1. Proposed title of this feature request

ADMSS support on Multi Cluster Engine

2. What is the nature and description of the request?

To be able to avoid MTU fragmentation for inter-site connections without modifying the WAN links/routes in the middle. A Hub cluster can be on site A and workload cluster can be on site B and MTU fragmentation needs to be avoided. See [^End-to-End-PMTU_Values.jpg].

We should be able to set advmss network route setting in siteconfig/clusterinstance if Spoke cluster RHCOS supported this setting on nmstate. we can set it in siteconfig for example on mce v2.8.3 but not in v2.8.4 or v2.9.1.
advmss supported in  RHCOS package nmstate>=2.2.47 (i.e., 4.16.46+, 4.18.22+, and 4.20.0+)

3. Why does the customer need this? (List the business requirements here)

Solution would be to update the MTU of the WAN link connecting two sites.
However, customer are often not able to change the MTU of their WAN links. Some customers networks may not easily want to support/accept bigger than 1500 MTU, because the WAN links are not in full control of the telco operator/department, there could be leased WAN links. And in some site routers they do not support bigger values than 9008 in the site router; 9008 as biggest supported MTU is quite common in site routers.

(Also note that decreasing the Machine Network MTU from 9100 is not an option.)
advmss set in a network route config in the siteconfig/clusterinstance is needed as below to unblock deployment issue (inspection, provisioning ...)

              routes:
                config:
                  - destination: 0.0.0.0/0
                    next-hop-address: 192.168.121.1
                    next-hop-interface: infra-bond.121
                    table-id: 254
                    advmss: 1350

4. List any affected packages or components.
Multi cluster engine

We need this on:
MCE v2.5.x
MCE v2.6.x

MCE v2.7.x

MCE v2.8.x

MCE v2.10.x