OSPF does not do the (ECMP based) forwarding. OSPF just provides info that might make it into the routing table and further into to the actual forwarding table. And yes, there can be ECMP for /32 destinations.
If a given (ingress) leaf switch has ECMP routes to a given remote (egress) leaf's loopback address, the flows between local and remote loopback address will be distributed among the ECMP links as the router's forwarding feature (e.g. Cisco's CEF) sees fit, based on its hashing alogorithm.
With SrcIP, DstIP, protocol number and DstPort (probably udp/4789 for VXLAN) being the same, then the UDP SrcPort will have to provide some entropy for the hashing algorithm.
It seems that more than one vendor decided to glean at the VXLAN encapsulated packet:
Jupiter Networks say this:
https://www.juniper.net/documentation/us/en/software/junos/evpn-vxlan/topics/topic-map/sdn-vxlan.html
The source port field in the UDP header is used to enable ECMP load
balancing of the VXLAN traffic in the Layer 3 network. This field is
set to a hash of the inner packet fields, which results in a variable
that ECMP can use to distinguish between tunnels (flows).
None of the other fields that flow-based ECMP normally uses are
suitable for use with VXLANs. All tunnels between the same two VTEPs
have the same outer source and destination IP addresses, and the UDP
destination port is set to port 4789 by definition. Therefore, none of
these fields provide a sufficient way for ECMP to differentiate flows.
Huawei for example say that in VXLAN transport, they're setting the UDP SrcPort from a hash value derived from the encapsulated Ethernet frame:
https://support.huawei.com/enterprise/en/doc/EDOC1100086966
The VXLAN header and the original Ethernet frame are used as UDP data.
In the UDP header, the destination port number (VXLAN Port) is fixed
at 4789, and the source port number (UDP Src. Port) is calculated
using the hash algorithm based on the original Ethernet frame.
And Cisco seems to do the same in some of their products. From a document about the Nexus 3600. Although I struggle to find the same information in the same verbosity for the Nexus 9300 series, I think it's a safe bet to assume that other product ranges do the same:
(emphasis by me)
ECMP and LACP Load Sharing with VXLANs
Encapsulated VXLAN packets are forwarded between VTEPs based
on the native forwarding decisions of the transport network.
Most data center transport networks are designed and deployed with
multiple redundant paths that take advantage of various multipath
load-sharing technologies to distribute traffic loads on all
available paths.
A typical VXLAN transport network is an IP-routing network that
uses the standard IP equal cost multipath (ECMP) to balance
the traffic load among multiple best paths. To avoid out-of-sequence
packet forwarding, flow-based ECMP is commonly deployed. An ECMP flow is
defined by the source and destination IP addresses and optionally, the
source and destination TCP or UDP ports in the IP packet header.
All the VXLAN packet flows between a pair of VTEPs have the same
outer source and destination IP addresses, and all VTEP devices
must use one identical destination UDP port that can be either the
Internet Allocated Numbers Authority (IANA)-allocated UDP port 4789
or a customer-configured port. The only variable element in the
ECMP flow definition that can differentiate VXLAN flows from the
transport network standpoint is the source UDP port. A similar
situation for Link Aggregation Control Protocol (LACP) hashing
occurs if the resolved egress interface that is based on the routing
and ECMP decision is an LACP port channel. LACP uses the VXLAN outer-
packet header for link load-share hashing, which results in the
source UDP port being the only element that can uniquely identify a
VXLAN flow.
In the Cisco Nexus 3600 platform switches implementation
of VXLANs, a hash of the inner frame's header is used as the VXLAN
source UDP port. As a result, a VXLAN flow can be unique. The IP
address and UDP port combination is in its outer header while the
packet traverses the underlay transport network.
TL;DR:
ECMP for /32 routes is far from useless in a BGP EVPN scenario, as long as the forwarding engine's hashing feature is provided with a good source of entropy (in extenso: many different and varying UDP SrcPorts).
Vendors seem to derive the UDP Src Port from a hash value calculated from suitably selected header fields of the VXLAN-encapsulated inner packet/frame.
This ensures that all packets belonging to a given "inner flow" are mapped to a single "external VXLAN flow", and ECMP's load-sharing will not balance any single flow across multiple links.
With the many 100s of flows leaving a VTEP towards many (or a single) remote VTEPs, the widely varing UDP SrcPort numbers will ensure that the forwarding engine will load-share traffic quite well over the available uplinks.
Also: Hashing and load distribution among ECMP links is a question of the router's forwarding feature (CEF in Ciscos), not a question of the routing protocol.
If you're on Cisco Nexus, before to resorting to big things like Netflow, there's two small tools to play with:
show routing hash <SrcIP> <DstIp> ip-proto 17 <SrcPort> <DstPort>, to see where the packets are going, as in NX-OS 9.3: ECMP polarization and "ip load-sharing ... rotate" After all, there's just ~64k (minus some corner cases) possibilites for a UDP SrcPort.- from the same SE-NE post, be sure to pick up the detail about
ip load-sharing ... rotate, and see how it impacts traffic distribution when applied.
show interface counters table (available on some platforms) gives a nice clean list of how traffic is entering or leaving your switch. I'm pretty sure that once you start getting some load, you'll see it distributed quite evenly towards the spines.
