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Cisco Systems' Private VLANs: Scalable Security in a Multi-Client Environment
RFC 5517

Document Type RFC - Informational (February 2010)
Authors Marco Foschiano , Sanjib HomChaudhuri
Last updated 2018-12-20
RFC stream Independent Submission
IESG Responsible AD Mark Townsley
Send notices to (None)
RFC 5517
Independent Submission                                   S. HomChaudhuri
Request for Comments: 5517                                  M. Foschiano
Category: Informational                                    Cisco Systems
ISSN: 2070-1721                                            February 2010

                     Cisco Systems' Private VLANs:
            Scalable Security in a Multi-Client Environment


   This document describes a mechanism to achieve device isolation
   through the application of special Layer 2 forwarding constraints.
   Such a mechanism allows end devices to share the same IP subnet while
   being Layer 2 isolated, which in turn allows network designers to
   employ larger subnets and so reduce the address management overhead.

   Some of the numerous deployment scenarios of the aforementioned
   mechanism (which range from data center designs to Ethernet-to-the-
   home-basement networks) are mentioned in the following text to
   exemplify the mechanism's possible usages; however, this document is
   not intended to cover all such deployment scenarios nor delve into
   their details.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 5517                      Private VLANs                February 2010

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................2
      1.1. Security Concerns with Sharing a VLAN ......................3
      1.2. The Traditional Solution and Its Related Problems ..........3
   2. Private VLANs Architecture ......................................4
      2.1. VLAN Pairings and Their Port-Related Characteristics .......7
   3. Extending Private VLANs across Switches .........................9
   4. A More Flexible IP Addressing Scheme ............................9
   5. Routing Considerations .........................................10
   6. Security Considerations ........................................10
   7. Acknowledgements ...............................................11
   8. References .....................................................11
      8.1. Normative References ......................................11
      8.2. Informative References ....................................11

1.  Introduction

   In an Ethernet switch, a VLAN is a broadcast domain in which hosts
   can establish direct communication with one another at Layer 2.  If
   untrusted devices are introduced into a VLAN, security issues may
   arise because trusted and untrusted devices end up sharing the same
   broadcast domain.

   The traditional solution to this kind of problem is to assign a
   separate VLAN to each user concerned about Layer 2 security issues.
   However, the IEEE 802.1Q standard [802.1Q] specifies that the VLAN ID
   field in an Ethernet frame is 12 bits wide.  That allows for a
   theoretical maximum of 4094 VLANs in an Ethernet network (VLAN
   numbers 0 and 4095 are reserved).  If the network administrator
   assigns one VLAN per user, then that equates to a maximum of 4094
   users that can be supported.  The private VLANs technology described
   in this memo addresses this scalability problem by offering more
   granular and more flexible Layer 2 segregation, as explained in the
   following sections.

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1.1.  Security Concerns with Sharing a VLAN

   Companies who have Internet presence can either host their servers in
   their own premises or, alternatively, they can locate their servers
   at the Internet Service Provider's premises.  A typical ISP would
   have a server farm that offers web-hosting functionality for a number
   of customers.  Co-locating the servers in a server farm offers ease
   of management but, at the same time, may raise security concerns.

   Let us assume that the ISP puts all the servers in one big VLAN.
   Servers residing in the same VLAN can listen to Layer 2 broadcasts
   from other servers.  Once a server learns the Media Access Control
   (MAC) address associated to the IP address of another computer in the
   same VLAN, it can establish direct Layer 2 communication with that
   device without having to go through a Layer 3 gateway/firewall.  If,
   for example, an attacker gets access to one of the servers, he or she
   can use that compromised host to launch an attack on other servers in
   the server farm.  To protect themselves from malicious attacks, ISP
   customers want their machines to be isolated from other machines in
   the same server farm.

   The security concerns become even more apparent in metropolitan area
   networks.  Metropolitan Service Providers may want to provide Layer 2
   Ethernet access to homes, rental communities, businesses, etc.  In
   this scenario, the subscriber next door could very well be a
   malicious network user.

