Virtual Subnet: A L3VPN-based Subnet Extension Solution
draft-xu-virtual-subnet-10
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| Authors | Xiaohu Xu , Susan Hares , Fan Yongbing , Christian Jacquenet | ||
| Last updated | 2013-02-24 | ||
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draft-xu-virtual-subnet-10
Network working group X. Xu
Internet Draft S. Hares
Category: Informational Huawei Technologies
Y. Fan
China Telecom
C. Jacquenet
France Telecom
Expires: August 2013 February 25, 2013
Virtual Subnet: A L3VPN-based Subnet Extension Solution
draft-xu-virtual-subnet-10
Abstract
This document describes a Layer3 Virtual Private Network (L3VPN)-
based subnet extension solution referred to as Virtual Subnet, which
can be used as a kind of Layer3 network virtualization overlay
approach for data center interconnect.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 25, 2013.
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Copyright Notice
Copyright (c) 2013 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
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Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
Table of Contents
1. Introduction ................................................ 3
2. Terminology ................................................. 5
3. Solution Description......................................... 5
3.1. Unicast ................................................ 5
3.1.1. Intra-subnet Unicast .............................. 5
3.1.2. Inter-subnet Unicast .............................. 6
3.2. Multicast .............................................. 8
3.3. CE Host Discovery ...................................... 9
3.4. ARP/ND Proxy ........................................... 9
3.5. CE Host Mobility ....................................... 9
3.6. Forwarding Table Scalability .......................... 10
3.6.1. MAC Table Reduction on Data Center Switches ...... 10
3.6.2. PE Router FIB Reduction .......................... 10
3.6.3. PE Router RIB Reduction .......................... 11
3.7. ARP/ND Cache Table Scalability on Default Gateways .... 13
3.8. ARP/ND and Unknown Uncast Flood Avoidance ............. 13
3.9. Path Optimization ..................................... 13
4. Security Considerations .................................... 14
5. IANA Considerations ........................................ 14
6. Acknowledgements ........................................... 14
7. References ................................................. 14
7.1. Normative References .................................. 14
7.2. Informative References ................................ 14
Authors' Addresses ............................................ 15
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1. Introduction
For business continuity purposes, Virtual Machine (VM) migration
across data centers is commonly used in those situations such as
data center maintenance, data center migration, data center
consolidation, data center expansion, and data center disaster
avoidance. It's generally admitted that IP renumbering of servers
(i.e., VMs) after the migration is usually complex and costly at the
risk of extending the business downtime during the process of
migration. To allow the migration of a VM from one data center to
another without IP renumbering, the subnet on which the VM resides
needs to be extended across these data centers.
In Infrastructure-as-a-Service (IaaS) cloud data center environments,
to achieve subnet extension across multiple data centers in a
scalable way, the following requirements SHOULD be considered for
any data center interconnect solution:
1) VPN Instance Space Scalability
In a modern cloud data center environment, thousands or even tens
of thousands of tenants could be hosted over a shared network
infrastructure. For security and performance isolation purposes,
these tenants need to be isolated from one another. Hence, the
data center interconnect solution SHOULD be capable of providing
a large enough Virtual Private Network (VPN) instance space for
tenant isolation.
2) Forwarding Table Scalability
With the development of server virtualization technologies, a
single cloud data center containing millions of VMs is not
uncommon. This number already implies a big challenge for data
center switches, especially for core/aggregation switches, from
the perspective of forwarding table scalability. Provided that
multiple data centers of such scale were interconnected at layer2,
this challenge would be even worse. Hence an ideal data center
interconnect solution SHOULD prevent the forwarding table size of
data center switches from growing by folds as the number of data
centers to be interconnected increases. Furthermore, if any kind
of L2VPN or L3VPN technologies is used for interconnecting data
centers, the scale of forwarding tables on PE routers SHOULD be
taken into consideration as well.
3) ARP/ND Cache Table Scalability on Default Gateways
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[NARTEN-ARMD] notes that the ARP/ND cache tables maintained by
data center default gateways in cloud data centers can raise both
scalability and security issues. Therefore, an ideal data center
interconnect solution SHOULD prevent the ARP/ND cache table size
from growing by multiples as the number of data centers to be
connected increases.
