diff --git a/moto/ec2/exceptions.py b/moto/ec2/exceptions.py
index 6033387cf..cc8535e84 100644
--- a/moto/ec2/exceptions.py
+++ b/moto/ec2/exceptions.py
@@ -677,3 +677,21 @@ class InvalidTaggableResourceType(EC2ClientError):
                 resource_type
             ),
         )
+
+
+class GenericInvalidParameterValueError(EC2ClientError):
+    def __init__(self, attribute, value):
+        super(GenericInvalidParameterValueError, self).__init__(
+            "InvalidParameterValue",
+            "invalid value for parameter {0}: {1}".format(attribute, value),
+        )
+
+
+class InvalidSubnetCidrBlockAssociationID(EC2ClientError):
+    def __init__(self, association_id):
+        super(InvalidSubnetCidrBlockAssociationID, self).__init__(
+            "InvalidSubnetCidrBlockAssociationID.NotFound",
+            "The subnet CIDR block with association ID '{0}' does not exist".format(
+                association_id
+            ),
+        )
diff --git a/moto/ec2/models.py b/moto/ec2/models.py
index 509c2ad7c..c2fa8480c 100644
--- a/moto/ec2/models.py
+++ b/moto/ec2/models.py
@@ -38,6 +38,7 @@ from os import listdir
 
 from .exceptions import (
     CidrLimitExceeded,
+    GenericInvalidParameterValueError,
     UnsupportedTenancy,
     DependencyViolationError,
     EC2ClientError,
@@ -95,6 +96,7 @@ from .exceptions import (
     InvalidVPCRangeError,
     InvalidVpnGatewayIdError,
     InvalidVpnConnectionIdError,
+    InvalidSubnetCidrBlockAssociationID,
     MalformedAMIIdError,
     MalformedDHCPOptionsIdError,
     MissingParameterError,
@@ -172,6 +174,7 @@ from .utils import (
     random_vpn_gateway_id,
     random_vpn_connection_id,
     random_customer_gateway_id,
+    random_subnet_ipv6_cidr_block_association_id,
     is_tag_filter,
     tag_filter_matches,
     rsa_public_key_parse,
@@ -284,6 +287,7 @@ class NetworkInterface(TaggedEC2Resource, CloudFormationModel):
         self.instance = None
         self.attachment_id = None
         self.description = description
+        self.source_dest_check = True
 
         self.public_ip = None
         self.public_ip_auto_assign = public_ip_auto_assign
@@ -478,10 +482,14 @@ class NetworkInterfaceBackend(object):
 
         found_eni.instance.detach_eni(found_eni)
 
-    def modify_network_interface_attribute(self, eni_id, group_ids):
+    def modify_network_interface_attribute(
+        self, eni_id, group_ids, source_dest_check=None
+    ):
         eni = self.get_network_interface(eni_id)
         groups = [self.get_security_group_from_id(group_id) for group_id in group_ids]
         eni._group_set = groups
+        if source_dest_check:
+            eni.source_dest_check = source_dest_check
 
     def get_all_network_interfaces(self, eni_ids=None, filters=None):
         enis = self.enis.values()
@@ -3633,6 +3641,7 @@ class Subnet(TaggedEC2Resource, CloudFormationModel):
         subnet_id,
         vpc_id,
         cidr_block,
+        ipv6_cidr_block,
         availability_zone,
         default_for_az,
         map_public_ip_on_launch,
@@ -3650,7 +3659,9 @@ class Subnet(TaggedEC2Resource, CloudFormationModel):
         self.default_for_az = default_for_az
         self.map_public_ip_on_launch = map_public_ip_on_launch
         self.assign_ipv6_address_on_creation = assign_ipv6_address_on_creation
-        self.ipv6_cidr_block_associations = []
+        self.ipv6_cidr_block_associations = {}
+        if ipv6_cidr_block:
+            self.attach_ipv6_cidr_block_associations(ipv6_cidr_block)
 
         # Theory is we assign ip's as we go (as 16,777,214 usable IPs in a /8)
         self._subnet_ip_generator = self.cidr.hosts()
@@ -3802,6 +3813,22 @@ class Subnet(TaggedEC2Resource, CloudFormationModel):
         except KeyError:
             pass  # Unknown IP
 