   It is therefore very important to offer Layer 2 traffic isolation
   among customers.  Customer A would not want his Layer 2 frames being
   broadcast to customer B, who happens to be in the same VLAN.  Also,
   customer A would not want customer B to bypass a router or a firewall
   and establish direct Layer 2 communication with him/her.

1.2.  The Traditional Solution and Its Related Problems

   The traditional solution would be to assign a separate VLAN to each
   customer.  That way, each user would be assured of Layer 2 isolation
   from devices belonging to other users.

   However, with the VLAN-per-customer model, if an ISP wanted to offer
   web-hosting services to, say, 4000 customers, it would consume 4000
   VLANs.  Theoretically, the maximum number of VLANs that an 802.1Q-
   compliant networking device can support is 4094.  In reality, many
   devices support a much smaller number of active VLANs.  Even if all
   devices supported all 4094 VLANs, there would still be a scalability
   problem when the 4095th customer signed up.

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   A second problem with assigning a separate VLAN per customer is
   management of IP addresses.  Since each VLAN requires a separate
   subnet, there can be potential wastage of IP addresses in each
   subnet.  This issue has been described by RFC 3069 [RFC3069] and will
   not be discussed in detail in this document.

2.  Private VLANs Architecture

   The private VLANs architecture is similar to but more elaborate than
   the aggregated VLAN model proposed in RFC 3069.  The concepts of
   'super VLAN' and 'sub VLAN' used in that RFC are functionally similar
   to the concepts of 'primary VLAN' and 'secondary VLAN' used in this

   On the other hand, the private VLANs technology differs from the
   mechanism described in [RFC4562] because instead of using a MAC-
   address-based 'forced forwarding' scheme it uses a VLAN-based one.

   A regular VLAN is a single broadcast domain.  The private VLANs
   technology partitions a larger VLAN broadcast domain into smaller
   sub-domains.  So far, two kinds of special sub-domains specific to
   the private VLANs technology have been defined: an 'isolated' sub-
   domain and a 'community' sub-domain.  Each sub-domain is defined by
   assigning a proper designation to a group of switch ports.

   Within a private VLAN domain, three separate port designations exist.
   Each port designation has its own unique set of rules, which regulate
   a connected endpoint's ability to communicate with other connected
   endpoints within the same private VLAN domain.  The three port
   designations are promiscuous, isolated, and community.

   An endpoint connected to a promiscuous port has the ability to
   communicate with any endpoint within the private VLAN.  Multiple
   promiscuous ports may be defined within a single private VLAN domain.
   In most networks, Layer 3 default gateways or network management
   stations are commonly connected to promiscuous ports.

   Isolated ports are typically used for those endpoints that only
   require access to a limited number of outgoing interfaces on a
   private-VLAN-enabled device.  An endpoint connected to an isolated
   port will only possess the ability to communicate with those
   endpoints connected to promiscuous ports.  Endpoints connected to
   adjacent isolated ports cannot communicate with one another.  For
   example, within a web-hosting environment, isolated ports can be used
   to connect hosts that require access only to default gateways.

   A community port is a port that is part of a private VLAN community,
   which is a grouping of ports connected to devices belonging to the

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   same entity (for example, a group of hosts of the same ISP customer
   or a pool of servers in a data center).  Within a community,
   endpoints can communicate with one another and can also communicate
   with any configured promiscuous port.  Endpoints belonging to one
   community cannot instead communicate with endpoints belonging to a
   different community or with endpoints connected to isolated ports.

   The aforementioned three port designations directly correspond to
   three different VLAN types (primary, isolated, and community) with
   well-defined, port-related characteristics, which are described in
   detail in Section 2.1 below.

   Figure 1 below illustrates the private VLAN model from a switch port
   classification perspective.

                                     |    R    |
                 |                        p1            |
                 |                                      |
            =====| t1                                   |
                 |                switch                |
                 |                                      |
                 |                                      |
                 |i1         i2          c1          c2 |
                  |          |           |           |
                  |          |           |           |
                  |          |           |           |
                  A          B           C           D

                 A, B - Isolated devices
                 C, D - Community devices
                 R - Router (or other L4-L7 device)
                 i1, i2 - Isolated switch ports
                 c1, c2 - Community switch ports
                 p1 - Promiscuous switch port
                 t1 - Inter-switch link port (a VLAN-aware port)

             Figure 1. Private VLAN classification of switch ports

   With reference to Figure 1, each of the port types is described

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   Isolated ports: An isolated port, e.g., i1 or i2, cannot talk to any
      other port in the private VLAN domain except for promiscuous ports
      (e.g., p1).  If a customer device needs to have access only to a
      gateway router, then it should be attached to an isolated port.

