INTAREA                                                      E. Nordmark
Internet-Draft                                            March 30, 2017
Intended status: Standards Track
Expires: October 1, 2017

           IP over Intentionally Partially Partitioned Links


   IP makes certain assumptions about the L2 forwarding behavior of a
   multi-access IP link.  However, there are several forms of
   intentional partitioning of links ranging from split-horizon to
   Private VLANs that violate some of those assumptions.  This document
   specifies that link behavior and how IP handles links with those

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   This Internet-Draft will expire on October 1, 2017.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Keywords and Terminology  . . . . . . . . . . . . . . . . . .   3
   3.  Private VLAN  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Bridge Configuration for Private VLANs  . . . . . . . . .   4
     3.2.  Resulting Bridge Behavior . . . . . . . . . . . . . . . .   6
   4.  IP over IPPL  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  IPv6 over IPPL  . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IPv4 over IPPL  . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Multiple routers  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Multicast over IPPL . . . . . . . . . . . . . . . . . . . . .  10
   9.  DHCP Implications . . . . . . . . . . . . . . . . . . . . . .  12
   10. Redirect Implications . . . . . . . . . . . . . . . . . . . .  12
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   14. Appendix: Layer 2 Learning Implications . . . . . . . . . . .  13
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     15.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   IPv4 and IPv6 can in general handle two forms of links; point-to-
   point links when only have two IP nodes (self and remote), and multi-
   access links with one or more nodes attached to the link.  For the
   multi-access links IP in general, and particular protocols like ARP
   and IPv6 Neighbor Discovery, makes a few assumptions about transitive
   and reflexive connectivity i.e., that all nodes attached to the link
   can send packets to all other nodes.

   There are cases where for various reasons and deployments one wants
   what looks like one link from the perspective of IP and routing, yet
   the L2 connectivity is restrictive.  A key property is that an IP
   subnet prefix is assigned to the link, and IP routing sees it as a
   regular multi-access link with link-local unicast and multicast
   addresses functioning as expected.  But a host attached to the link
   might not be able to send packets to all other hosts attached to the
   link.  The motivation for this is outside the scope of this document,
   but in summary the motivation to preserve the single link view as
   seen by IP routing is to conserve IP(v4) address space, and the
   motivation to restrict communication on the link could be due to
   (security) policy or to wireless connectivity approaches.

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   This intentional and partial partition appears in a few different
   forms.  For DSL [TR-101] and Cable [DOCSIS-MULPI] the pattern is to
   have a single access router on the link, and all the hosts can send
   and receive from the access router, but host-to-host communication is
   blocked.  A richer set of restrictions are possible for Private VLANs
   (PVLAN) [RFC5517], which has a notion of three different ports i.e.
   attachment points: isolated, community, and promiscuous.  Note that
   other techniques operate at L2/L3 boundary like [RFC4562] but those
   are out of scope for this document.

   The possible connectivity patterns for Private VLANs appears to be a
   super-set of the DSL and Cable use of split horizon, thus this
   document specifies the PVLAN behavior, shows the impact on IP/ARP/ND,
   and specifies how IP/ARP/ND must operate to work with PVLAN.

   If private VLANs, or the split horizon subset, has been configured at
   layer 2 for the purposes of IPv4 address conservation, then that
   layer 2 configuration will affect IPv6 even though IPv6 might not
   have the same need for address conservation.

   The cases covered in this document are where the link has been
   intentionally partitioned, which is different from the cases where a
   collection of links are joined to have a common IP subnet prefix.  An
   example of the differences is the expected behavior for packets sent
   to link-local IP addresses.  The issues for such multi-link subnets
   are described in [RFC4903].

2.  Keywords and Terminology

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

   a The following terms from [RFC4861] are used without modifications:

   node           a device that implements IP.
   router         a node that forwards IP packets not explicitly
                  addressed to itself.
   host           any node that is not a router.
   link           a communication facility or medium over which nodes
                  can communicate at the link layer, i.e., the layer
                  immediately below IP.  Examples are Ethernets (simple
                  or bridged), PPP links, X.25, Frame Relay, or ATM
                  networks as well as Internet-layer (or higher-layer)
                  "tunnels", such as tunnels over IPv4 or IPv6 itself.
   interface      a node's attachment to a link.
   neighbors      nodes attached to the same link.

