[Babel-users] Babel Admin Distance

[Juliusz Chroboczek]

Added babel-users to CC.

> During discussion about this the other night, I guessed at an admin
> distance of 100 for Babel.  In conversation this morning with Don
> Slice he indicated that a number of 100 might be a bit low as that do
> we really trust Babel more than OSPF?  Do people have opinions on what
> the Admin Distance should be set too?

Here are the real-world cases I know.

Our Babel network here in Paris uses FRR on its edge routers,
redistributing a default route from RIPng into Babel, and announcing
a static route over RIPng to the rest of the universe.  If there are two
edge routers, we want the RIPng route to be used instead of the Babel
route announced by the other edge router, so Babel must have a higher AD
than RIPng.  (My employer happens to use RIPng for IPv6 routing, but
the same argument would apply to OSPF or IS-IS.)

The Italian confederation of mesh networks uses a mixture of OLSR,
BATMAN-adv and Babel in its individual meshes, and uses Babel for routing
between the individual meshes (mainly due to the minuscule amount of
traffic it generates as compared to the traditional mesh protocols).  I'm
not entirely clear how redistribution is done in the confederation, but
I believe that they'd prefer Babel to have a lower AD than OLSR.

The Slovenian mesh network (wlan-si) uses both Babel and OLSR.  If a given
prefix is carried by both Babel and OLSR, I believe that they prefer the
Babel route.  This, again, implies that Babel's AD should be smaller than
that of OLSR.

The Nexedi overlay network uses Babel for routing between LANs, mainly due
to its ability to choose the best among a set of parallel tunnels.  They
don't do any routing within the LANs, so any AD will do for them.

The above examples would seem to imply that Babel's AD should be between
that of traditional IGPs and that of traditional mesh protocols.  However,
I don't see any reason why a deployment wouldn't use Babel to distribute
externals between OSPF and IS-IS islands (e.g. in cases where Babel is
being used to pick the best among parallel tunnels, as in Nexedi's case),
so having Babel's AD lower than that of traditional IGPs would appear to
make sense in some cases.

-- Juliusz