   Community ports: A community port, e.g., c1 or c2, is part of a group
      of ports.  The ports within a community can have Layer 2
      communications with one another and can also talk to any
      promiscuous port.  If an ISP customer has, say, 2 devices that
      he/she wants to be isolated from other customers' devices but to
      be able to communicate among themselves, then community ports
      should be used.

   Promiscuous ports: As the name suggests, a promiscuous port (p1) can
      talk to all other types of ports.  A promiscuous port can talk to
      isolated ports as well as community ports and vice versa.  Layer 3
      gateways, DHCP servers, and other 'trusted' devices that need to
      communicate with the customer endpoints are typically connected
      via promiscuous ports.

   Please note that isolated, community, and promiscuous ports can
   either be access ports or hybrid/trunk ports (according to the
   terminology presented in Annex D of the IEEE 802.1Q specification, up
   to its 2004 revision).

   The table below summarizes the communication privileges between the
   different private VLAN port types.

   |             | isolat-| promis-| commu-| commu-| interswitch |
   |             | ted    | cuous  | nity1 | nity2 | link port   |
   | isolated    | deny   | permit | deny  | deny  | permit      |
   | promiscuous | permit | permit | permit| permit| permit      |
   | community1  | deny   | permit | permit| deny  | permit      |
   | community2  | deny   | permit | deny  | permit| permit      |
   | interswitch |        |        |       |       |             |
   | link port   | deny(*)| permit | permit| permit| permit      |

                                  Table 1

   (*) Please note that this asymmetric behavior is for traffic
       traversing inter-switch link ports over an isolated VLAN only.

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       Traffic from an inter-switch link port to an isolated port will
       be denied if it is in the isolated VLAN.  Traffic from an inter-
       switch link port to an isolated port will be permitted if it is
       in the primary VLAN (see below for the different VLAN

   N.B.: An inter-switch link port is simply a regular port that
         connects two switches (and that happens to carry two or more

2.1.  VLAN Pairings and Their Port-Related Characteristics

   In practice, the Layer 2 communication constraints described in the
   table above can be enforced by creating sub-domains within the same
   VLAN domain.  However, a sub-domain within a VLAN domain cannot be
   easily implemented with only one VLAN ID.  Instead, a mechanism of
   pairing VLAN IDs can be used to achieve this notion.  Specifically,
   sub-domains can be represented by pairs of VLAN numbers:

     <Vp,Vs>   Vp is the primary VLAN ID               ------
               Vs is the secondary VLAN ID             | Vp |
               where Vs can be:                       /      \
                  - Vi (an isolated VLAN)            /        \
                  - Vc (a community VLAN)           /          \
                                                 ------       ------
                                                 | Vi |       | Vc |
                                                 ------       ------
                                                 <Vp,Vi>      <Vp,Vc>

                  Figure 2. A private VLAN domain can be
                implemented with one or more VLAN ID pairs.

   A private VLAN domain is built with at least one pair of VLAN IDs:
   one (and only one) primary VLAN ID (Vp) plus one or more secondary
   VLAN IDs (Vs).  Secondary VLANs can be of two types: isolated VLANs
   (Vi) or community VLANs (Vc).

   A primary VLAN is the unique and common VLAN identifier of the whole
   private VLAN domain and of all its VLAN ID pairs.

   An isolated VLAN is a secondary VLAN whose distinctive characteristic
   is that all hosts connected to its ports are isolated at Layer 2.
   Therefore, its primary quality is that it allows a design based on
   private VLANs to use a total of only two VLAN identifiers (i.e., a
   single private VLAN pairing) to provide port isolation and serve any
   number of end users (vs. a traditional design in which one separate
   plain VLAN ID would be assigned to each port).

