me down this rabbit hole. default-information originate in OSPF, with a default route coming from iBGP. Here we see I am getting the route from iBGP on int_source, but I am not recieving it on reciever:

Now, when we add the command bgp redistribute-internal to the bgp process, we do get the route.

So, is default-information originate a form of redistribution?

In RIP, it definitely is not a form of redistribution. RIP advertises the route regardless. EIGRP does not have a concept of default origination. OSPF does, and with iBGP sourced routes, it acts like it is redistributing. Looking around, I could find only one output to support this theory. If anyone knows of another command that shows this, I would love to hear it.

Notice how the show ip protocols says “Redistributing External Routes from,”. It does not specify the source protocol. In this case, it is redistributing from RIP.

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