On Wed, Oct 17, 2018 at 01:07:32PM +0200, Sebastian Benoit wrote:
> open...@kene.nu(open...@kene.nu) on 2018.10.17 12:44:02 +0200:
> > Hello,
> >
> > On Tue, Oct 16, 2018 at 4:56 PM Sebastian Benoit <benoit-li...@fb12.de>
> > wrote:
> > >
> > > Tommy Nevtelen(to...@nevtelen.com) on 2018.10.16 15:11:51 +0200:
> > > > On Tue, Oct 16, 2018 at 10:21:37AM +0200, Claudio Jeker wrote:
> > > > > On Tue, Oct 16, 2018 at 09:13:20AM +0200, open...@kene.nu wrote:
> > > > > > Hello,
> > > > > >
> > > > > > Only relying on OSPF hellos effectively makes it mimic BGP with its
> > > > > > keepalives. I will ponder the value of transporting the underlay in
> > > > > > OSPF, effectively transporting loopback peering addresses for BGP in
> > > > > > OSPF. I am not sure that it will make my life easier but will
> > > > > > consider
> > > > > > it.
> > > > >
> > > > > OSPF is generally faster at converging after reroute and it is
> > > > > possible to
> > > > > set the router-dead-time to minimal which will give you a 1 second
> > > > > timeout. Also the default of 40sec is lower than the 90sec of BGP.
> > > > > Additionally OSPF may give you multipath routes so the failover for
> > > > > BGP
> > > > > may be not noticable. Also GRE has a way to emulate link state but to
> > > > > be
> > > > > honest if I use OSPF on a GRE link I will not turn it on (unless
> > > > > requested).
> > > >
> > > > I guess the brewing BFD support would speed this up for BGP when it
> > > > arrives
> > > > and make OSPF less useful if speed is the thing that needs to be solved.
> > > >
> > > > Also I've been thinking about the following config in ospfd
> > > >
> > > > rtlabel label external-tag number
> > > > Map route labels to external route tags and vice versa.
> > > > The
> > > > external route tag is a non-negative 32-bit number
> > > > attached to AS-external OSPF LSAs.
> > > >
> > > > What exactly does this mean? As I understand it is to map rtlabels to
> > > > LSA
> > > > Type 5 tags. But what do you do with it then? Could this be used for
> > > > what
> > > > this thread is talking about or is it totally off?
> > >
> > > If you do this on two (or more routers) you distribute the routes and they
> > > end up in the fib with that rtlabel (note the "and vice versa").
> > >
> > > You can do all the things you can do with route labels, for example use
> > > them in pf filters.
> > >
> > > And yes, you could also use it to redistribute them into bgp (although
> > > that
> > > needs to happen on another router i think):
> > >
> > > ospfd ---type5 lsa---> ospfd --> fib with rtlabel --> bgpd ...
> > > hostA hostB hostB hostB
> > >
> > > /Benno
> >
> > I might be wrong here but in prder to have ospfd generate type 5 LSAs
> > one needs both a BGP speaker that announces the prefix in question
> > into ospf and two different ospf areas in your network?
>
> No need for a bgp speaker: a route in the routing table with the label is
> enough.
>
> So here is a rough description of how ospfd and bgpd announce new prefixes
> (i.e. the ones not learned from other routers).
>
> ospfd looks at whats in the kernel routing table (FIB), and depending on its
> configuration (the interface statements _and_ redistribute commands) it adds
> routes into its ospfd internal RIB. That RIB is then propagated into the
> ospf network (skip ospf protocol details here).
>
> ospfd needs routes in the FIB to inject new routes into OSPF.
>
> By inject, i mean "add a new network to the routes in the RIB".
>
> BGPD on the other hand has the "network <prefix>" statement.
>
> It adds the prefix given there to its RIB ("injects it into BGP") without
> the need for a route in the FIB.
>
> The additional options (network priority ..., network rtlabel ...) are
> only there for convinience and special cases, they are not really needed
> for standard bgp setups.
>
> > Or can I make ospfd generate type5 LSAs in some other way? I see that
> > rtlabels would do it but that implies I have an already existing route
> > in the fib which preemptively I tag in some way. In my case the routes
> > are generated by interface statements in ospfd.conf (so type1 and 2
> > LSAs).
>
> If you use interface statements, those interfaces have IPs on them, which you
> will find have routes in the routing table.
>
> Also, there is "ifconfig <if> rtlabel ..."
>
> I don't know if ospfd will pick those route-labels up (with the rtlabel
> statement). I think it should, but havent used that or looked at the code.
>
The rtlabel / external-tag mapping only works for Type-5 LSA which are
prefixes ospfd picked up via any of the redistribute statements. Any prefix
covered by interface statements end up as non Type-5 LSA and so will not
use the mapping. This includes stub networks.
I have not tried to use "ifconfig <if> rtlabel ..." with ospfd together
but I see no reason why it should not work if used with redistribute
connected for example.
--
:wq Claudio