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   A community VLAN is a secondary VLAN that is associated to a group of
   ports that connect to a certain "community" of end devices with
   mutual trust relationships.

   While only one isolated VLAN is allowed in a private VLAN domain,
   there can be multiple distinct community VLANs.

   Please note that this VLAN pairing scheme simply requires that all
   traffic transported within primary and secondary VLANs be tagged
   according to the IEEE 802.1Q standard (see for example [802.1Q],
   Section B.1.3), with at most a single standard VLAN tag.  No special
   double-tagging is necessary due to the 1:1 correspondence between a
   secondary VLAN and its associated primary VLAN.

   (Also note that this document makes use of the "traditional" VLAN
   terminology, whereas the IEEE 802.1ag standard [802.1ag] amends key
   sections of IEEE 802.1Q-2005 to make the distinction between "VLANs"
   and "VLAN IDs" so that every "VLAN" can be assigned one or more VLAN
   IDs, similarly to the pairing scheme described in this document.)

   The ports in a private VLAN domain derive their special
   characteristics (as described in Section 2) from the VLAN pairing(s)
   they are configured with.  In particular, a promiscuous port is a
   port that can communicate with all other private VLAN port types via
   the primary VLAN and any associated secondary VLANs, whereas isolated
   or community ports can communicate over their respective secondary
   VLANs only.

   For example, with reference to Figure 1, a router R connected to the
   promiscuous port can have Layer 2 communication with a device A
   connected to an isolated port and also with a device C connected to a
   community port.  Devices C and D can also have Layer 2 communication
   between themselves since they are part of the same community VLAN.
   However, devices A and B cannot communicate at Layer 2 due to the
   special port segregation property of the isolated VLAN.  Also,
   devices A and C cannot communicate at Layer 2 since they belong to
   different secondary VLANs.

   The impact of these enforced forwarding restrictions is two-fold.
   Firstly, service providers can assign multiple customers to the same
   isolated VLAN, thereby conserving VLAN IDs.  Secondly, end users can
   be assured that their Layer 2 traffic cannot be sniffed by other end
   users sharing the same isolated VLAN or connected to a different
   secondary VLAN.

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3.  Extending Private VLANs across Switches

   Some switch vendors have attempted to provide a port isolation
   feature within a VLAN by implementing special logic at the port
   level.  However, when implemented at the port level, the isolation
   behavior is restricted to a single switch.

   When a VLAN spans multiple switches, there is no standard mechanism
   to propagate port-level isolation information to other switches and,
   consequently, the isolation behavior fails in other switches.

   In this document, the proposal is to implement the port isolation
   information implicitly at the VLAN level.  A particular VLAN ID can
   be configured to be the isolated VLAN.  All switches in the network
   would give special "isolated VLAN" treatment to frames tagged with
   this particular VLAN ID.  Thereby, the isolated VLAN behavior can be
   maintained consistently across all switches in a Layer 2 network.

   In general, isolated, community, and primary VLANs can all span
   multiple switches, just like regular VLANs.  Inter-switch link ports
   need not be aware of the special VLAN type and will carry frames
   tagged with these VLANs just like they do any other frames.

   One of the objectives of the private VLANs architecture is to ensure
   that traffic from an isolated port in one switch does not reach
   another isolated or community port in a different switch even after
   traversing an inter-switch link.  By implicitly embedding the
   isolation information at the VLAN level and by transporting it along
   with the packet, it is possible to maintain a consistent behavior
   throughout the network.  Therefore, the mechanism discussed in
   Section 2, which will restrict Layer 2 communication between two
   isolated ports in the same switch, will also restrict Layer 2
   communication between two isolated ports in two different switches.

