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Network Working Group                                       D. McPhersonRequest for Comments: 3069                          Amber Networks, Inc.Category: Informational                                         B. Dykes                                                         Onesecure, Inc.                                                           February 2001VLAN Aggregation for Efficient IP Address AllocationStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This document introduces the concept of Virtual Local Area Network   (VLAN) aggregation as it relates to IPv4 address allocation.  A   mechanism is described by which hosts that reside in the same   physical switched infrastructure, but separate virtual broadcast   domains, are addressed from the same IPv4 subnet and share a common   default gateway IP address, thereby removing the requirement of a   dedicated IP subnet for each virtual Local Area Network (LAN) or   Metropolitan Area Network (MAN).   Employing such a mechanism significantly decreases IPv4 address   consumption in virtual LANs and MANs.  It may also ease   administration of IPv4 addresses within the network.1. Introduction   The VLAN [802.1Q] aggregation technique described in this document   provides a mechanism by which hosts that reside within the same   physical switched infrastructure, but separate virtual broadcast   domains, may be addressed from the same IPv4 subnet and may share a   common default gateway IPv4 address.   Such a mechanism provides several advantages over traditional IPv4   addressing architectures employed in large switched LANs today.  The   primary advantage, that of IPv4 address space conservation, can be   realized when considering the diagram in Figure 1:McPherson & Dykes            Informational                      [Page 1]

RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001   Figure 1:    +------+    +------+    +------+    +------+    +------+    |      |    |      |    |      |    |      |    |      |    | A.1  |    | A.2  |    | B.1  |    | C.1  |    | B.2  |    |      |    |      |    |      |    |      |    |      |    +------+    +------+    +------+    +------+    +------+        \          |           |           |            /          \        |           |           |          /            \ +-----------------------------------+ /              |                                   |              |          Ethernet Switch(es)      |              |                                   |              +-----------------------------------+                               |                               |                          +--------+                          |        |                          | Router |                          |        |                          +--------+   In the Figure 1 hosts A.1 and A.2 belong to customer A, VLAN A.   Hosts B.1 and B.2 belong to customer B, VLAN B.  Host C.1 belongs to   customer C and resides in it's own virtual LAN, VLAN C.   Traditionally, an IP subnet would be allocated for each customer,   based on initial IP requirements for address space utilization, as   well as on projections of future utilization.  For example, a scheme   such as that illustrated in Table 1 may be used.   Table 1:                                Gateway     Usable   Customer     Customer   IP Subnet       Address     Hosts    Hosts     ========   ============    =======     ======   ========     A          1.1.1.0/28      1.1.1.1     14       13     B          1.1.1.16/29     1.1.1.17    6        5     C          1.1.1.24/30     1.1.1.25    2        1   Customer A's initial deployment consists of 2 hosts, though they   project growth of up to 10 hosts.  As a result, they're allocated the   IP subnet 1.1.1.0/28 which provides 16 IP addresses.  The first IP   address, 1.1.1.0, represents the subnetwork number.  The last IP   address, 1.1.1.15, represents the directed broadcast address.  The   first usable address of the subnet, 1.1.1.1, is assigned to the   router and serves as the default gateway IP address for the subnet.   The customer is left 13 IP addresses, even though their requirement   was only for 10 IP addresses.McPherson & Dykes            Informational                      [Page 2]

RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001   Customer B's initial deployment consists of 2 hosts, though they   project growth of up to 5 hosts.  As a result, they're allocated the   IP subnet 1.1.1.16/29 which provides 8 IP addresses.  The first IP   address, 1.1.1.16, represents the subnetwork number.  The last IP   address, 1.1.1.23, represents the directed broadcast address.  The   first usable address of the subnet, 1.1.1.17, is assigned to the   router and serves as the default gateway IP address for the subnet.   The customer is left 5 with IP addresses.   Customer C's initial deployment consists of 1 host, and they have no   plans of deploying additional hosts.  As a result, they're allocated   the IP subnet 1.1.1.24/30 which provides 4 IP addresses.  The first   IP address, 1.1.1.24, represents the subnetwork number.  The last IP   address, 1.1.1.27, represents the directed broadcast address.  The   first usable address of the subnet, 1.1.1.25, is assigned to the   router and serves as the default gateway IP address for the subnet.   The customer is left 1 IP address.   The sum of address requirements for all three customers is 16.  The   most optimal address allocation scheme here requires 28 IP addresses.   Now, if customer A only grows to use 3 of his available address, the   additional IP addresses can't be used for other customers.   Also, assume customer C determines the need to deploy one additional   host, and as such, requires one additional IP address.  Because all   of the addresses within the existing IP subnet 1.1.1.24/30 are used,   and the following address space has been allocated to other   customers, a new subnet is required.  Ideally, the customer would be   allocated a /29 and renumber host C.1 into the new subnet.  However,   the customer is of the opinion that renumbering is not a viable   option.  As such, another IP subnet is allocated to the customer,   this time perhaps a /29, providing two additional addresses for   future use.   As you can see, the number of IP addresses consumed by the subnetwork   number, directed broadcast address, and a unique gateway address for   each subnet is quite significant.  Also, the inherent constraints of   the addressing architecture significantly reduce flexibility.2. Discussion   If within the switched environment, on the routed side of the   network, we introduce the notion of sub-VLANs and super-VLANs, a much   more optimal approach to IP addressing can be realized.McPherson & Dykes            Informational                      [Page 3]

RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001   Essentially, what occurs is that each sub-VLAN (customer) remains   within a separate broadcast domain.  One or more sub-VLANs belong to   a super-VLAN, and utilize the default gateway IP address of the   super-VLAN.  Hosts within the sub-VLANs are numbered out of IP   subnets associated with the super-VLAN, and their IP subnet masking   information reflects that of the super-VLAN subnet.   If desired, the super-VLAN router performs functions similar to Proxy   ARP to enable communication between hosts that are members of   different sub-VLANs.   This model results in a much more efficient address allocation   architecture.  It also provides network operators with a mechanism to   provide standard default gateway address assignments.   Let's again consider Figure 1, now utilizing the super-VLAN sub-VLAN   model.  Table 2 provides the new addressing model.   Table 2:                                Gateway     Usable   Customer     Customer   IP Subnet       Address     Hosts    Hosts     ========   ============    =======     ======   ========     A          1.1.1.0/24      1.1.1.1     10       .2-.11     B          1.1.1.0/24      1.1.1.1     5        .12-.16     C          1.1.1.0/24      1.1.1.1     1        .17   Customer A's initial deployment consists of 2 hosts, though they   project growth of up to 10 hosts.  As a result, they're allocated the   IP address range 1.1.1.2 - 1.1.1.11.  The gateway address for the   customer is 1.1.1.1, the subnet is 1.1.1.0/24.   Customer B's initial deployment consists of 2 hosts, though they   project growth of up to 5 hosts.  As a result, they're allocated the   IP address range 1.1.1.12 - 1.1.1.16.  The gateway address for the   customer is 1.1.1.1, the subnet is 1.1.1.0/24.   Customer C's initial deployment consists of 1 host, and they have no   plans of deploying additional hosts.  As a result, they're allocated   the IP address 1.1.1.17.  The gateway address for the customer is   1.1.1.1, the subnet is 1.1.1.0/24.   The sum of address requirements for all three customers is 16.  As a   result, only 16 addresses are allocated within the subnet.  These 16   addresses, combined with the global default gateway address of   1.1.1.1, as well as the subnetwork number of 1.1.1.0 and directed   broadcast of 1.1.1.255, result in a total of 19 addresses used.  This   leaves 236 additional usable hosts address with the IP subnet.McPherson & Dykes            Informational                      [Page 4]

RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001   Now, if customer A only grows to use 3 of his available addresses,   the additional IP addresses can be used for other customers.   Also, assume customer C determines the need to deploy one additional   host, and as such, requires one additional IP address.  The customer   is simply allocated the next available IP address within the subnet,   their default gateway remains the same.   The benefits of such a model are obvious, especially when employed in   large LANs or MANs.3. Use of Directed Broadcasts   This specification provides no support for directed broadcasts.   Specifically, the <net, subnet, -1> directed broadcast address can   only apply to one of the Layer 2 broadcast domains.   Though use of directed broadcast is frowned upon in the Internet   today, there remain a number of applications, primarily in the   enterprise arena, that continue to use them.  As such, care should be   taken to understand the implications of using these applications in   conjunction with the addressing model outlined in this specification.4. Multicast Considerations   It is assumed that the Layer 2 multicast domain will be the same as   the Layer 2 broadcast domain (i.e., VLAN).  As such, this means that   for an IP multicast packet to reach all potential receivers in the IP   subnet the multicast router(s) attached to the IP subnet need to   employ something akin to IP host routes for the sender in order for   the Reverse Path Forwarding check to work.5. Deployment Considerations   Extreme Networks has a working implementation of this model that has   been deployed in service provider data center environments for over a   year now.  Other vendors are rumored to be developing similar   functionality.6. Security Considerations   One obvious issue that does arise with this model is the   vulnerabilities created by permitting arbitrary allocation of   addresses across disparate broadcast domains.  It is advised that   address space ranges be made sticky.  That is, when an address or   range of addresses is allocated to a given sub-VLAN, reception of IPMcPherson & Dykes            Informational                      [Page 5]

RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001   or ARP packets on a sub-VLAN with a source IP address that isn't   allocated to the sub-VLAN should be discarded, and perhaps trigger a   logging message or other administrative event.   Implementation details are intentionally omitted as all functions in   this document should remain local to the super-VLAN router.  As such,   no interoperability issues with existing protocols should result.7. Acknowledgements   Thanks to Mike Hollyman and Erik Nordmark for their feedback.8. References   [802.1Q]  IEEE 802.1Q, "Virtual LANs".9. Authors' Addresses   Danny McPherson   Amber Networks, Inc.   48664 Milmont Drive   Fremont, CA  94538   EMail: danny@ambernetworks.com   Barry Dykes   OneSecure, Inc.   2000 S. Colorado Blvd Suite 2-1100   Denver, CO.  80222   EMail:  bdykes@onesecure.comMcPherson & Dykes            Informational                      [Page 6]

RFC 3069       VLAN Aggregation for IP Address Allocation  February 200110.  Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.McPherson & Dykes            Informational                      [Page 7]