Hi guys,
On 2/18/21 12:16 AM, Han Zhou wrote:
On Wed, Feb 17, 2021 at 2:36 PM Tim Rozet <tro...@redhat.com> wrote:
On Tue, Feb 16, 2021 at 4:21 AM Han Zhou <hz...@ovn.org> wrote:
On Wed, Feb 10, 2021 at 2:17 PM Lorenzo Bianconi <
lorenzo.bianc...@redhat.com> wrote:
Increase priority for automatic routes (routes created assigning IP
addresses to OVN logical router interfaces) in order to always prefer
them
over static routes since the router has a direct link to the
destination
address (possible use-case can be found here [0]).
[0] https://bugzilla.redhat.com/show_bug.cgi?id=1891516
Hi Lorenzo, Tim,
While the patch may solve the problem in the bug report, I think there
is something more fundamental to be discussed. The problem is caused by
ovn-k8s's use of "src-ip" in static routes which overrides the
direct-connected route. I think the implementation of "src-ip" support in
the static route is somehow flawed. The priorities of the flows generated
by static routes are calculated according to the prefix length, so that
longest prefix routes are prioritised when there are multiple route
matches, which is correct when comparing matches among "dst-ip" routes or
among "src-ip" routes, but is not correct between "dst-ip" and "src-ip"
routes. Comparison of prefix length between these two types of static
routes doesn't make sense, because they match by different fields (src-ip
v.s. dst-ip). For example, when there are static routes:
1. 192.168.0.0/24 via 100.64.0.1 src-ip
2. 10.0.0.0/20 via 100.64.0.2 dst-ip
In this example, a packet from 192.168.0.1 to 10.0.0.1 matches both
routes, but it is unreasonable to say it should follow the 1st route just
because it has longer prefix length. Instead, we should prioritize one type
over the other. It seems in physical router implementation policy based
routing always has higher priority than destination routing, so we should
probably enforce it in a similar way in OVN, i.e. set "src-ip" flows with
higher priority than all the "dst-ip" flows. In fact, the policy routing
table already supported this behavior because it is examined before the
static route table.
Since the "src-ip" feature in the static route table is flawed, and can
be replaced by the policy routing table, I'd suggest to deprecate it. For
I agree.
correctness, users (like ovn-k8s) should use the policy routing table
instead for the src-ip based routing rules. Users have full control of how
they want the packets to be routed. For the use case mentioned in the bug
report, it should have two rules in the policy routing table:
100 ip.dst == 100.64.0.0/16 accept # for directly connected destination,
skip the src-ip rules
90 ip.src == 10.244.0.0/24 nexthop 100.64.0.1 # src-ip rules
Would this better satisfy the need of ovn-k8s?
I believe this is correct. src-ip matching should be done in the policy
table so traditional dest based routing is handled in default routing
table. Need to go double check though.
One example, but should be the same for all "traditional" routers, "The
Policy-Based Routing feature is a process whereby a device puts packets
through a route map before routing the packets.":
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_pi/configuration/15-mt/iri-15-mt-book/iri-pbr.html
If the above is agreed, the priority change of directly connected routes
from this patch is irrelevant to the problem reported in the bug, because
policy routing rules are examined before the static routing table anyway,
so no matter how high the route priority is, it wouldn't matter. In
addition, it seems to me no real use cases would benefit from this change,
but I could be wrong and please correct me if so.
I disagree with this. Trying to override a directly connected route is
fundamentally wrong, which is why real routers specifically stop a user
from being able to do this. What if a user who had a router attached to
100.64.0.0/16, adds a /32 route for 100.64.0.1 via another
interface/subnet? That would take precedence over the directly attached
route in OVN iiuc and pretty much guarantee improper networking. Directly
connected routes should always take precedence, and therefore the default
route lflows that get installed should always have the highest possible
priority.
I think there's some confusion here.
In "traditional" routers, longest prefix match has priority in the route
selection process.
If two routes (from different protocols, including "directly connected")
are available for the *same* prefix *and* length then, and only then,
the one with the lowest administrative distance is selected. Most of
the times "directly connected" routes have a lower admin distance by
default compared to static routes so they are preferred *if* such a
selection has to happen.
The admin distance is also usually configurable (although not usually
recommended) and allows users to override the default preference of routes.
Back to the example above, in a "traditional" router, when the two
routes are configured, with default admin distances:
100.64.0.2/16 directly connected, via dev1
100.64.0.1/32 static, via 42.42.42.42
All traffic with destination IP in 100.64.0.3-100.64.255.254 is
forwarded via dev1 (the directly connected route is selected).
Traffic with destination IP 100.64.0.2 is processed locally.
Traffic with destination IP 100.64.0.1 is forwarded via the static route
towards next-hop 42.42.42.42.
Hi Tim,
Thanks for your inputs. Here are my thoughts:
In the scenario we discussed, it is in fact the same output interfaces but
just different nexthops in the 10.64.0.0/16 subnet. In this case, adding a
more specific route with a specific nexthop on the directly connected
subnet doesn't seem to be violating any routing principle, right? Use the
topology in the bug report as an example ((In the diagram of the bug report
I think there is a typo: 100.64.0.1 should be the DR's output port IP, and
100.64.0.2 & 100.64.0.3 belong to GR1 and GR2). 10.64.0.0/16 is directly
connected, but the adjacent L2 network doesn't have to have all the /16 IPs
directly connected. Some of the nodes can reside behind a L3 hop. For
example, if we know that 10.64.1.0/24 is behind a router with IP
10.64.255.1, we can still add a route: 10.64.1.0/24 via 10.64.255.1, which
should work with the current OVN implementation, while this patch would in
fact break it.
On the other hand, what I wanted to emphasize is not that we want to
support the above use case I mentioned - it doesn't look like a good design
for me either although it seems valid. My point is that maybe we shouldn't
have the special adjustment for the priority just to restrict such use
cases, which doesn't bring us any extra benefit, while making the code less
generic. OVN providing the possibility of overriding the routes with longer
prefix length doesn't mean the user has to use it, and if someone relies on
this capability for their special needs then I assume they should have a
clear understanding of what they are doing - it is totally up to the user.
If we do believe that any configuration that tries to add more specific
routes than the directly connected routes is invalid and want to protect
against them (I am not convinced yet), then probably we should consider
As mentioned above, I don't think more specific static routes are an
invalid config.
As a matter of fact, there are examples of using them on "traditional"
routers to have part of the traffic go through additional services, e.g.:
https://community.cisco.com/t5/other-network-architecture/how-do-you-promote-a-static-route-over-a-directly-connected/m-p/644482/highlight/true#M179363
even more scenarios, e.g. what if there are two interfaces configured with
overlapping directly connected subnets, e.g. 100.64.0.0/16 on LRP1 and
100.64.123.0/24 on LRP2 (both LRP1 and LRP2 are on the same LR)? Currently
if a packet with dest IP 100.64.123.* arrives we would expect it to be
routed to LRP2, but with this patch they will have the same priority and
the behavior will become unpredictable.
So for the above reasons I'm a little concerned about making the specific
change for the priorities of directly connected routes because it seems
would make things more complex without obvious benefits.
I agree, I don't think it's OK to bump priority of directly connected
routes.
Thanks,
Han
Regards,
Dumitru
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