Due to the non-decreasing interest to the post about Private VLANs, I decided to make another one, more detailed – including a diagram and verification techniques.

Introduction

To begin with, recall that VLAN is essentially a broadcast domain. Private VLANs (PVANs) allow splitting the domain into multiple isolated broadcast "subdomains", introducing sub-VLANs inside a VLAN. As we know, Ethernet VLANs can not communicate directly with each other - they require a L3 device to forward packets between separate broadcast domains. The same restriction applies to PVLANS - since the subdomains are isolated at Level 2, they need to communicate using an upper level (L3/packet forwarding) device - such as router.

In reality, different VLANs normally map to different IP subnets. When we split a VLAN using PVLANs, hosts in different PVLANs still belong to the same IP subnet, yet now they need to use a router (L3 device) to talk to each other (for example, by using Local Proxy ARP). In turn, the router may either permit or forbid communications between sub-VLANs using access-lists. Commonly, these configurations arise in "shared" environments, say ISP co-location, where it's beneficial to put multiple customers into the same IP subnet, yet provide a good level of isolation between them.

Private VLANs Terminology

The following is the reference diagram that we are going to use to illustrate Private VLAN concepts and functionality.

For our sample configuration, we take VLAN 1000 and divide it into three PVLANs - sub-VLAN 1012 (R1 and R2), sub-VLAN 1034 (R3 and R4) and sub-VLAN 1055 (router R5 only). Router R6 will be used as layer 3 device, to resolve the layer 3 communication issue. We name VLAN 1000 as “Primary” and classify the ports, assigned to this VLAN, based on their types:

• Promiscuous (“P”) port: Usually connects to a router. This port type is allowed to send and receive L2 frames from any other port on the VLAN.
• Isolated (“I”) port: This type of port is only allowed to communicate with “P”-ports - i.e., they are "stub" port. You commonly see these ports connecting to hosts.
• Community (“C”) port: Community ports are allowed to talk to their buddies, sharing the same community (group) and to “P”-ports.

In order to implement sub-VLAN behavior, we need to define how packets are forwarded between different types of ports. We group the VLANs in "Primary" and "Secondary".

• Primary VLAN (VLAN 1000 in our example). This VLAN is used to forward frames downstream from “P”-ports to all other port types (“I” and “C” ports) in the system. Essentially, Primary VLAN embraces all ports in the domain, but only transports frames from the router to hosts (from “P” to “I” and “C”).
• Secondary Isolated VLAN: forwards frames from "I" ports to "P" ports. Since Isolated ports do not exchange frames with each other, we can use just ONE isolated VLAN to connect all I-Port to the P-port.
• Secondary Community VLANs: Transport frames between community ports (C-ports) within to the same group (community) and forward frames upstream to the P-ports of the primary VLAN.

How Private VLANs Work

Here are the key aspects of Private VLAN functioning:

• The Primary VLAN delivers frames downstream from the router (promisc port) to all mapped hosts.
• The Isolated VLAN transports frames from the stub hosts upstream to the router
• The Community VLANs allow bi-directional frame exchange withing a single group, in addition to forwarding frames upstream towards "P"-ports.
• Ethernet MAC address learning and forwarding procedure remain the same, as well as broadcast/multicast flooding procedure within boundaries of primary/secondary VLANs.

Private VLANs could be trunked. The secondary VLAN numbers are used to tag frames, just as with regular VLANs, and the primary VLAN traffic is trunked as well. However, you need to configure Private VLAN specific settings (bindings, mappings) on every participating swtich, as it's not possible to use VTPv2 to dissiminate that information . This due to the fact that VTPv2 has no TLVs to carry private VLANs information. VTPv3 was designed to overcome this limitation among others.

Configuring Private VLANs

We have primary VLAN 1000, Isolated VLAN 1005 (R5) Community VLAN 1012 (R1, R2) and Community VLAN 1034 (R3, R4).