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   This document defines the following set of terms:

   bridge         a layer-2 device which implements 802.1Q
   port           a bridge's attachment to another bridge or to a node.

3.  Private VLAN

   A private VLAN is a structure which uses two or more 802.1Q (VLAN)
   values to separate what would otherwise be a single VLAN, viewed by
   IP as a single broadcast domain, into different types of ports with
   different L2 forwarding behavior between the different ports.  A
   private VLAN consists of a single primary VLAN and multiple secondary

   From the perspective of both a single bridge and a collection of
   interconnected bridges there are three different types of ports use
   to attach nodes plus an inter-bridge port:

   o  Promiscuous: A promiscuous port can send packets to all ports that
      are part of the private VLAN.  Such packets are sent using the
      primary VLAN ID.
   o  Isolated: Isolated VLAN ports can only send packets to promiscuous
      ports.  Such packets are sent using an isolated VLAN ID.
   o  Community: A community port is associated with a per-community
      VLAN ID, and can send packets to both ports in the same community
      VLAN and promiscuous ports.
   o  Inter-bridge: A port used to connect a bridge to another bridge.

   Private VLANs is an implementation of asymmetric VLANs and Rooted-
   Multipoint connectivity.  Private VLANs were an integral part of
   [IEEE802.1Q-1998].  The mapping between the mechanisms in that
   standard plus the above Private VLAN notion of different types of
   ports to the L2 forwarding behavior are somewhat complex and
   described in the following sections.

3.1.  Bridge Configuration for Private VLANs

   This text is reproduced from [IEEE802.1-LIAISON] to ensure this
   specification together with [IEEE802.1Q-1998] provide a complete
   standard on which we can describe the IP implications.  Note that
   this section uses slightly different terminology than above e.g.,
   "root port" instead of "promiscuous port".

   "Private VLANs" as described in this document are a combination of
   the "Multi-Netted Server" and the "Rooted-Multipoint" use cases
   described in 802.1Q annex F.1.3 "Asymmetric VLANs and Rooted-
   Multipoint Connectivity".  The "Multi-Netted Server" example
   describes how a bridged network allows a server to communicate with

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   multiple mutually-isolated groups of clients by allocating a VLAN ID
   per group.  The "Rooted-Multipoint" example describes an optimization
   that allows all groups containing a single client to share a single
   VLAN ID while still remaining isolated from each other.

   In the details for basic private VLANs below, all clause numbers are
   IEEE Std 802.1Q-2014.  Clause 12 is used as a reference for
   management.  The MIBs in clause 17 are constructed as an
   implementation of the management model in clause 12, as are the YANG
   models currently being developed.

   o  Select a set of VLAN IDs (VIDs) - let's call them the Branch VID
      (communication from Leaf ports [hosts] to Root ports [routers]),
      the Trunk VID (from Root ports to each other and to Leaf ports),
      and zero or more Party VIDs (from Community ports to Root ports
      and other Community ports in the same community).
   o  Assign all VIDs to the same FID ( - this activates
      Shared VLAN Learning and needs to be done in all bridges.
   o  For all Leaf ports:

      *  Configure the Branch VID as the PVID (the default input VID,
      *  Configure the port to accept only untagged or priority tagged
         frames (
      *  Configure the port to disable ingress filtering
      *  Create a permanent VLAN registration entry (
         specifying a fixed registration (8.8.2:b:1:i), frames to be
         output untagged (8.8.2:b:2) for the Trunk VID.
   o  For all Community ports:

      *  Configure the Party VID for this port's community as the PVID
         (the default input VID,
      *  Configure the port to accept only untagged or priority tagged
         frames (
      *  Configure the port to disable ingress filtering
      *  Create a permanent VLAN registration entry (
         specifying a fixed registration (8.8.2:b:1:i), frames to be
         output untagged (8.8.2:b:2) for the Trunk VID and the Party
   o  For all Root ports:

      *  Configure the Trunk VID as the PVID (the default input VID,
      *  Configure the port to accept only untagged or priority tagged
         frames (

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      *  Configure the port to disable ingress filtering
      *  Create a permanent VLAN registration entry (
         specifying a fixed registration (8.8.2:b:1:i), frames to be
         output untagged (8.8.2:b:2) for the Trunk, Branch, and all
         Party VIDs.  Alternatively, they can use the MVRP protocol
         (11.2) to configure the VIDs dynamically.