4) ARP/ND and Unknown Unicast Flood Suppression or Avoidance
It's well-known that the flooding of Address Resolution Protocol
(ARP)/Neighbor Discovery (ND) broadcast/multicast and unknown
unicast traffic within a large Layer2 network are likely to
affect performances of networks and hosts. As multiple data
centers each containing millions of VMs are interconnected
together across the Wide Area Network (WAN) at layer2, the impact
of flooding as mentioned above will become even worse. As such,
it becomes increasingly desirable for data center operators to
suppress or even avoid the flooding of ARP/ND broadcast/multicast
and unknown unicast traffic across data centers.
5) Path Optimization
A subnet usually indicates a location in the network. However,
when a subnet has been extended across multiple geographically
dispersed data center locations, the location semantics of such
subnet is not retained any longer. As a result, the traffic from
a cloud user (i.e., a VPN user) which is destined for a given
server located at one data center location of such extended
subnet may arrive at another data center location firstly
according to the subnet route, and then be forwarded to the
location where the service is actually located. This suboptimal
routing would obviously result in the unnecessary consumption of
the bandwidth resources which are intended for data center
interconnection. Furthermore, in the case where the traditional
VPLS technology [RFC4761, RFC4762] is used for data center
interconnect and default gateways of different data center
locations are configured within the same virtual router
redundancy group, the returning traffic from that server to the
cloud user may be forwarded at layer2 to a default gateway
located at one of the remote data center premises, rather than
the one placed at the local data center location. This suboptimal
routing would also unnecessarily consume the bandwidth resources
which are intended for data center interconnect.
This document describes a L3VPN-based subnet extension solution
referred to as Virtual Subnet (VS), which can meet all of the
requirements of cloud data center interconnect as described above.
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Since VS mainly reuses existing technologies including BGP/MPLS IP
VPN [RFC4364] and ARP/ND proxy [RFC925][RFC1027][RFC4389], it allows
those service providers offering IaaS public cloud services to
interconnect their geographically dispersed data centers in a much
scalable way, and more importantly, data center interconnection
design can rely upon their existing MPLS/BGP IP VPN infrastructures
and their experiences in the delivery and the operation of MPLS/BGP
IP VPN services.
Please note that VS is targeted at scenarios where the traffic
across data centers is routable IP traffic.
2. Terminology
This memo makes use of the terms defined in [RFC4364], [RFC2338]
[MVPN] and [VA-AUTO].
3. Solution Description
3.1. Unicast
3.1.1. Intra-subnet Unicast
+--------------------+
+-----------------+ | | +-----------------+
|VPN_A:1.1.1.1/24 | | | |VPN_A:1.1.1.1/24 |
| \ | | | | / |
| +------+ \++---+-+ +-+---++/ +------+ |
| |Host A+----+ PE-1 | | PE-2 +----+Host B| |
| +------+\ ++-+-+-+ +-+-+-++ /+------+ |
| 1.1.1.2/24 | | | | | | 1.1.1.3/24 |
| | | | | | | |
| DC West | | | IP/MPLS Backbone | | | DC East |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
| |
VRF_A : V VRF_A : V
+------------+---------+--------+ +------------+---------+--------+
| Prefix | Nexthop |Protocol| | Prefix | Nexthop |Protocol|
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.1/32 |127.0.0.1| Direct | | 1.1.1.1/32 |127.0.0.1| Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.2/32 | 1.1.1.2 | Direct | | 1.1.1.2/32 | PE-1 | IBGP |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.3/32 | PE-2 | IBGP | | 1.1.1.3/32 | 1.1.1.3 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.0/24 | 1.1.1.1 | Direct | | 1.1.1.0/24 | 1.1.1.1 | Direct |
+------------+---------+--------+ +------------+---------+--------+
Figure 1: Intra-subnet Unicast Example
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As shown in Figure 1, two CE hosts (i.e., Hosts A and B) belonging
to the same subnet (i.e., 1.1.1.0/24) are located at different data
centers (i.e., DC West and DC East) respectively. PE routers (i.e.,
PE-1 and PE-2) which are used for interconnecting these two data
centers create host routes for their local CE hosts respectively and
then advertise them via L3VPN signaling. Meanwhile, ARP proxy is
enabled on VRF attachment circuits of these PE routers.
Now assume host A sends an ARP request for host B before
communicating with host B. Upon receiving the ARP request, PE-1
acting as an ARP proxy returns its own MAC address as a response.
