[ovs-discuss] Need assistance

[Gk Gk]

Hi,

I had a discussion with Michelson on the IRC regarding my questions on the
OVN man page.  On page 8, under the Distributed gateway ports section, the
page discusses the physical mtu issues while sending the traffic through
the tunnel.

They man page suggests two options. One is to set the
reside-on-redirect-chassis option. If we set that option  they said this,

"*The first solution is the reside−on−redirect−chassis option for logical
router ports. Setting this option on a LRP from (e.g.) LS1 to LR1 disables
forwarding from LS1 to LR1 except on the gateway chassis. On chas- sis
other than the gateway chassis, this single change means that packets that
would otherwise have been forwarded to LR1 are instead forwarded to LN1.
The instance of LN1 on the gateway chassis then receives the packet and
forwards it to LR1. The packet traverses the LR1 logical router pipeline,
possibly undergoes NAT, and eventually ends up at LSlocal’s localnet port.
The packet never traverses a tunnel, avoiding the MTU issue.*

*This option has the further consequence of centralizing ‘‘distributed’’
logical router LR1, since no packets are forwarded from LS1 to LR1 on any
chassis other than the gateway chassis. Therefore, east-west traffic passes
through the gateway chassis, not just north-south. (The naive ‘‘fix’’ of
allowing east-west traffic to flow directly between chassis over LN1 does
not work because routing sets the Ethernet source address to LR1’s source
address. Seeing this single Ethernet source address originate from all of
the chassis will con- fuse the physical switch.)*"


I have few questions on the above excerpt from the man page.  I have
attached a diagram for the reference.  If you see the diagram attached ,
if  we set the reside-on-redirect-chassis option, then the east-west
traffic from VM1 goes thru LN1 port on LS1 to the gateway  chassis where
the LR router is. This east-west traffic from VM1 will never go through the
LR1 on HV1 due to this setting. In this scenario, can you explain me how it
will change the ethernet source address of the east-west traffic to the LR1
source address, and how will the traffic emerge from all chassis with the
same ethernet source address ?



Thanks

Kumar
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