External Connectivity in noNAT case still allocates subnets to BDs

[pforai]

Hi,

When configuring external connectivity in a no-NAT scenario it is required to supply a neutron network and subnet for the external segment.

The following should serve as an example where a subnet pool for internal PTG connectivity is created within a address scope bound to an APIC pre-existing VRF. The address scope has an additional address pool for external connectivity (as implicit creation is not possible).

The APIC now allocates BD subnets from the external address scope even in a no-NAT case. This is a bit counter intuitive as a no-NAT strategy should not require additional subnets and/or consume fabric side resources for each internal PTG network.

This is the configuration we've applied:

neutron address-scope-create --shared asv4-common 4 --apic:distinguished_names type=dict VRF=uni/tn-common/ctx-shared-services

# Create a subnetpool belonging to that VRF, used for allocating subnets on external networks
neutron subnetpool-create --address-scope asv4-common  --pool-prefix 100.64.0.0/10 --default-prefixlen 28 --shared v4-external-pool

# Create another subnetpool also belonging to that VRF< used for allocating subnets for PTGs
neutron subnetpool-create --address-scope asv4-common  --pool-prefix 172.16.0.0/24 --default-prefixlen 28 --shared v4-internal-pool

# Create the network for the L3 out in common tenant at out-L3Out-Shared-Services
neutron net-create L3Out-Ext-net --router:external True --shared --apic:distinguished_names type=dict ExternalNetwork=uni/tn-common/out-L3Out-Shared-Services/instP-Ext-Network --apic:nat_type "" 

# Allocate a subnet for the L3 out, using a subnet from the subnetpool created above
neutron subnet-create --name L3Out-Ext-subnet L3Out-Ext-net --subnetpool v4-external-pool --dns-nameserver 8.8.8.8

# Create an external segment for that L3 out
gbp external-segment-create L3Out-Shared-Services --shared True --subnet_id $(neutron subnet-show L3Out-Ext-subnet -c id -f value)  --port-address-translation false --external-route destination=0.0.0.0/0,nexthop=```

This is running the RPMs for AIM from Cisco for Pike.