Network working group X. Xu
Internet Draft S. Hares
Category: Informational Huawei Technologies
Y. Fan
China Telecom
C. Jacquenet
France Telecom
Expires: April 2013 October 15, 2012
Virtual Subnet: A L3VPN-based Subnet Extension Solution
draft-xu-virtual-subnet-09
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Abstract
This document describes a Layer3 Virtual Private Network (L3VPN)-
based subnet extension solution referred to as Virtual Subnet, which
mainly reuses existing Border Gateway Protocol (BGP)/Multi-Protocol
Label Switch (MPLS) IP Virtual Private Network (VPN)[RFC4364] and
Address Resolution Protocol(ARP)/Neighbor Discovery (ND) proxy
[RFC925][RFC1027][RFC4389]technologies. Virtual Subnet provides a
scalable approach for interconnecting cloud data centers.
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 .............................................. 9
3.3. CE Host Discovery ...................................... 9
3.4. ARP/ND Proxy ........................................... 9
3.5. CE Host Mobility ...................................... 10
3.6. Forwarding Table Scalability .......................... 11
3.6.1. MAC Table Reduction on Data Center Switches ...... 11
3.6.2. PE Router FIB Reduction .......................... 11
3.6.3. PE Router RIB Reduction .......................... 13
3.7. ARP/ND Cache Table Scalability on Default Gateways .... 14
3.8. ARP/ND and Unknown Uncast Flood Avoidance ............. 14
3.9. Active-active Multi-homing ............................ 15
3.10. Path Optimization .................................... 15
4. Security Considerations .................................... 15
5. IANA Considerations ........................................ 16
6. Acknowledgements ........................................... 16
7. References ................................................. 16
7.1. Normative References .................................. 16
7.2. Informative References ................................ 16
Authors' Addresses ............................................ 17
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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/Neighbor 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) Active-active Multi-homing
In order to utilize the bandwidth of all available paths between
the data center and the transport network in addition to
providing resilient connectivity between them, active-active
multi-homing is increasingly advocated by data center operators
as a replacement of the traditional active-standby multi-homing
approach.
6) 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
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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.
Since VS mainly reuses existing technologies including BGP/MPLS IP
VPN [RFC4364] and ARP/ND proxy [RFC925][RFC1027][RFC4389], it allows
service providers who are offering IaaS cloud services to the public
to interconnect their geographically dispersed data centers in a
much more scalable way, and more importantly, data center
interconnection design can rely upon their existing MPLS/BGP IP VPN
infrastructures therefore taking benefit from years of experience 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. In such scenario, data
center operators who are implementing data center interconnect could
benefit from the advantages that such host route-based subnet
extension solution uniquely provides, such as MAC table reduction on
data center switches, ARP/ND cache table reduction on data center
default gateways, path optimization for inter-subnet traffic, and so
on.
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
As shown in Figure 1, two CE hosts (i.e., Hosts A and B) which are
configured within the same subnet (i.e., 1.1.1.0/24) are located in
two 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
the above two data centers create host routes for their local CE
hosts respectively and then redistribute these routes into BGP.
Meanwhile, ARP proxy is enabled on the VRF attachment circuits of
these PE routers.
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+--------------------+
+-----------------+ | | +-----------------+
|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
Now assume host A sends an ARP request for host B before
communicating with host B. Upon receiving the ARP request, the ARP
proxy embedded in PE-1 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 or Local Area Network (LAN). In fact, such subnet is
a virtual subnet which is emulated by using host routes, rather than
a real subnet.
3.1.2. Inter-subnet Unicast
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 its next-hop being pointed to GW, and 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 (i.e., outside of 1.1.1.0/24). Upon
receiving this ARP request, the ARP proxy embedded in PE-1 returns
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its own MAC address as a response. Host A then sends a packet
towards Host B to PE-1. PE-1 forwards such packet towards PE-2
according to the default route learnt from PE-2, which in turn
forwards that packet to GW according to the default route as well.
In contrast, if host B 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, it will receive an ARP response from GW. As
such, the packet destined for the destination host will be forwarded
directly to GW. Note that since the outgoing interface of the best-
match route for the target host (i.e., 1.1.1.4) is the same as the
one over which the ARP packet arrived, PE-2 would not respond to
this ARP request.