Step 1:

First, disable VTP, i.e. enable VTP transparent mode. After disabling VTP, create Primary and Secondary VLANs and bind them into PVLAN domain:

SW1:

vtp mode transparent

!

! Creating primary VLAN, which is shared among secondary’s

!

vlan 1000

  private-vlan primary
!

! Community VLAN for R1 and R2: allows a “subVLAN” within a Primary VLAN

!

vlan 1012

  private-vlan community

!

! Community VLAN for R3 and R4

!

vlan 1034

  private-vlan community
!

! Isolated VLAN: Connects all stub hosts to router.

! Remember - only one isolated vlan per primary VLAN.

! In our case, isolates R5 only.

!

vlan 1055

  private-vlan isolated
!

!  Associating the primary with secondary’s

!

vlan 1000

 private-vlan association 1012,1034,1055

This step is needed is to group PVLANs into a shared domain and establish a formal association (for syntax checking and VLAN type verifications). Repeat the same operations on SW2, since VTP has been disabled.

Step 2:

Configure host ports and bind them to the respective isolated PVLANs. Note that a host port belongs to different VLANs at the same time: downstream primary and upstream secondary. Also, enable trunking between switches, to allow private VLANs traffic to pass between switches.

SW1:

!

! Community port (links R1 to R2 and “P”-ports)

!

interface FastEthernet0/1

 description == R1

 switchport private-vlan host-association 1000 1012

 switchport mode private-vlan host

 spanning-tree portfast
!

! Community port (links R3 to R4 and “P”-ports)

!

interface FastEthernet0/3

 description == R3

 switchport private-vlan host-association 1000 1034

 switchport mode private-vlan host

 spanning-tree portfast
!

! Isolated port (uses isolated VLAN to talk to “P”-ports)

!

interface FastEthernet0/5

 description == R5

 switchport private-vlan host-association 1000 1055

 switchport mode private-vlan host

 spanning-tree portfast
!

! Trunk port

!

interface FastEthernet 0/13

 switchport trunk encapsulation dot1q

 switchport mode trunk
SW2:

interface FastEthernet0/2

 description == R2

 switchport private-vlan host-association 1000 1012

 switchport mode private-vlan host

 spanning-tree portfast

!

interface FastEthernet0/4

 description == R4

 switchport private-vlan host-association 1000 1034

 switchport mode private-vlan host

 spanning-tree portfast
!

! Trunk port

!

interface FastEthernet 0/13

 switchport trunk encapsulation dot1q

 switchport mode trunk
Next, Verify the configuration on SW1:
Rack1SW1#show vlan id 1012
VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1012 VLAN1012                         active    Fa0/13
VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1012 enet  101012     1500  -      -      -        -    -        0      0   
Remote SPAN VLAN

----------------

Disabled
Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

1000    1012      community         Fa0/1
Rack1SW1#show vlan id 1034
VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1034 VLAN1034                         active    Fa0/13
VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1034 enet  101034     1500  -      -      -        -    -        0      0   
Remote SPAN VLAN

----------------

Disabled
Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

1000    1034      community         Fa0/3
Rack1SW1#show vlan id 1055
VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1055 VLAN1055                         active    Fa0/13
VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1055 enet  101055     1500  -      -      -        -    -        0      0   
Remote SPAN VLAN

----------------

Disabled
Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

1000    1055      isolated          Fa0/5
Rack1SW1#show interfaces fastEthernet 0/13 trunk 
Port        Mode         Encapsulation  Status        Native vlan

Fa0/13      desirable    802.1q         trunking      1
Port        Vlans allowed on trunk

Fa0/13      1-4094
Port        Vlans allowed and active in management domain

Fa0/13      1,1000,1012,1034,1055
Port        Vlans in spanning tree forwarding state and not pruned

Fa0/13      1,1000,1012,1034,1055
Verify on SW2:
Rack1SW2#show vlan id 1000
VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1000 VLAN1000                         active    Fa0/13
VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1000 enet  101000     1500  -      -      -        -    -        0      0   
Remote SPAN VLAN