4. A More Flexible IP Addressing Scheme

   The common practice of deploying multiple VLANs in a network for
   security reasons and of allocating a subnet to each VLAN has led to a
   certain number of inefficiencies in network designs, such as the
   suboptimal utilization of the IP addressing space (as exemplified in
   the introduction of RFC 3069 [RFC3069]).  Moreover, each subnet
   requires addresses to be set aside for internetworking purposes (a
   subnetwork address, a directed broadcast address, default gateway
   address(es), etc.).  So a high number of used VLANs traditionally
   translates into a significant number of special addresses to be

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   On the other hand, in a private VLAN domain, all members can share a
   common address space that is part of a single subnet associated to
   the primary VLAN.  An end device can be assigned an IP address
   statically or by using a DHCP server connected to a promiscuous port.
   Since IP addresses are no longer allocated on a smaller subnet basis
   but are assigned from a larger address pool shared by all members in
   the private VLAN domain, address allocation becomes much more
   efficient: fewer addresses are consumed for internetworking purposes,
   while most of the address space is allotted to end devices, leaving
   ample flexibility in the way available addresses are (re-)assigned.

5.  Routing Considerations

   The entire private VLANs architecture confines secondary VLANs within
   the 2nd layer of the OSI model.  With reference to Figure 2, the
   secondary VLANs are internal to a private VLAN domain.  Layer 3
   entities are not directly aware of their existence: to them it
   appears as if all the end devices are part of the primary VLAN.

   With reference to Figure 1, the isolation behavior between devices A
   and B is at the Layer 2 level only.  Devices A and B can still
   communicate at the Layer 3 level via the router R.  Since A and B are
   part of the same subnet, the router assumes that they should be able
   to talk directly to each other.  That however is prevented by the
   isolated VLAN's specific behavior.  So, in order to enable A and B to
   communicate via the router, a proxy-ARP-like functionality needs to
   be supported on the router interface.

   With regard to the specific version of the IP protocol in use, all
   routing considerations apply to both IPv4 and IPv6 for the case of
   unicast traffic.  On the other hand, due to their complexity,
   considerations about multicast bridging and routing within a private
   VLAN domain transcend the scope of this introductory document, and
   are therefore omitted.

6.  Security Considerations

   In a heterogeneous Layer 2 network that is built with switches from
   multiple vendors, the private VLAN feature should be supported and
   configured on all the switches.  If a switch S in that network does
   not support this feature, then there may be undesired forwarding of
   packets, including permanent flooding of Layer 2 unicast frames.
   That is because switch S is not aware of the association between
   primary and secondary VLANs and consequently cannot apply the
   segregation rules and constraints characteristic of the private VLANs
   architecture (an example of one such constraint is explained in
   [802.1Q], Section B.1.3).  This impact is limited to traffic within

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   the private VLAN domain and will not affect the regular Layer 2
   forwarding behavior on other VLANs.

   If the private VLAN feature is properly deployed, it can be used at
   Layer 2 to segregate individual users or groups of users from each
   other: this segregation allows a network designer to more effectively
   constrain Layer 2 forwarding so as to, for instance, block or contain
   unwanted inter-device communication like port scans or Address
   Resolution Protocol (ARP) poisoning attacks.

7.  Acknowledgements

   Many people have contributed to the private VLANs architecture.  We
   would particularly like to thank, in alphabetical order, Senthil
   Arunachalam, Jason Chen, Tom Edsall, Michael Fine, Herman Hou, Kannan
   Kothandaraman, Milind Kulkarni, Heng-Hsin Liao, Tom Nosella, Prasanna
   Parthasarathy, Ramesh Santhanakrishnan, Mukundan Sudarsan, Charley
   Wen, and Zhong Xu for their significant contributions.

8.  References

8.1.  Normative References

   [802.1Q]   Institute of Electrical and Electronics Engineers,
              "Virtual Bridged Local Area Networks", IEEE Standard
              802.1Q, 2005 Edition, May 2006.

   [802.1ag]  Institute of Electrical and Electronics Engineers,
              "Connectivity Fault Management", IEEE Standard 802.1ag,
              2007 Edition, December 2007.

8.2.  Informative References

   [RFC3069]  McPherson, D. and B. Dykes, "VLAN Aggregation for
              Efficient IP Address Allocation", RFC 3069, February 2001.

   [RFC4562]  Melsen, T. and S. Blake, "MAC-Forced Forwarding: A Method
              for Subscriber Separation on an Ethernet Access Network",
              RFC 4562, June 2006.

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Authors' Addresses

   Marco Foschiano
   Cisco Systems, Inc.
   Via Torri Bianche 7
   Vimercate, MI, 20059, Italy

   Sanjib HomChaudhuri

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