Host A then sends IP packets for host B to PE-1. Strictly according
to the normal L3VPN forwarding procedure, PE-1 tunnels such packets
towards PE-2 which in turn forwards them to host B. Thus, hosts A
and B can communicate with each other as if they were located within
the same subnet. In fact, such subnet is a virtual subnet which is
emulated by using host routes.
3.1.2. Inter-subnet Unicast
+--------------------+
+-----------------+ | | +-----------------+
|VPN_A:1.1.1.1/24 | | | |VPN_A:1.1.1.1/24 |
| \ | | | | / |
| +------+ \++---+-+ +-+---++/ +------+ |
| |Host A+------+ PE-1 | | PE-2 +-+----+Host B| |
| +------+\ ++-+-+-+ +-+-+-++ | /+------+ |
| 1.1.1.2/24 | | | | | | | 1.1.1.3/24 |
| GW=1.1.1.4 | | | | | | | GW=1.1.1.4 |
| | | | | | | | +------+ |
| | | | | | | +----+ GW +--|
| | | | | | | /+------+ |
| | | | | | | 1.1.1.4/24 |
| | | | | | | |
| DC West | | | IP/MPLS Backbone | | | DC East |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
| |
VRF_A : V VRF_A : V
+------------+---------+--------+ +------------+---------+--------+
| Prefix | Nexthop |Protocol| | Prefix | Nexthop |Protocol|
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.1/32 |127.0.0.1| Direct | | 1.1.1.1/32 |127.0.0.1| Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.2/32 | 1.1.1.2 | Direct | | 1.1.1.2/32 | PE-1 | IBGP |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.3/32 | PE-2 | IBGP | | 1.1.1.3/32 | 1.1.1.3 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.4/32 | PE-2 | IBGP | | 1.1.1.4/32 | 1.1.1.4 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.0/24 | 1.1.1.1 | Direct | | 1.1.1.0/24 | 1.1.1.1 | Direct |
+------------+---------+--------+ +------------+---------+--------+
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| 0.0.0.0/0 | PE-2 | IBGP | | 0.0.0.0/0 | 1.1.1.4 | Static |
+------------+---------+--------+ +------------+---------+--------+
Figure 2: Inter-subnet Unicast Example (1)
As shown in Figure 2, only one data center (i.e., DC East) is
deployed with a default gateway (i.e., GW). PE-2 which is connected
to GW would either be configured with or learn from GW a default
route with next-hop being pointed to GW. Meanwhile, this route is
distributed to other PE routers (i.e., PE-1) as per normal [RFC4364]
operation. Assume host A sends an ARP request for its default
gateway (i.e., 1.1.1.4) prior to communicating with a destination
host outside of its subnet. Upon receiving this ARP request, PE-1
acting as an ARP proxy returns its own MAC address as a response.
Host A then sends a packet for Host B to PE-1. PE-1 tunnels such
packet towards PE-2 according to the default route learnt from PE-2,
which in turn forwards that packet to GW.
+--------------------+
+-----------------+ | | +-----------------+
|VPN_A:1.1.1.1/24 | | | |VPN_A:1.1.1.1/24 |
| \ | | | | / |
| +------+ \++---+-+ +-+---++/ +------+ |
| |Host A+----+-+ PE-1 | | PE-2 +-+----+Host B| |
| +------+\ | ++-+-+-+ +-+-+-++ | /+------+ |
| 1.1.1.2/24 | | | | | | | | 1.1.1.3/24 |
| GW=1.1.1.4 | | | | | | | | GW=1.1.1.4 |
| +------+ | | | | | | | | +------+ |
|--+ GW-1 +----+ | | | | | | +----+ GW-2 +--|
| +------+\ | | | | | | /+------+ |
| 1.1.1.4/24 | | | | | | 1.1.1.4/24 |
| | | | | | | |
| DC West | | | IP/MPLS Backbone | | | DC East |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
| |
VRF_A : V VRF_A : V
+------------+---------+--------+ +------------+---------+--------+
| Prefix | Nexthop |Protocol| | Prefix | Nexthop |Protocol|
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.1/32 |127.0.0.1| Direct | | 1.1.1.1/32 |127.0.0.1| Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.2/32 | 1.1.1.2 | Direct | | 1.1.1.2/32 | PE-1 | IBGP |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.3/32 | PE-2 | IBGP | | 1.1.1.3/32 | 1.1.1.3 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.4/32 | 1.1.1.4 | Direct | | 1.1.1.4/32 | 1.1.1.4 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.0/24 | 1.1.1.1 | Direct | | 1.1.1.0/24 | 1.1.1.1 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 0.0.0.0/0 | 1.1.1.4 | Static | | 0.0.0.0/0 | 1.1.1.4 | Static |
+------------+---------+--------+ +------------+---------+--------+
Figure 3: Inter-subnet Unicast Example (2)
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As shown in Figure 3, in the case where each data center is deployed
with a default gateway, CE hosts will get ARP responses directly
from their local default gateways, rather than from their local PE
routers when sending ARP requests for their default gateways.