----------------

Disabled
Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

1000    1012      community         Fa0/2, Fa0/6

1000    1034      community         Fa0/4, Fa0/6

1000    1055      isolated          Fa0/6
Rack1SW2#show vlan id 1012
VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1012 VLAN1012                         active    Fa0/13
VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1012 enet  101012     1500  -      -      -        -    -        0      0   
Remote SPAN VLAN

----------------

Disabled
Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

1000    1012      community         Fa0/2, Fa0/6
Rack1SW2#show vlan id 1034
VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1034 VLAN1034                         active    Fa0/13
VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1034 enet  101034     1500  -      -      -        -    -        0      0   
Remote SPAN VLAN

----------------

Disabled
Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

1000    1034      community         Fa0/4, Fa0/6
Rack1SW2#show vlan id 1055
VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1055 VLAN1055                         active    Fa0/13
VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1055 enet  101055     1500  -      -      -        -    -        0      0   
Remote SPAN VLAN

----------------

Disabled
Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

1000    1055      isolated          Fa0/6
Rack1SW2#show interface fastEthernet 0/13 trunk 
Port        Mode         Encapsulation  Status        Native vlan

Fa0/13      desirable    802.1q         trunking      1
Port        Vlans allowed on trunk

Fa0/13      1-4094
Port        Vlans allowed and active in management domain

Fa0/13      1,1000,1012,1034,1055
Port        Vlans in spanning tree forwarding state and not pruned

Fa0/13      1,1000,1012,1034,1055

Step 3:

Create a promiscuous port and configure downstream mappings. Here we add secondary VLANs for which traffic is received by this particular “P”-port. Primary VLAN is used to send traffic downstream to all “C” and “I” ports per their associations.

SW2:

!

! Promiscuous port, mapped to all secondary VLANs

!

interface FastEthernet0/6

 description == R6

 switchport private-vlan mapping 1000 1012,1034,1055

 switchport mode private-vlan promiscuous

 spanning-tree portfast
Verify the promiscuous port configuration:
Rack1SW2#show int fa 0/6 switch | beg private

Administrative Mode: private-vlan promiscuous

Operational Mode: private-vlan promiscuous

Administrative Trunking Encapsulation: negotiate

Operational Trunking Encapsulation: native

Negotiation of Trunking: Off

Access Mode VLAN: 1 (default)

Trunking Native Mode VLAN: 1 (default)

Administrative Native VLAN tagging: enabled

Voice VLAN: none

Administrative private-vlan host-association: none

Administrative private-vlan mapping: 1000 (VLAN1000) 1012 (VLAN1012) 1034 (VLAN1034) 1055 (VLAN1055)

Administrative private-vlan trunk native VLAN: none

Administrative private-vlan trunk Native VLAN tagging: enabled

Administrative private-vlan trunk encapsulation: dot1q

Administrative private-vlan trunk normal VLANs: none

Administrative private-vlan trunk private VLANs: none

Operational private-vlan:

  1000 (VLAN1000) 1012 (VLAN1012) 1034 (VLAN1034) 1055 (VLAN1055)

If you need to configure an SVI on a switch to communicate with private VLAN members, you should add an interface corresponding to Primary VLAN only. Obviously that's because all secondary VLANs are "subordinates" of primary. After an SVI has been created, you have to map the required secondary VLANs to the SVI (just like with a promiscuous port) in order to make communications possible. You may exclude some mappings from SVI interface, and limit it to communicating only with certain secondary VLANs.

SW1:

!

! SW1 SVI is mapped to all secondary VLANs

!

interface Vlan 1000

 ip address 10.0.0.7 255.255.255.0

 private-vlan mapping 1012,1034,1055
SW2:

!