   The above configuration assumes the router attached to a Root port is
   transmitting untagged frames and is participating only in this set of
   VIDs.  If the router is participating in other VLANs as well, then it
   transmits all frames for this Private VLAN using the Trunk VID, and
   the Root port configuration consists simply of creating the permanent
   VLAN registration entries for all VIDs specifying a fixed
   registration and frames to be output tagged.

   Note that the set of Trunk, Branch, and all Party VIDs, together,
   implement a single VLAN with special connectivity properties - not
   separate VLANs.  The connectivity of that VLAN is:

   o  Frames transmitted from Root ports can be received by all ports on
      the VLAN.
   o  Frames transmitted from Leaf ports can only be received by Root
      ports on the VLAN.
   o  Frames transmitted from Community ports can only be received by
      Root ports and other Community ports (in the same community) on
      the VLAN.

3.2.  Resulting Bridge Behavior

   Once a bridge or a set of interconnected bridges have been configured
   with both the primary and isolated VLAN ID, and zero or more
   community VLAN IDs associated with the private VLAN, it results in
   the following L2 forwarding behaviors for the bridge:

   o  A packet received on an isolated port will not be L2 forwarded out
      an isolated or community port; it will (subject to bandwidth/
      resource issues) be L2 forwarded out promiscuous and inter-bridge
   o  A packet received on a community port will not be L2 forwarded out
      an isolated port or a community port with a different VLAN ID; it
      will be L2 forwarded out promiscuous and inter-bridge ports as
      well as community ports that have the same community VLAN ID.
   o  A packet received on a promiscuous port will be L2 forwarded out
      all types of ports in the private VLAN.
   o  A packet received on an inter-bridge port with an isolated VLAN ID
      will be L2 forwarded as a packet received on an isolated port.

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   o  A packet received on an inter-bridge port with a community VLAN ID
      will be L2 forwarded as a packet received on a community port
      associated with that VLAN ID.
   o  A packet received on an inter-bridge port with a promiscuous VLAN
      ID will be L2 forwarded as a packet received on a promiscuous

   In addition to the above VLAN filtering and implied MAC address
   learning rules, the L2 packet forwarding is also subject to the
   normal 802.1Q rules with blocking ports due to spanning-tree protocol

4.  IP over IPPL

   When IP is used over Intentionally Partially Partitioned links like
   private VLANs the normal usage is to attached routers (and
   potentially other shared resources like servers) to promiscuous
   ports, while attaching other hosts to either community or isolated
   ports.  If there is a single host for a given tenant or other domain
   of separation, then it is most efficient to attach that host to an
   isolated port.  If there are multiple hosts in the private VLAN that
   should be able to communicate at layer 2, then they should be
   assigned a common community VLAN ID and attached to ports with that

   The above configuration means that hosts will not be able to
   communicate with each other unless they are in the same community.
   However, mechanisms outside of the scope of this document can be used
   to allow IP communication between such hosts e.g., by having firewall
   or gateway in or beyond the routers connected to the promiscuous
   ports.  When such a policy is in place it is important that all
   packets which cross communities are sent to a router, which can have
   access-control lists or deeper firewall rules to decide which packets
   to IP forward.

5.  IPv6 over IPPL

   IPv6 Neighbor Discovery [RFC4861] can be used to get all the hosts on
   the link to send all unicast packets except those send to link-local
   destination addresses to the routers.  That is done by setting the
   L-flag (on-link) to zero for all of the Prefix Information options.
   Note that this is orthogonal to whether SLAAC (Stateless Address
   Auto-Configuration) [RFC4862] or DHCPv6 [RFC3315] is used for address
   auto-configuration.  Setting the L-flag to zero is RECOMMENDED
   configuration for private VLANs.