+------+
+------+ PE-3 +------+
+-----------------+ | +------+ | +-----------------+
|VPN_A:1.1.1.1/24 | | | |VPN_A:1.1.1.1/24 |
| \ | | | | / |
| +------+ \++---+-+ +-+---++/ +------+ |
| |Host A+------+ PE-1 | | PE-2 +------+Host B| |
| +------+\ ++-+-+-+ +-+-+-++ /+------+ |
| 1.1.1.2/24 | | | | | | 1.1.1.3/24 |
| GW=1.1.1.1 | | | | | | GW=1.1.1.1 |
| | | | | | | |
| DC West | | | IP/MPLS Backbone | | | DC East |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
| |
VRF_A : V VRF_A : V
+------------+---------+--------+ +------------+---------+--------+
| Prefix | Nexthop |Protocol| | Prefix | Nexthop |Protocol|
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.1/32 |127.0.0.1| Direct | | 1.1.1.1/32 |127.0.0.1| Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.2/32 | 1.1.1.2 | Direct | | 1.1.1.2/32 | PE-1 | IBGP |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.3/32 | PE-2 | IBGP | | 1.1.1.3/32 | 1.1.1.3 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.0/24 | 1.1.1.1 | Direct | | 1.1.1.0/24 | 1.1.1.1 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 0.0.0.0/0 | PE-3 | IBGP | | 0.0.0.0/0 | PE-3 | IBGP |
+------------+---------+--------+ +------------+---------+--------+
Figure 4: Inter-subnet Unicast Example (3)
Alternatively, as shown in Figure 4, PE routers themselves could be
directly configured as default gateways of their locally connected
CE hosts as long as these PE routers have routes for outside
networks.
3.2. Multicast
To support IP multicast between CE hosts of the same virtual subnet,
MVPN technology [MVPN] could be directly reused. For example, PE
routers attached to a given VPN join a default provider multicast
distribution tree which is dedicated for that VPN. Ingress PE
routers, upon receiving multicast packets from their local CE hosts,
forward them towards remote PE routers through the corresponding
default provider multicast distribution tree.
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More details about how to support multicast and broadcast in VS will
be explored in a later version of this document.
3.3. CE Host Discovery
PE routers SHOULD be able to discover their local CE hosts and keep
the list of these hosts up to date in a timely manner so as to
ensure the availability and accuracy of the corresponding host
routes originated from them. PE routers could accomplish local CE
host discovery by some traditional host discovery mechanisms using
ARP or ND protocols. Furthermore, Link Layer Discovery Protocol
(LLDP) described in [802.1AB] or VSI Discovery and Configuration
Protocol (VDP) described in [802.1Qbg], or even interaction with the
data center orchestration system could also be considered as a means
to dynamically discover local CE hosts.
3.4. ARP/ND Proxy
Acting as ARP or ND proxies, PE routers SHOULD only respond to an
ARP request or Neighbor Solicitation (NS) message for the target
host when there is a corresponding host route in the associated VRF
and the outgoing interface of that route is different from the one
over which the ARP request or the NS message arrived.
In the scenario where a given VPN site (i.e., a data center) is
multi-homed to more than one PE router via an Ethernet switch or an
Ethernet network, VRRP [RFC5798] is usually enabled on these PE
routers. In this case, only the PE router being elected as the VRRP
Master is allowed to perform the ARP/ND proxy function.
3.5. CE Host Mobility
During the VM migration process, the PE router to which the moving
VM is now attached would create a host route for that CE host upon
receiving a notification message of VM attachment while the PE
router to which the moving VM was previously attached would withdraw
the corresponding host route when receiving a notification message
of VM detachment. Meanwhile, the latter PE router could optionally
broadcast a gratuitous ARP/ND message on behalf of that CE host with
source MAC address being one of its own. In the way, the ARP/ND
entry of that moved CE host which has been cached on any local CE
host would be updated accordingly.