! SW2 SVI is mapped to 1012/1034 only, so it’s cant communicate with R5

!

interface Vlan1000

 ip address 10.0.0.8 255.255.255.0

 private-vlan mapping 1012,1034

Now to verify the configuration, configure R1-R6 interfaces in subnet “10.0.0.0/24” and ping broadcast addresses.

Rack1R1#ping 10.0.0.255 repeat 1
Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 10.0.0.255, timeout is 2 seconds:
Reply to request 0 from 10.0.0.7, 4 ms

Reply to request 0 from 10.0.0.2, 4 ms

Reply to request 0 from 10.0.0.6, 4 ms

Reply to request 0 from 10.0.0.8, 4 ms
Rack1R3#ping 10.0.0.255 repeat 1
Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 10.0.0.255, timeout is 2 seconds:
Reply to request 0 from 10.0.0.7, 4 ms

Reply to request 0 from 10.0.0.4, 4 ms

Reply to request 0 from 10.0.0.6, 4 ms

Reply to request 0 from 10.0.0.8, 4 ms
Rack1R5#ping 10.0.0.255 repeat 1
Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 10.0.0.255, timeout is 2 seconds:
Reply to request 0 from 10.0.0.7, 1 ms

Reply to request 0 from 10.0.0.6, 1 ms
Rack1R6#ping 10.0.0.255 repeat 1
Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 10.0.0.255, timeout is 2 seconds:
Reply to request 0 from 10.0.0.1, 4 ms

Reply to request 0 from 10.0.0.7, 4 ms

Reply to request 0 from 10.0.0.2, 4 ms

Reply to request 0 from 10.0.0.5, 4 ms

Reply to request 0 from 10.0.0.3, 4 ms

Reply to request 0 from 10.0.0.4, 4 ms

Reply to request 0 from 10.0.0.8, 4 ms

Lastly, there is another feature, called protected port or “Private VLAN edge”. The feature is pretty basic and is available even on low-end Cisco switches. It allows isolating ports in the same VLAN. Specifically, all ports in a VLAN, marked as protected are prohibited from sending frames to each other (but still allowed to send frames to other (non-protected) ports within the same VLAN). Usually, ports configured as protected are also configured not to receive unknown unicast (frame with destination MAC address not in switch's MAC table) and multicast frames flooding for added security.

Example:

interface range FastEthernet 0/1 - 2

 switchport mode access

 switchport protected

 switchport block unicast

 switchport block multicast

You may want to see the updated version of this post at:

http://blog.internetworkexpert.com/2008/07/14/private-vlans-revisited/

---

Private VLAN concepts are quite simple, but Cisco's implemenation and configuration steps are a bit confusing - with all the "mappings" and "associations" stuff. Here comes a short overview of how private VLANs work.

To begin with, let's look at the concept of VLAN as a broadcast domain. What Private VLANs (PVANs) do, is split the domain into multiple isolated broadcast subdomains. It's a simple nesting concept - VLANs inside a VLAN. As we know, Ethernet VLANs are not allowed to communicate directly, they need L3 device to forward packets between broadcast domains. The same concept applies to PVLANS - since the subdomains are isolated at level 2, they need to communicate using an upper level (L3 and packet forwarding) entity - such as router. However, there is difference here. Regular VLANs usually correspond to a single IP subnet. When we split VLAN using PVLANs, hosts in different PVLANs still belong to the same IP subnet, but they need to use router (another L3 device) to talk to each other (for example, by means of local Proxy ARP). In turn, router may either permit or forbid communications between sub-VLANs using access-lists.
Why would anyone need Private VLANs? Commonly, this kind of configurations arise in "shared" environments, say ISP co-location, where it's beneficial to put multiple customers into the same IP subnet, yet provide a good level of isolation between them.