   If the policy includes allowing some packets that are sent to link-
   local destinations to cross between different tenants, then some for

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   of NS/NA proxy is needed in the routers, and the routers need to IP
   forward packets addressed to link-local destinations out the same
   interface as REQUIRED in [RFC2460].  If the policy allows for some
   packets sent to global IPv6 address to cross between tenants then the
   routers would IP forward such packets out the same interface.
   However, with the L=0 setting those global packets will be sent to
   the default router, while the link-local destinations would result in
   a Neighbor Solicitation to resolve the IPv6 to link-layer address
   binding.  Handling such a NS when there are multiple promiscuous
   ports hence multiple routers risks creating loops.  If the router
   already has a neighbor cache entry for the destination it can respond
   with an NA on behalf of the destination.  However, if it does not it
   MUST NOT send a NS on the link, since the NA will be received by the
   other router(s) on the link which can cause an unbounded flood of
   multicast NS packets (all with hoplimit 255), in particular of the
   host IPv6 address does not respond.  Note that such an NS/NA proxy is
   defined in [RFC4389] under some topological assumptions such as there
   being a distinct upstream and downstream direction, which is not the
   case of two or more peer routers on the same IPPL.  For that reason
   NS/NA packet proxies as in [RFC4389] MUST NOT be used with IPPL.

   IPv6 includes Duplicate Address Detection [RFC4862], which assumes
   that a link-local IPv6 multicast can be received by all hosts which
   are attached to the same link.  That is not the case in a private
   VLAN, hence there could potentially be undetected duplicate IPv6
   addresses.  However, the DAD proxy approach [RFC6957] defined for
   split-horizon behavior can safely be used even when there are
   multiple promiscuous ports hence multiple routers attached to the
   link, since it does not rely on sending Neighbor Solicitations
   instead merely gathers state from received packets.  The use of
   [RFC6957] with private VLAN is RECOMMENDED.

   The Router Advertisements in a private VLAN MUST be sent out on a
   promiscuous VLAN ID so that all nodes on the link receive them.

6.  IPv4 over IPPL

   IPv4 [RFC0791] and ARP [RFC0826] do not have a counterpart to the
   Neighbor Discovery On-link flag.  Hence nodes attached to isolated or
   community ports will always ARP for any destination which is part of
   its configured subnet prefix, and those ARP request packets will not
   be L2 forwarded by the bridges to the target nodes.  Thus the routers
   attached to the promiscuous ports MUST provide a robust proxy ARP
   mechanism if they are to allow any (firewalled) communication between
   nodes from different tenants or separation domains.

   For the ARP proxy to be robust it MUST avoid loops where router1
   attached to the link sends an ARP request which is received by

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   router2 (also attached to the link), resulting in an ARP request from
   router2 to be received by router1.  Likewise, it MUST avoids a
   similar loop involving IP packets, where the reception of an IP
   packet results in sending a ARP request from router1 which is proxied
   by router2.  At a minimum, the reception of an ARP request MUST NOT
   result in sending an ARP request, and the routers MUST either be
   configured to know each others MAC addresses, or receive the VLAN
   tagged packets so they can avoid proxying when the packet is received
   with the promiscuous VLAN ID.  Note that should there be an IP
   forwarding loop due to proxying back and forth, the IP TTL will
   expire avoiding unlimited loops.

   Any proxy ARP approach MUST work correctly with Address Conflict
   Detection [RFC5227].  ACD depends on ARP probes only receiving
   responses if there is a duplicate IP address, thus the ARP probes
   MUST NOT be proxied.  These ARP probes have a Sender Protocol Address
   of zero, hence they are easy to identify.

   When proxying an ARP request (with a non-zero Sender Protocol
   Address) the router needs to respond by placing its own MAC address
   in the Sender Hardware Address field.  When there are multiple
   routers attached to the private VLAN this will not only result in
   multiple ARP replies for each ARP request, those replies would have a
   different Sender Hardware Address.  That might seem surprising to the
   requesting node, but does not cause an issue with ARP implementations
   that follow the pseudo-code in [RFC0826].

   If the two or more routers attached to the private VLAN implement
   VRRP [RFC5798] the routers MAY use their VRRP MAC address as the
   Sender Hardware Address in the proxied ARP replies, since this
   reduces the risk nodes that do not follow the pseudo-code in
   [RFC0826].  However, if they do so it can cause flapping of the MAC
   tables in the bridges between the routers and the ARPing node.  Thus
   such use is NOT RECOMMENDED in general topologies of bridges but can
   be used when there are no intervening bridges.