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3.6. Forwarding Table Scalability
3.6.1. MAC Table Reduction on Data Center Switches
In a VS environment, the MAC learning domain associated with a given
virtual subnet which has been extended across multiple data centers
is partitioned into segments and each segment is confined within a
single data center. Therefore data center switches only need to
learn local MAC addresses, rather than learning both local and
remote MAC addresses.
3.6.2. PE Router FIB Reduction
+------+
+------+RR/APR+------+
+-----------------+ | +------+ | +-----------------+
|VPN_A:1.1.1.1/24 | | | |VPN_A:1.1.1.1/24 |
| \ | | | | / |
| +------+ \++---+-+ +-+---++/ +------+ |
| |Host A+------+ PE-1 | | PE-2 +------+Host B| |
| +------+\ ++-+-+-+ +-+-+-++ /+------+ |
| 1.1.1.2/24 | | | | | | 1.1.1.3/24 |
| | | | | | | |
| DC West | | | IP/MPLS Backbone | | | DC East |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
| |
VRF_A : V VRF_A : V
+------------+---------+--------+------+ +------------+---------+--------+------+
| Prefix | Nexthop |Protocol|In_FIB| | Prefix | Nexthop |Protocol|In_FIB|
+------------+---------+--------+------+ +------------+---------+--------+------+
| 1.1.1.1/32 |127.0.0.1| Direct | Yes | | 1.1.1.1/32 |127.0.0.1| Direct | Yes |
+------------+---------+--------+------+ +------------+---------+--------+------+
| 1.1.1.2/32 | 1.1.1.2 | Direct | Yes | | 1.1.1.2/32 | PE-1 | IBGP | No |
+------------+---------+--------+------+ +------------+---------+--------+------+
| 1.1.1.3/32 | PE-2 | IBGP | No | | 1.1.1.3/32 | 1.1.1.3 | Direct | Yes |
+------------+---------+--------+------+ +------------+---------+--------+------+
| 1.1.1.0/25 | RR | IBGP | Yes | | 1.1.1.0/25 | RR | IBGP | Yes |
+------------+---------+--------+------+ +------------+---------+--------+------+
|1.1.1.128/25| RR | IBGP | Yes | |1.1.1.128/25| RR | IBGP | Yes |
+------------+---------+--------+------+ +------------+---------+--------+------+
| 1.1.1.0/24 | 1.1.1.1 | Direct | Yes | | 1.1.1.0/24 | 1.1.1.1 | Direct | Yes |
+------------+---------+--------+------+ +------------+---------+--------+------+
Figure 5: FIB Reduction Example
To reduce the FIB size of PE routers, Virtual Aggregation (VA) [VA-
AUTO] technology can be used. Take the VPN instance A shown in
Figure 5 as an example, the procedures of FIB reduction are as
follows:
1) Multiple more specific prefixes (e.g., 1.1.1.0/25 and
1.1.1.128/25) corresponding to the prefix of virtual subnet (i.e.,
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1.1.1.0/24) are configured as Virtual Prefixes (VPs) and a Route-
Reflector (RR) is configured as an Aggregation Point Router (APR)
for these VPs. PE routers as RR clients advertise host routes for
their own local CE hosts to the RR which in turn, as an APR,
installs those host routes into its FIB and then attach the "can-
suppress" tag to those host routes before reflecting them to its
clients.
2) Those host routes which have been attached with the "can
suppress" tag would not be installed into FIBs by clients who are
VA-aware since they are not APRs for those host routes. In
addition, the RR as an APR would advertise the corresponding VP
routes to all of its clients, and those of which who are VA-aware
in turn would install these VP routes into their FIBs.
3) Upon receiving a packet from a local CE host, if no matching host
route found, the ingress PE router will forward the packet to the
RR according to one of the VP routes learnt from the RR, which in
turn forwards the packet to the relevant egress PE router
according to the host route learnt from that egress PE router. In
a word, the FIB table size of PE routers can be greatly reduced at
the cost of path stretch. Note that in the case where the RR is
not available for transferring L3VPN traffic between PE routers
for some reason (e.g., the RR is implemented on a server, rather
than a router), the APR function could actually be performed by a
given PE router other than the RR as long as that PE router has
installed all host routes belonging to the virtual subnet into its
FIB. Thus, the RR only needs to attach a "can-suppress" tag to the
host routes learnt from its clients before reflecting them to the
other clients. Furthermore, PE routers themselves could directly
attach the "can-suppress" tag to those host routes for their local
CE hosts before distributing them to remote peers as well.