+    def attach_ipv6_cidr_block_associations(self, ipv6_cidr_block):
+        association = {
+            "associationId": random_subnet_ipv6_cidr_block_association_id(),
+            "ipv6CidrBlock": ipv6_cidr_block,
+            "ipv6CidrBlockState": "associated",
+        }
+        self.ipv6_cidr_block_associations[
+            association.get("associationId")
+        ] = association
+        return association
+
+    def detach_subnet_cidr_block(self, association_id):
+        association = self.ipv6_cidr_block_associations.get(association_id)
+        association["ipv6CidrBlockState"] = "disassociated"
+        return association
+
 
 class SubnetBackend(object):
     def __init__(self):
@@ -3819,6 +3846,7 @@ class SubnetBackend(object):
         self,
         vpc_id,
         cidr_block,
+        ipv6_cidr_block=None,
         availability_zone=None,
         availability_zone_id=None,
         context=None,
@@ -3852,6 +3880,10 @@ class SubnetBackend(object):
         if not subnet_in_vpc_cidr_range:
             raise InvalidSubnetRangeError(cidr_block)
 
+        # The subnet size must use a /64 prefix length.
+        if ipv6_cidr_block and "::/64" not in ipv6_cidr_block:
+            raise GenericInvalidParameterValueError("ipv6-cidr-block", ipv6_cidr_block)
+
         for subnet in self.get_all_subnets(filters={"vpc-id": vpc_id}):
             if subnet.cidr.overlaps(subnet_cidr_block):
                 raise InvalidSubnetConflictError(cidr_block)
@@ -3895,6 +3927,7 @@ class SubnetBackend(object):
             subnet_id,
             vpc_id,
             cidr_block,
+            ipv6_cidr_block,
             availability_zone_data,
             default_for_az,
             map_public_ip_on_launch,
@@ -3937,6 +3970,27 @@ class SubnetBackend(object):
         else:
             raise InvalidParameterValueError(attr_name)
 
+    def get_subnet_from_ipv6_association(self, association_id):
+        subnet = None
+        for s in self.get_all_subnets():
+            if association_id in s.ipv6_cidr_block_associations:
+                subnet = s
+        return subnet
+
+    def associate_subnet_cidr_block(self, subnet_id, ipv6_cidr_block):
+        subnet = self.get_subnet(subnet_id)
+        if not subnet:
+            raise InvalidSubnetIdError(subnet_id)
+        association = subnet.attach_ipv6_cidr_block_associations(ipv6_cidr_block)
+        return association
+
+    def disassociate_subnet_cidr_block(self, association_id):
+        subnet = self.get_subnet_from_ipv6_association(association_id)
+        if not subnet:
+            raise InvalidSubnetCidrBlockAssociationID(association_id)
+        association = subnet.detach_subnet_cidr_block(association_id)
+        return subnet.id, association
+
 
 class FlowLogs(TaggedEC2Resource, CloudFormationModel):
     def __init__(
@@ -4750,12 +4804,13 @@ class RouteBackend(object):
         nat_gateway = None
         transit_gateway = None
         egress_only_igw = None
+        interface = None
 
         route_table = self.get_route_table(route_table_id)
 
         if interface_id:
             # for validating interface Id whether it is valid or not.
-            self.get_network_interface(interface_id)
+            interface = self.get_network_interface(interface_id)
 
         else:
             if gateway_id:
@@ -4787,7 +4842,7 @@ class RouteBackend(object):
             nat_gateway=nat_gateway,
             egress_only_igw=egress_only_igw,
             transit_gateway=transit_gateway,
-            interface=None,
+            interface=interface,
             vpc_pcx=self.get_vpc_peering_connection(vpc_peering_connection_id)
             if vpc_peering_connection_id
             else None,
diff --git a/moto/ec2/responses/elastic_network_interfaces.py b/moto/ec2/responses/elastic_network_interfaces.py
index a46b4858c..f4dce86c7 100644
--- a/moto/ec2/responses/elastic_network_interfaces.py
+++ b/moto/ec2/responses/elastic_network_interfaces.py
@@ -66,8 +66,11 @@ class ElasticNetworkInterfaces(BaseResponse):
     def modify_network_interface_attribute(self):
         eni_id = self._get_param("NetworkInterfaceId")
         group_ids = self._get_multi_param("SecurityGroupId")
+        source_dest_check = self._get_param("SourceDestCheck")
         if self.is_not_dryrun("ModifyNetworkInterface"):
-            self.ec2_backend.modify_network_interface_attribute(eni_id, group_ids)
+            self.ec2_backend.modify_network_interface_attribute(
+                eni_id, group_ids, source_dest_check
+            )
             return MODIFY_NETWORK_INTERFACE_ATTRIBUTE_RESPONSE
 