7.  Multiple routers

   In addition to the above issues when multiple routers are attached to
   the same PVLAN, the routers need to avoid potential routing loops for
   packets entering the subnet.  When such a packet arrives the router
   might need to send a ARP request (or Neighbor Solicitation) for the
   host, which can trigger the other router to send a proxy ARP (or
   Neighbor Advertisement).  The host, if present, will also respond to
   the ARP/NS.  This issue is described in [PVLAN-HOSTING] in the
   particular case of HSRP.

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   When multiple routers are attached to the same PVLAN, whether they
   are using VRRP, HSRP, or neither, they SHOULD NOT proxy ARP/ND
   respond to a request from another router.  At a minimum a router MUST
   be configurable with a list of IP addresses to which it should not
   proxy respond.  Thus the user can configure that list with the IP
   address(es) of the other router(s) attached to the PVLAN.

8.  Multicast over IPPL

   Layer 2 multicast or broadcast is used by protocols like ARP
   [RFC0826], IPv6 Neighbor Discovery [RFC4861] and Multicast DNS
   [RFC6762] with link-local scope.  The first two have been discussed

   Multicast DNS can be handled by implementing using some proxy such as
   [I-D.ietf-dnssd-hybrid] but that is outside of the scope of this

   IP Multicast which spans across multiple IP links and that have
   senders that are on community or isolated ports require additional IP
   forwarding mechanisms in the routers that are attached to the
   promiscuous ports, since the routers need to IP forward such packets
   out to any allowed receivers in the private VLAN without resulting in
   packet duplication.  For multicast senders on isolated ports such IP
   forwarding would result in the sender receiving the packet it
   transmitted.  For multicast senders on community ports, any receivers
   in the same community VLAN are subject to receiving duplicate
   packets; one copy directly from layer 2 from the sender and a second
   copy IP forwarded by the multicast router.

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                      |     Eth2      |
                      |    Router     | #4 route to other subnets
                      |     Eth1      | Members on Eth1 interface
             ^                |
             | #3 to VID 10   | | #5 to promisc VID
                              | v
                      |               | #6 bridge in promisc VID
                      |    Bridge     |
                      |               | #2 bridge in VID 10
                         |  |  |  |
        +----------------+  |  |  +----------------+
        |             +-----+  +-----+             |
        |             |              |             | | #3 to VID 10
        | ^ #1 to     |              |             | v
        | | VID 10    |              |             |
        |             | | # 7 to     | | # 7 to    | | #7 to promisc VID
        |             | v promisc    | v promisc   | v
        |             |   VID        |   VID       |
        |             |              |             |
        |             |              |             |
        |             |              |             |
        |             |              |             |
   +----+-----+ +-----+----+    +----+-----+ +-----+----+
   | Community| | Community|    | Isolated | | Community|
   |   VID 10 | |   VID 20 |    |   VID 99 | |  VID 10  |
   |   Host 1 | |   Host 2 |    |   Host 3 | |  Host 4  |
   +----------+ +-----+----+    +----------+ +----------+

             Figure 1: Example upstream multicast duplication

   The example in the figure shows where the router has been configured
   to route multicast packets out the ingress PVLAN interface so that
   receivers on isolated ports and in other communities will receive
   packets sent by Host 1.  But that has the side effect of Host 4,
   which is in the same community as Host 1 will receive both a bridged
   and a routed packet.  Alternatively, if the router is configured to
   not route multicast out the ingress PVLAN interface, then Host 2 and
   Host 3 would not receive the packet.

   For that reason it is NOT RECOMMENDED to configure outbound multicast
   IP forwarding from private VLANs.

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9.  DHCP Implications

   With IPv4 both a static configuration and a DHCPv4 configuration will
   assign a subnet prefix to any hosts including those attached to the
   isolated or community ports.  Hence the above robust proxy ARP is
   needed even in the case of DHCPv4.

   With IPv6 static configuration, or SLAAC (Stateless Address Auto-
   Configuration) [RFC4862] or DHCPv6 [RFC3315] can be used to configure
   the IPv6 addresses on the interfaces.  However, when DHCPv6 is used
   to configure the IPv6 addresses it does not configure any notion of
   an on-link prefix length.  Thus in that case the on-link
   determination comes from the Router Advertisement.  Hence the above
   approach of setting L=0 in the Prefix Information Option will result
   in packets being sent to the default router(s).