4) Provided a given local CE host sends an ARP request for a remote
CE host, the PE router that receives such request will install the
host route for that remote CE host into its FIB, in case there is
a host route for that CE host in its RIB and has not yet been
installed into the FIB. Therefore, the subsequent packets destined
for that remote CE host will be forwarded directly to the egress
PE router. To save the FIB space, FIB entries corresponding to
remote host routes which have been attached with "can-suppress"
tags would expire if they have not been used for forwarding
packets for a certain period of time.
3.6.3. PE Router RIB Reduction
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+------+
+------+ RR +------+
+-----------------+ | +------+ | +-----------------+
|VPN_A:1.1.1.1/24 | | | |VPN_A:1.1.1.1/24 |
| \ | | | | / |
| +------+ \++---+-+ +-+---++/ +------+ |
| |Host A+------+ PE-1 | | PE-2 +------+Host B| |
| +------+\ ++-+-+-+ +-+-+-++ /+------+ |
| 1.1.1.2/24 | | | | | | 1.1.1.3/24 |
| | | | | | | |
| DC West | | | IP/MPLS Backbone | | | DC East |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
| |
VRF_A : V VRF_A : V
+------------+---------+--------+ +------------+---------+--------+
| Prefix | Nexthop |Protocol| | Prefix | Nexthop |Protocol|
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.1/32 |127.0.0.1| Direct | | 1.1.1.1/32 |127.0.0.1| Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.2/32 | 1.1.1.2 | Direct | | 1.1.1.3/32 | 1.1.1.3 | Direct |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.0/25 | RR | IBGP | | 1.1.1.0/25 | RR | IBGP |
+------------+---------+--------+ +------------+---------+--------+
|1.1.1.128/25| RR | IBGP | |1.1.1.128/25| RR | IBGP |
+------------+---------+--------+ +------------+---------+--------+
| 1.1.1.0/24 | 1.1.1.1 | Direct | | 1.1.1.0/24 | 1.1.1.1 | Direct |
+------------+---------+--------+ +------------+---------+--------+
Figure 6: RIB Reduction Example
To reduce the RIB size of PE routers, BGP Outbound Route Filtering
(ORF) mechanism is used to realize on-demand route announcement.
Take the VPN instance A shown in Figure 6 as an example, the
procedures of RIB reduction are as follows:
1) PE routers as RR clients advertise host routes for their local CE
hosts to a RR which however doesn't reflect these host routes by
default unless it receives explicit ORF requests for them from its
clients. The RR is configured with routes for more specific
subnets (e.g., 1.1.1.0/25 and 1.1.1.128/25) corresponding to the
virtual subnet (i.e., 1.1.1.0/24) with next-hop being pointed to
Null0 and then advertises these routes to its clients via BGP.
2) Upon receiving a packet from a local CE host, if no matching host
route found, the ingress PE router will forward the packet to the
RR according to one of the subnet routes learnt from the RR, which
in turn forwards the packet to the relevant egress PE router
according to the host route learnt from that egress PE router. In
a word, the RIB table size of PE routers can be greatly reduced at
the cost of path stretch.
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3) Just as the approach mentioned in section 3.6.2, in the case
where the RR is not available for transferring L3VPN traffic
between PE routers for some reason, a PE router other than the RR
could advertise the more specific subnet routes as long as that PE
router has installed all host routes belonging to that virtual
subnet into its FIB.
4) Provided a given local CE host sends an ARP request for a remote
CE host, the ingress PE router that receives such request will
request the corresponding host route from its RR by using the ORF
mechanism (e.g., a group ORF containing Route-Target (RT) and
prefix information) in case there is no host route for that CE
host in its RIB yet. Once the host route for the remote CE host is
learnt from the RR, the subsequent packets destined for that CE
host would be forwarded directly to the egress PE router. Note
that the RIB entries of remote host routes could expire if they
have not been used for forwarding packets for a certain period of
time. Once the expiration time for a given RIB entry is
approaching, the PE router would notify its RR not to pass the
updates for corresponding host route by using the ORF mechanism.