     def reset_network_interface_attribute(self):
@@ -97,9 +100,7 @@ CREATE_NETWORK_INTERFACE_RESPONSE = """
         {% if eni.private_ip_address %}
           <privateIpAddress>{{ eni.private_ip_address }}</privateIpAddress>
         {% endif %}
-        {% if eni.instance %}
-          <sourceDestCheck>{{ eni.instance.source_dest_check }}</sourceDestCheck>
-        {% endif %}
+        <sourceDestCheck>{{ eni.source_dest_check }}</sourceDestCheck>
         <groupSet>
         {% for group in eni.group_set %}
             <item>
@@ -132,7 +133,7 @@ DESCRIBE_NETWORK_INTERFACES_RESPONSE = """<DescribeNetworkInterfacesResponse xml
            <subnetId>{{ eni.subnet.id }}</subnetId>
            <vpcId>{{ eni.subnet.vpc_id }}</vpcId>
            <availabilityZone>us-west-2a</availabilityZone>
-           <description>{{ eni.description }}</description>
+           <description>{{ eni.description or "" }}</description>
            <ownerId>190610284047</ownerId>
            <requesterManaged>false</requesterManaged>
            {% if eni.attachment_id %}
@@ -145,9 +146,7 @@ DESCRIBE_NETWORK_INTERFACES_RESPONSE = """<DescribeNetworkInterfacesResponse xml
              <privateIpAddress>{{ eni.private_ip_address }}</privateIpAddress>
            {% endif %}
            <privateDnsName>ip-10-0-0-134.us-west-2.compute.internal</privateDnsName>
-           {% if eni.instance %}
-             <sourceDestCheck>{{ eni.instance.source_dest_check }}</sourceDestCheck>
-           {% endif %}
+           <sourceDestCheck>{{ eni.source_dest_check }}</sourceDestCheck>
            <groupSet>
            {% for group in eni.group_set %}
                <item>
diff --git a/moto/ec2/responses/route_tables.py b/moto/ec2/responses/route_tables.py
index 14c733445..137bc92e1 100644
--- a/moto/ec2/responses/route_tables.py
+++ b/moto/ec2/responses/route_tables.py
@@ -210,6 +210,10 @@ DESCRIBE_ROUTE_TABLES_RESPONSE = """
                   <transitGatewayId>{{ route.transit_gateway.id }}</transitGatewayId>
                   <state>active</state>
                 {% endif %}
+                {% if route.interface %}
+                  <networkInterfaceId>{{ route.interface.id }}</networkInterfaceId>
+                  <state>active</state>
+                {% endif %}
               </item>
             {% endfor %}
           </routeSet>
diff --git a/moto/ec2/responses/subnets.py b/moto/ec2/responses/subnets.py
index f18aaf2ec..5698e1340 100644
--- a/moto/ec2/responses/subnets.py
+++ b/moto/ec2/responses/subnets.py
@@ -9,6 +9,7 @@ class Subnets(BaseResponse):
     def create_subnet(self):
         vpc_id = self._get_param("VpcId")
         cidr_block = self._get_param("CidrBlock")
+        ipv6_cidr_block = self._get_param("Ipv6CidrBlock")
         availability_zone = self._get_param("AvailabilityZone")
         availability_zone_id = self._get_param("AvailabilityZoneId")
         tags = self._get_multi_param("TagSpecification")
@@ -22,6 +23,7 @@ class Subnets(BaseResponse):
         subnet = self.ec2_backend.create_subnet(
             vpc_id,
             cidr_block,
+            ipv6_cidr_block,
             availability_zone,
             availability_zone_id,
             context=self,
@@ -55,6 +57,26 @@ class Subnets(BaseResponse):
                 )
                 return MODIFY_SUBNET_ATTRIBUTE_RESPONSE
 