+--------------------+
+-----------------+ | | +-----------------+
|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 |
+------------+---------+--------+ +------------+---------+--------+
| 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 3, in this case where each data center is
deployed with a default gateway, CE hosts will get ARP responses
from their local default gateways, rather than from their local PE
routers when sending ARP requests for their default gateways.
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+--------------------+
+-----------------+ | | +-----------------+
|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)
Alternatively, as shown in Figure 4, PE routers themselves could be
directly configured as the default gateways of their locally
connected CE hosts as long as these PE routers have routes for the
outside networks.
+------+
+------+ 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 |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
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| |
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)
3.2. Multicast
To support IP multicast between CE hosts of the same virtual subnet,
the 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.
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.
More details about the local CE host discovery approach will be
explored in a later version of this document or a separate draft.
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
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host for which there is a 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. Otherwise, PE
routers would not respond.
In the case where it's hard to guarantee each PE router has learnt
all of its own local CE hosts entirely, upon receipt of an ARP
request or a NS message for an unknown target host for which there
is no corresponding host route in the associated VRF yet, ingress PE
routers could propagate a BGP UPDATE message containing the IP
address of the target host or even that of the requesting host so as
to trigger remote PE routers receiving that message to send an ARP
request or a NS message for the target host on their own attachment
circuits on behalf of the requesting host. As such, the target host
which has been silently attached to a given PE router (e.g., there
is no any kind of host attachment notification received by the PE
router.) could be discovered accordingly. The details of this
special BGP update message will be disclosed in a separate draft.
In scenarios 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] SHOULD be enabled on these PE
routers for the sake of the availability of the network connectivity.
In this case, only the PE router which is acting as the VRRP Master
SHOULD perform the ARP/ND proxy function and respond with the
virtual MAC address, instead of its physical MAC address.
3.5. CE Host Mobility
After moving from one VPN site to another, a CE host (e.g., a VM)
will send a gratuitous ARP/ND message. Upon receiving that message,
the PE router connected to the site where the VM moves to will
create a host route for that CE host and then advertise it to remote
PE routers.
Upon learning such route, the PE router that previously connected
the CE host would immediately check whether that CE host is still
connected to it by some means (e.g., ARP/ND PING and/or ICMP PING).
If not, the PE router would accordingly withdraw the corresponding
host route which has been advertised before. Meanwhile, the PE
router would broadcast a gratuitous ARP/ND message on behalf of that
CE host. As such, the ARP/ND entry of that CE host which was 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 of the segments 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 as required in the case where the
traditional VPLS technology [RFC4761, RFC4762] is used for data
center interconnect.
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:
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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.,
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.
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3.6.3. PE Router RIB Reduction
+------+
+------+ 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.
Alternatively, if dedicated default gateways are directly connected
to PE routers of the VS as shown in Figure 3, all remote CE hosts of
a given virtual subnet share the same MAC address (i.e., the MAC
address of the local PE router) from the point of view of default
gateways, because of the use of the ARP/ND proxy function embedded
in PE routers. Therefore, ARP/ND entries of those remote CE hosts
could be aggregated into one ARP/ND entry (i.e., 1.1.1.0/24-> the
MAC address of the PE router in the IPv4 case). Accordingly, default
gateways are required to use the longest-matching algorithm for
ARP/ND cache lookup instead of the existing exact-matching algorithm.
Thus, the ARP/ND cache table size of DC gateways can be reduced
greatly as well.
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 of the segments is confined
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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. Active-active Multi-homing
For PE router redundancy purposes, a VPN site could be connected to
more than one PE router. In this case, VRRP SHOULD be enabled on
these PE routers and only the PE router which is acting as the VRRP
Master SHOULD perform the ARP proxy functionality. However, all PE
routers, either as a VRRP master or a VRRP slave, are allowed to
advertise host routes for their local CE hosts. Hence, from the
perspective of remote PE routers, there will be multiple host routes
for a given CE host located within that multi-homed site. In other
words, active-active multi-homing is available for the inbound
traffic of a given multi-homed site.
3.10. 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 the 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).
As such, the 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 the cloud data center and
forwarded to cloud user sites, each PE router acting as a default
gateway would forward traffic received from its local CE hosts
directly to the remote PE routers (i.e., PE-3) according to the
best-match route in the corresponding VRF. As a result, traffic from
data centers to enterprise sites is forwarded along the optimal path
without consuming the bandwidth resources intended for data center
interconnect.
4. Security Considerations
TBD.
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5. IANA Considerations
There is no requirement for IANA.
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.
[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.
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[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)
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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