3.7. ARP/ND Cache Table Scalability on Default Gateways
In case where data center default gateway functions are implemented
on PE routers of the VS as shown in Figure 4, since the ARP/ND cache
table on each PE router only needs to contain ARP/ND entries of
local CE hosts, the ARP/ND cache table size will not grow as the
number of data centers to be connected increases.
3.8. ARP/ND and Unknown Uncast Flood Avoidance
In VS, the flooding domain associated with a given virtual subnet
that has been extended across multiple data centers, has been
partitioned into segments and each segment is confined within a
single data center. Therefore, the performance impact on networks
and servers caused by the flooding of ARP/ND broadcast/multicast and
unknown unicast traffic is alleviated.
3.9. Path Optimization
Take the scenario shown in Figure 4 as an example, to optimize the
forwarding path for traffic between cloud users and cloud data
centers, PE routers located at cloud data centers (i.e., PE-1 and
PE-2), which are also data center default gateways, propagate host
routes for their local CE hosts respectively to remote PE routers
which are attached to cloud user sites (i.e., PE-3).
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As such, traffic from cloud user sites to a given server on the
virtual subnet which has been extended across data centers would be
forwarded directly to the data center location where that server
resides, since traffic is now forwarded according to the host route
for that server, rather than the subnet route.
Furthermore, for traffic coming from cloud data centers and
forwarded to cloud user sites, each PE router acting as a default
gateway would forward the traffic received from its local CE hosts
according to the best-match route in the corresponding VRF. As a
result, traffic from data centers to cloud user sites is forwarded
along the optimal path as well.
4. Security Considerations
This document doesn't introduce additional security risk to BGP/MPLS
L3VPN, nor does it provide any additional security feature for
BGP/MPLS L3VPN.
5. IANA Considerations
There is no requirement for any IANA action.
6. Acknowledgements
Thanks to Dino Farinacci, Himanshu Shah, Nabil Bitar, Giles Heron,
Ronald Bonica, Monique Morrow for their valuable comments and
suggestions on this document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[RFC4364] Rosen. E and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[MVPN] Rosen. E and Aggarwal. R, "Multicast in MPLS/BGP IP VPNs",
draft-ietf-l3vpn-2547bis-mcast-10.txt, Work in Progress,
Janurary 2010.
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[VA-AUTO] Francis, P., Xu, X., Ballani, H., Jen, D., Raszuk, R., and
L. Zhang, "Auto-Configuration in Virtual Aggregation",
draft-ietf-grow-va-auto-05.txt, Work in Progress, December
2011.
[RFC925] Postel, J., "Multi-LAN Address Resolution", RFC-925, USC
Information Sciences Institute, October 1984.
[RFC1027] Smoot Carl-Mitchell, John S. Quarterman, "Using ARP to
Implement Transparent Subnet Gateways", RFC 1027, October
1987.
[RFC4389] D. Thaler, M. Talwar, and C. Patel, "Neighbor Discovery
Proxies (ND Proxy) ", RFC 4389, April 2006.
[RFC5798] S. Nadas., "Virtual Router Redundancy Protocol", RFC 5798,
March 2010.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling", RFC
4761, January 2007.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[802.1AB] IEEE Standard 802.1AB-2009, "Station and Media Access
Control Connectivity Discovery", September 17, 2009.
[802.1Qbg] IEEE Draft Standard P802.1Qbg/D2.0, "Virtual Bridged
Local Area Networks -Amendment XX: Edge Virtual Bridging",
Work in Progress, December 1, 2011.
[NARTEN-ARMD] Narten, T., Karir, M., and I. Foo, "Problem Statement
for ARMD", draft-ietf-armd-problem-statement-01.txt, Work
in Progress, February 2012.
Authors' Addresses
Xiaohu Xu
Huawei Technologies,
Beijing, China.
Phone: +86 10 60610041
Email: xuxiaohu@huawei.com
Susan Hares
Huawei Technologies (FutureWei group)
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2330 Central Expressway
Santa Clara, CA 95050
Phone: +1-734-604-0332
Email: Susan.Hares@huawei.com
shares@ndzh.com
Yongbing Fan
Guangzhou Institute, China Telecom
Guangzhou, China.
Phone: +86 20 38639121
Email: fanyb@gsta.com
Christian Jacquenet
France Telecom
Rennes
France
Email: christian.jacquenet@orange.com
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