+    def associate_subnet_cidr_block(self):
+        ipv6_cidr_block = self._get_param("Ipv6CidrBlock")
+        subnet_id = self._get_param("SubnetId")
+
+        association = self.ec2_backend.associate_subnet_cidr_block(
+            subnet_id, ipv6_cidr_block
+        )
+        template = self.response_template(ASSOCIATE_SUBNET_CIDR_BLOCK_RESPONSE)
+        return template.render(subnet_id=subnet_id, association=association)
+
+    def disassociate_subnet_cidr_block(self):
+        association_id = self._get_param("AssociationId")
+
+        subnet_id, association = self.ec2_backend.disassociate_subnet_cidr_block(
+            association_id
+        )
+        template = self.response_template(DISASSOCIATE_SUBNET_CIDR_BLOCK_RESPONSE)
+        result = template.render(subnet_id=subnet_id, association=association)
+        return result
+
 
 CREATE_SUBNET_RESPONSE = """
 <CreateSubnetResponse xmlns="http://ec2.amazonaws.com/doc/2013-10-15/">
@@ -71,7 +93,19 @@ CREATE_SUBNET_RESPONSE = """
     <mapPublicIpOnLaunch>{{ subnet.map_public_ip_on_launch }}</mapPublicIpOnLaunch>
     <ownerId>{{ subnet.owner_id }}</ownerId>
     <assignIpv6AddressOnCreation>{{ 'false' if not subnet.assign_ipv6_address_on_creation or subnet.assign_ipv6_address_on_creation == 'false' else 'true'}}</assignIpv6AddressOnCreation>
-    <ipv6CidrBlockAssociationSet>{{ subnet.ipv6_cidr_block_associations }}</ipv6CidrBlockAssociationSet>
+    <ipv6CidrBlockAssociationSet>
+    {% for ipv6_association in subnet.ipv6_cidr_block_associations.values() %}
+    {% if ipv6_association.ipv6CidrBlockState == "associated" %}
+      <item>
+        <ipv6CidrBlock>{{ ipv6_association.ipv6CidrBlock }}</ipv6CidrBlock>
+        <associationId>{{ ipv6_association.associationId }}</associationId>
+        <ipv6CidrBlockState>
+          <state>{{ ipv6_association.ipv6CidrBlockState }}</state>
+        </ipv6CidrBlockState>
+      </item>
+    {% endif %}
+    {% endfor %}
+    </ipv6CidrBlockAssociationSet>
     <subnetArn>arn:aws:ec2:{{ subnet._availability_zone.name[0:-1] }}:{{ subnet.owner_id }}:subnet/{{ subnet.id }}</subnetArn>
     <tagSet>
         {% for tag in subnet.get_tags() %}
@@ -109,7 +143,19 @@ DESCRIBE_SUBNETS_RESPONSE = """
         <mapPublicIpOnLaunch>{{ subnet.map_public_ip_on_launch }}</mapPublicIpOnLaunch>
         <ownerId>{{ subnet.owner_id }}</ownerId>
         <assignIpv6AddressOnCreation>{{ 'false' if not subnet.assign_ipv6_address_on_creation or subnet.assign_ipv6_address_on_creation == 'false' else 'true'}}</assignIpv6AddressOnCreation>
-        <ipv6CidrBlockAssociationSet>{{ subnet.ipv6_cidr_block_associations }}</ipv6CidrBlockAssociationSet>
+        <ipv6CidrBlockAssociationSet>
+        {% for ipv6_association in subnet.ipv6_cidr_block_associations.values() %}
+        {% if ipv6_association.ipv6CidrBlockState == "associated" %}
+          <item>
+            <ipv6CidrBlock>{{ ipv6_association.ipv6CidrBlock }}</ipv6CidrBlock>
+            <associationId>{{ ipv6_association.associationId }}</associationId>
+            <ipv6CidrBlockState>