   Hence no special considerations are needed for DHCPv4 or DHCPv6.

10.  Redirect Implications

   ICMP redirects can be used for both IPv4 and IPv6 to indicate a
   better first-hop router to hosts, and in addition for IPv6 can be
   used to indicate the direct link-layer address to use to send to a
   node which is on the link.  ICMP redirects to another router which
   attached to a promiscuous port would work since the host can reach
   it.  However, communication will fail if that port is not
   promiscuous.  In addition, the IPv6 redirect to an on-link host is
   likely to be problematic since a host is likely to be attached to an
   isolated or community port.

   For those reasons it is RECOMMENDED that the sending of IPv4 and IPv6
   redirects is disabled on the routers attached to the IPPL.

11.  Security Considerations

   In general DAD is subject to a Denial of Service attack since a
   malicious host can claim all the IPv6 addresses [RFC3756].  Same
   issue applies to IPv4/ARP when Address Conflict Detection [RFC5227]
   is implemented.

12.  IANA Considerations

   There are no IANA actions needed for this document.

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13.  Acknowledgements

   The author is grateful for the comments from Mikael Abrahamsson, Fred
   Baker, Wes Beebee, Hemant Singh, Dave Thaler, Pascal Thubert, and
   Sowmini Varadhan, and also the IEEE 802.1 Working Group in general
   and Norm Finn and Steve Haddock in particular for their careful
   review and providing the text in [IEEE802.1-LIAISON].

14.  Appendix: Layer 2 Learning Implications

   While not in scope for this document, there are some observations
   relating to the interaction of IPPL (and private VLANs in particular)
   and layer 2 learning which are worth mentioning.  Depending on the
   details of how the deployed Ethernet bridges perform learning, a side
   effect of using a different .1Q tag for packets sent from the routers
   than for packets sent towards the routers mean that the 802.1Q
   learning and aging process in intermediate bridges might age out the
   MAC address entry for the routers MAC address.  If that happens
   packets sent towards the router will be flooded at layer two.  The
   observed behavior is that an ARP request for the router's IP address
   will result in re-learning the MAC address.  Thus some operators work
   around this issue by configuring the ARP aging time to be shorter
   than the MAC aging time.

15.  References

15.1.  Normative References

              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Virtual Bridged Local Area Networks", IEEE Std
              802.1Q-1998, 1998, <

              (Access Controlled link within page)

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC6957]  Costa, F., Combes, J-M., Ed., Pougnard, X., and H. Li,
              "Duplicate Address Detection Proxy", RFC 6957,
              DOI 10.17487/RFC6957, June 2013,

15.2.  Informative References

              "DOCSIS 3.0: MAC and Upper Layer Protocols Interface
              Specification", August 2015, <

              Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", draft-ietf-dnssd-hybrid-06 (work in
              progress), March 2017.

              "Asymmetric (private) VLANs - comments on draft-nordmark-
              intarea-ippl", March 2017, <

              "PVLANs in a Hosting Environment", March 2010,

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   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <>.

   [RFC3756]  Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
              Neighbor Discovery (ND) Trust Models and Threats",
              RFC 3756, DOI 10.17487/RFC3756, May 2004,

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
              2006, <>.

   [RFC4562]  Melsen, T. and S. Blake, "MAC-Forced Forwarding: A Method
              for Subscriber Separation on an Ethernet Access Network",
              RFC 4562, DOI 10.17487/RFC4562, June 2006,

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              DOI 10.17487/RFC4903, June 2007,

   [RFC5227]  Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
              DOI 10.17487/RFC5227, July 2008,

   [RFC5517]  HomChaudhuri, S. and M. Foschiano, "Cisco Systems' Private
              VLANs: Scalable Security in a Multi-Client Environment",
              RFC 5517, DOI 10.17487/RFC5517, February 2010,

   [RFC5798]  Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
              Version 3 for IPv4 and IPv6", RFC 5798,
              DOI 10.17487/RFC5798, March 2010,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [TR-101]   "Migration to Ethernet-Based DSL Aggregation", The
              Broadband Forum Technical Report TR-101, July 2011,

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Author's Address

   Erik Nordmark
   Santa Clara, CA


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