'isself' should match the ports only if the hosts are being string matched. In cases where an instance is being addressed through a proxy port, port matching is an incorrect check - it invalidates a genuine match.

e.g. a mongod instance listening on port 27017 may be get redirected traffic from another port through IPtables routing. Such instances would fail to get added to a replica set for no good reason. Port proxies maybe required in multi-tenant cloud based systems where bind-able IPs/ports are NAT'ed.

Take this example replica set config to setup a one node cluster. The server itself can be addressed using a DNS name and an externally proxied port, but we cannot form a cluster if the same is used for the rs config.

config = { 
                "id"             : "test", 
                "members" : [ { 
                                         "_id" : 0, 
                                         "host" : "public_dns_to_ec2_node:38132" 
                                        } ] 
                }

Internally, the mongod was bound to 127.0.0.1:27017 and we have IPtables proxy rule to route all tcp traffic from 38132 to 127.0.0.1:27017, e.g.

-A OUTPUT -d 10.203.77.175/32 -m tcp -p tcp --dport 38132 -j DNAT --to-destination 127.0.0.1:27017
-A PREROUTING -d 10.203.77.175/32 -m tcp -p tcp --dport 38132 -j DNAT --to-destination 127.0.0.1:27017

where 'public_dns_to_ec2_node' routes to 10.203.77.175 through public IP DNS registration.

With the above example it is clear that a fix is needed to not unnecessarily match the ports as integers. Or we cannot even create a one node cluster, leave aside a usable 3-node replica set.