automatic default route propagation into RIP: default metric

Hello All,

I'd like to find documentation regarding default metrics for
'default-information originate'd routes. I've looked through the IOS
12.2 IP Command Reference set and Jeff Doyle's "Routing TCP/IP Vol 1",
without success.

I'm observing that the default metric appears to be >= 16, thus is
unreachable for RIP.

ISP---A---B---C

RouterA and RouterB are running OSPF, with RouterA having a static
default route pointing to ISP propagated into OSPF via the
default-information originate command (default metric type and metric,
not that it matters, right?).

Now RouterB and RouterC run RIP, and the goal is to get C to obtain
the default route automatically from the OSPF route:

RouterB:
O*E2 0.0.0.0/0 [110/1] via 10.0.0.1, 00:00:08, Ethernet0

RouterB also has the default-information originate command
(12.1(5)T10), again using the defaults.

The 0.0.0.0 route is being propagated into the RIP domain, though with
an unreachable metric:

RouterB#debug ip rip
2d23h: RIP: sending v2 update to 224.0.0.9 via Serial0 (10.1.3.193)
2d23h: RIP: build update entries
2d23h: 0.0.0.0/0 via 0.0.0.0, metric 16, tag 0
2d23h: 10.0.0.0/22 via 0.0.0.0, metric 1, tag 0


I explicitly played with the OSPF def-info orig metric type and
metric, with the expected results in RouterB's routing table and 'sh
ip ospf database external' output; however it seems the RIP
advertisement always has 16 regardless of what the OSPF-originated
metric or metric-type is.

I figured I should be able to set RouterC's RIP def-info orig to use a
suitable metric, though there are no options allowed save the
route-map (i.e. no metric). About the only thing I could do was set
the default-metric, which works as-expected, assuming I expected a
redistribution config item to effect default route propagation. (Doyle
notes on p714 that the default metric for redistributed routes is 0,
is this also the default for def-info originated routes??? and for
RIP, does 0 => 16?)

So:

Q1: What is the default metric for RIP def-info originated routes, and
why is it >=16? i.e. why bother making these routes unreachable??

Q2: How are you supposed to set the metric for def-info orig routes?
(route-map?)

Q3: Would it be true to say the 'default-information originate'
command is simply a specialised case of redistribution? i.e. all
options such as default-metric apply?


Thanks in advance,

Ben

 

route-map (i.e. no metric). About the only thing I could do was set

I had the same problem a long time ago and I missed that one in the
beginning, unfortunately because it is the key.

If you redistribute an ospf route to RIP by a redistribute, it seems
to get a metric 0. RIP metrics must be in the range 1-15 on the
other hand. The metric 0 is invalid and is marked as any other
invalid route (like say metric 16 would have been marked).
The RIP refuces it.

Generally, redistribution cannot convert metrics, so it has some
default values like zero and it is better to set them manually.
They might be invalid like in this case. This is a general
concept for all redistributions! (Some protocols like ospf
on the other hand accept the defaults while eg. rip does not)

If you had redistributed it by something like this things will change:
router rip
redistribute ospf 123 metric 1

It would be metric 1 on the router A doing the redistribution instead
of being invalid (zero) and it would propagate to B and C as
expected.

 

Thanks for confirming the situation Harri. I found it strange, but
RIP's redistribute command doesn't support setting the metric [1];
thus it appears it's only possible to either set the default-metric or
use a route map. I suppose, for the reasons you've noted, it's best to
be in the habit of setting the default-metric...

[1] http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123tcr/123tip2r/ip2_o1gt.htm#1037093

Thanks,

Ben