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Bug 2238494

Summary:	E/W routed traffic does not handle different network MTU sizes
Product:	Red Hat Enterprise Linux Fast Datapath	Reporter:	Rodolfo Alonso <ralonsoh>
Component:	ovn22.12	Assignee:	lorenzo bianconi <lorenzo.bianconi>
Status:	CLOSED DEFERRED	QA Contact:	Jianlin Shi <jishi>
Severity:	medium	Docs Contact:
Priority:	medium
Version:	FDP 22.L	CC:	ctrautma, jiji, lorenzo.bianconi, mmichels, rsafrono
Target Milestone:	---	Keywords:	Reopened
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2024-03-19 20:35:21 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description Rodolfo Alonso 2023-09-12 08:29:13 UTC

Description of problem:
The E/W routed traffic between two networks (not GW networks) cannot handle networks with different MTU sizes. The flag "ovn_emit_need_to_frag" is configured in the Neutron configuration (although this is affecting only to the GW ports).

Even if the packets sent have DF (Do-Not-Fragment bit) unset, the ICMP packets are not reaching the port in the network with lower MTU.

Version-Release number of selected component (if applicable):
* RHEL 9.0
* OSP17.1
* ovn-nbctl 22.12.1
* Open vSwitch Library 3.0.90
* DB Schema 7.0.0

How reproducible:
See steps to reproduce.

Steps to Reproduce:
1. Deploy an OpenStack environment, using any OVN (geneve, vxlan) backend type: https://rhos-ci-jenkins.lab.eng.tlv2.redhat.com/view/QE/view/OSP17.1/
2. Create two networks and one router between them. The MTUs must be different
$ openstack network create --mtu 1500 net1
$ openstack network create --mtu 1200 net2
$ openstack subnet create --network net1 --subnet-range 10.0.201.0/28 snet1
$ openstack subnet create --network net2 --subnet-range 10.0.202.0/28 snet2
$ openstack router create router1
$ openstack router add subnet router1 snet1
$ openstack router add subnet router1 snet2
3. Spawn two VMs, each one in a different network
$ openstack server create --network net1 --flavor cirros --image cirros server1
$ openstack server create --network net2 --flavor cirros --image cirros server2
4. Access the VMs and ping from server1 (MTU 1500) to server2 (MTU 1200). Use the "ping -M dont" flag to unset the DF flag.

Actual results:
The datapath flow is received in the compute node of server2: https://paste.opendev.org/show/bVhsu1RdXnF7tquQN4Hg/. However, the TAP interface of server2 does not receive any packet. When the MTU is 1200 or lower, this TAP interface receives the ping packets; e.g.: https://paste.opendev.org/show/bdblIQENJeUp2kWio7Ac/

Expected results:
OVN should fragment the traffic and sent the fragments to the 1200 TAP interface, that should be received by server2.

Additional info:
---

Comment 3 OVN Bot 2024-02-14 21:16:15 UTC

This issue is being closed as an automatic process due to the issue's age. If you wish to re-open this issue, please do so in Jira (https://issues.redhat.com) in the 'FDP' project. Please be sure to set the component to the latest OVN version where this issue is known to occur. If this is a feature request or improvement, please set the component to 'OVN'.