For our sample configuration, we will take VLAN 100 and divide it into two PVLANs - sub-VLANs 101 and 102. Take the regular VLAN and call it primary (VLAN 100 in our example), then divide ports, assigned to this VLAN, by their types:

Promiscuous (P): Usually connects to a router - a type of a port which is allowed to send and receive frames from any other port on the VLAN
Isolated (I): This type of port is only allowed to communicate with P-ports - they are "stub". This type of ports usually connects to hosts
Community (C): Community ports are allowed to talk to their buddies, sharing the same group (of course they can talk to P-ports)

In order to implement sub-VLAN behavior, we need to define how packets are forwarded between different port types. First comes the Primary VLAN - simply the original VLAN (VLAN 100 in our example). This type of VLAN is used to forward frames downstream from P-ports to all other port types (I and C ports). In essense, Primary VLAN entails all port in domain, but is only used to transport frames from router to hosts (P to I and C). Next comes Secondary VLANs, which correspond to Isolated and Community port groups. They are used to transport frames in the opposite direction - from I and C ports to P-port.

Isolated VLAN: forwards frames from I ports to P ports. Since Isolated ports do not exchange frames with each other, we can use just ONE isolated VLAN to connect all I-Port to the P-port.
Community VLANs: Transport frames between community ports (C-ports) within to the same group (community) and forward frames uptstream to the P-ports of the primary VLAN.

This is how it works:

Primary VLANs is used to deliver frames downstream from router to all hosts; Isolated VLAN transports frames from stub hosts upstream to the router; Community VLANs allow frames exchange withing a single group and also forward frames in upstream direction towards P-port. All the basic MAC address learning and unknown unicast flooding princinples remain the same.

Let's move to the configuration part (Primary VLAN 100, Isolated VLAN 101 and Community VLAN 102).

Step 1:

Create Primary and Secondary VLANs and group them into PVLAN domain:

!

! Creating VLANs: Primary, subject to subdivision

!

vlan 100

 private-vlan primary
!

! Isolated VLAN: Connects all stub hosts to router

!

vlan 101

 private-vlan isolated
!

! Community VLAN: allows a subVLAN within a Primary VLAN

!

vlan 102

 private-vlan community
!

!  Associating

!

vlan 100

 private-vlan assoc 101,102

What this step is needed for, is to group PVLANs into a domain and establish a formal association (for syntax checking and VLAN type verifications).

Step 2:

Configure host ports and bind them to the respective isolated PVLANs. Note that a host port belongs to different VLANs at the same time: downstream primary and upstream secondary.

!

! Isolated port (uses isoalated VLAN to talk to P-port)

!

interface FastEthernet x/y

 switchport mode private-vlan host

 switchport private-vlan host-association 100 101
!

! Community ports: use community VLAN

!

interface range FastEthernet x/y - z

 switchport mode private-vlan host

 switchport private-vlan host-association 100 102

Step 3:

Create a promiscuous port, and configure downstream mapping. Here we add secondary VLANs for which traffic is received by this P-port. Primary VLAN is used to send traffic downstream to all C/I ports as per their associations.

!

! Router port

!

interface FastEthernet x/y

 switchport mode private-vlan promisc

 switchport private-vlan mapping 100 add 101,102

if you need to configure an SVI on the switch, you should add an interface correspoding to Primary VLAN only. Obviously that's because of all secondary VLANs being simply "subordiantes" of primary. In our case the config would look like this:

interface Vlan 100

 ip address 172.16.0.1 255.255.255.0

Lastly, there is another feature, worths to be mentioned, called protected port or Private VLAN edge. The feature is pretty basic and avaiable even on low-end Cisco switches, allows to isolate ports in the same VLAN. Specifically, all ports in a VLAN, marked as protected are prohibited from sending frames to each other (but still allowed to send frames to other (non-protected) ports within the same VLAN). Usually, ports configurated as protected, are also configured not to receive unknown unicast (frame with destination MAC address not in switch's MAC table) and multicast frames flooding for added security.

Example:

interface range FastEthernet 0/1 - 2

 switchport mode access

 switchport protected

 switchport block unicast

 switchport block multicast