+              <state>{{ ipv6_association.ipv6CidrBlockState }}</state>
+            </ipv6CidrBlockState>
+          </item>
+        {% endif %}
+        {% endfor %}
+        </ipv6CidrBlockAssociationSet>
         <subnetArn>arn:aws:ec2:{{ subnet._availability_zone.name[0:-1] }}:{{ subnet.owner_id }}:subnet/{{ subnet.id }}</subnetArn>
         {% if subnet.get_tags() %}
           <tagSet>
@@ -133,3 +179,31 @@ MODIFY_SUBNET_ATTRIBUTE_RESPONSE = """
   <requestId>7a62c49f-347e-4fc4-9331-6e8eEXAMPLE</requestId>
   <return>true</return>
 </ModifySubnetAttributeResponse>"""
+
+ASSOCIATE_SUBNET_CIDR_BLOCK_RESPONSE = """
+<AssociateSubnetCidrBlock xmlns="http://ec2.amazonaws.com/doc/2016-11-15/">
+  <requestId>59dbff89-35bd-4eac-99ed-be587EXAMPLE</requestId>
+  <subnetId>{{ subnet_id }}</subnetId>
+  <ipv6CidrBlockAssociation>
+    <ipv6CidrBlock>{{ association.ipv6CidrBlock }}</ipv6CidrBlock>
+    <ipv6CidrBlockState>
+      <state>{{ association.ipv6CidrBlockState }}</state>
+    </ipv6CidrBlockState>
+    <associationId>{{ association.associationId }}</associationId>
+  </ipv6CidrBlockAssociation>
+</AssociateSubnetCidrBlock>
+"""
+
+DISASSOCIATE_SUBNET_CIDR_BLOCK_RESPONSE = """
+<DisassociateSubnetCidrBlockResponse xmlns="http://ec2.amazonaws.com/doc/2016-11-15/">
+  <requestId>59dbff89-35bd-4eac-99ed-be587EXAMPLE</requestId>
+  <subnetId>{{ subnet_id }}</subnetId>
+  <ipv6CidrBlockAssociation>
+    <ipv6CidrBlock>{{ association.ipv6CidrBlock }}</ipv6CidrBlock>
+    <ipv6CidrBlockState>
+      <state>{{ association.ipv6CidrBlockState }}</state>
+    </ipv6CidrBlockState>
+    <associationId>{{ association.associationId }}</associationId>
+  </ipv6CidrBlockAssociation>
+</DisassociateSubnetCidrBlockResponse>
+"""
diff --git a/moto/ec2/utils.py b/moto/ec2/utils.py
index 5c0835bd5..72b2fce35 100644
--- a/moto/ec2/utils.py
+++ b/moto/ec2/utils.py
@@ -39,6 +39,7 @@ EC2_RESOURCE_TO_PREFIX = {
     "spot-instance-request": "sir",
     "spot-fleet-request": "sfr",
     "subnet": "subnet",
+    "subnet-ipv6-cidr-block-association": "subnet-cidr-assoc",
     "reservation": "r",
     "volume": "vol",
     "vpc": "vpc",
@@ -107,6 +108,12 @@ def random_subnet_id():
     return random_id(prefix=EC2_RESOURCE_TO_PREFIX["subnet"])
 
 
+def random_subnet_ipv6_cidr_block_association_id():
+    return random_id(
+        prefix=EC2_RESOURCE_TO_PREFIX["subnet-ipv6-cidr-block-association"]
+    )
+
+
 def random_subnet_association_id():
     return random_id(prefix=EC2_RESOURCE_TO_PREFIX["route-table-association"])
 
diff --git a/tests/terraform-tests.success.txt b/tests/terraform-tests.success.txt
index 82172fd3c..be35edeb4 100644
--- a/tests/terraform-tests.success.txt
+++ b/tests/terraform-tests.success.txt
@@ -88,6 +88,7 @@ TestAccAWSRouteTable_IPv4_To_NatGateway
 TestAccAWSRouteTable_IPv4_To_TransitGateway
 TestAccAWSRouteTable_IPv4_To_VpcPeeringConnection
 TestAccAWSRouteTable_IPv6_To_EgressOnlyInternetGateway
+TestAccAWSRouteTable_IPv6_To_NetworkInterface_Unattached
 TestAccAWSRouteTable_disappears
 TestAccAWSRouteTable_basic
 TestAccAWSSsmDocumentDataSource