Using BGP for routing in large-scale data centers
draft-lapukhov-bgp-routing-large-dc-00
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draft-lapukhov-bgp-routing-large-dc-00
IDR P. Lapukhov
Internet-Draft Microsoft Corp.
Intended status: Informational A. Premji
Expires: January 8, 2013 Arista Networks
July 7, 2012
Using BGP for routing in large-scale data centers
draft-lapukhov-bgp-routing-large-dc-00
Abstract
Some service providers build and operate data centers at the size
exceeding 100,000 servers. In this document, those data-centers are
referred to as "large-scale" to differentiate them from more common
smaller infrastructures. The data centers of that scale have unique
set of network design requirement, with primary focus on operational
simplicity and stability.
This document attempts to summarize the authors' experiences in
designing and supporting large data centers, using BGP as the only
control-plane protocol. The intent is to describe a proven and
stable routing design that could be leveraged by others in the
industry.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 8, 2013.
Copyright Notice
Copyright (c) 2012 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
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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Traditional data center designs . . . . . . . . . . . . . . . 3
2.1. Layer 2 Designs . . . . . . . . . . . . . . . . . . . . . 3
2.2. Fully routed network designs . . . . . . . . . . . . . . . 4
3. Document structure . . . . . . . . . . . . . . . . . . . . . . 5
4. Network design requirements . . . . . . . . . . . . . . . . . 5
4.1. Traffic patterns . . . . . . . . . . . . . . . . . . . . . 5
4.2. CAPEX minimization . . . . . . . . . . . . . . . . . . . . 6
4.3. OPEX minimization . . . . . . . . . . . . . . . . . . . . 6
4.4. Traffic Engineering . . . . . . . . . . . . . . . . . . . 6
5. Requirement List . . . . . . . . . . . . . . . . . . . . . . . 7
6. Network topology . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Clos topology overview . . . . . . . . . . . . . . . . . . 7
6.2. Clos topology properties . . . . . . . . . . . . . . . . . 8
6.3. Scaling Clos topology . . . . . . . . . . . . . . . . . . 9
7. Routing design . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Choosing the routing protocol . . . . . . . . . . . . . . 10
7.2. BGP configuration for Clos topology . . . . . . . . . . . 10
7.2.1. BGP Autonomous System numbering layout . . . . . . . . 11
7.2.2. Non-unique private BGP ASN's . . . . . . . . . . . . . 12
7.2.3. Prefix advertisement . . . . . . . . . . . . . . . . . 13
7.2.4. External connectivity . . . . . . . . . . . . . . . . 13
7.3. ECMP Considerations . . . . . . . . . . . . . . . . . . . 14
7.3.1. Basic ECMP . . . . . . . . . . . . . . . . . . . . . . 14
7.3.2. BGP ECMP over multiple ASN . . . . . . . . . . . . . . 15
7.4. BGP convergence properties . . . . . . . . . . . . . . . . 16
7.4.1. Convergence timing . . . . . . . . . . . . . . . . . . 16
7.4.2. Failure impact scope . . . . . . . . . . . . . . . . . 16
7.4.3. Third-party route injection . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
11. Informative References . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
This document presents a practical routing design to be used in
large-scale data centers, sometimes called hyperscale or warehousr-
scale. The most distinctive characterstic of these data center is
having 100,000 or more end hosts connected to the network. While
historically only a few companies have been operating networks of
that scale, recent trend in building large cloud data centers re-
ignated interest in network designs to support deployment of this
scale. In contrary to more traditional data center designs, the
approach proposed in this document does not depend on large Layer 2
domains and instead uses routing at every level of the network. The
reason to make that choice is based on the unique set of design
requirements, with primary focus on cost reduction. Furthermore,
analyzing the requirements the conclusion is that BGP best suits to
accomplish this goal due primarily to its simplicity and broad vendor
support.
2. Traditional data center designs
This section provides an overview of two types of traditional data
center designs - Layer-2 and fully routed Layer-3 topologies.
2.1. Layer 2 Designs
In the networking industry, common design choice for data centers is
using a mix of Ethernet-based Layer 2 technologies. Network topology
typically looks like a tree with redundant uplinks and three levels
of hierarchy (see Figure 1) commonly named Core, Aggregation and
Access. To accommodate bandwidth demands, every next level has
higher port density and bandwidth capacity. In this document, the
topology layers will be referenced as "tiers", e.g. Tier 1, Tier 2
and Tier 3 instead of Core, Aggregation or Access layers.
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+------+ +------+
| | | |
| |==| | Tier1
| | | |
+------+ +------+
| | | |
+---------+ | | +----------+
| +-------+--+------+--+-------+ |
| | | | | | | |
+----+ +----+ +----+ +----+
| | | | | | | |
| |=====| | | |=====| | Tier2
| | | | | | | |
+----+ +----+ +----+ +----+
| | | |
| | | |
| +-----+ | | +-----+ |
+-| |-+ +-| |-+ Tier3
+-----+ +-----+
| | | | | |
[Servers] [Servers]
Figure 1: Typical Data Center network layout
IP routing is normally used only at the upper layers of the topology,
e.g. Tier 1 or Tier 2. The main reasons for introducing such large
(sometimes called stretched) Level 2 domains, are the following:
o Supporting legacy applications that may require direct Layer 2
adjacency or use non-IP protocols
o Seamless mobility for virtual machines, to allow preserving IP
address when a virtual machine changes physical host
o Simplified IP addressing - less IP subnets is required for the
data-center
o Application load-balancing may require direct Layer 2 adjacency to
perform some functions such as Level 2 Direct Server Return (DSR)
2.2. Fully routed network designs
Network designs that leverage IP routing down to the access layer
(Tier 3) of the network gained some popularity, mostly due to
improved network stability, scalability (by means of information
hiding) and convergence times. A common choice of routing protocol
for data center designs would be an IGP, such as OSPF or ISIS. As
data centers grow in scale, and server count exceeds tens of
thousands, those fully routed designs become more attractive.
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BGP is the de-facto standard protocol for routing on the Internet,
having wide support from network equipment vendors and being well-
understood by network engineers world-wide. However, it is not
common to see BGP being used in data centers that employ fully routed
network design. There multiple reasons for that:
o BGP is perceived as "WAN protocol only" and often not being
considered for enterprise or data center application
o BGP is believed to converge "slower" than traditional IGPs
o BGP is assumed to have a dependency on the presence of an IGP,
which assists with recursive next-hop resolution
o BGP require a lot of configuration efforts as it does not support
any form of neighbor auto-discovery
In this document we argue benefits of choosing BGP as the single
routing protocol, including acceptable convergence time.
3. Document structure
The remaining of this document is organized as following. First the
design requirements for large scale data centers are presented.
Next, the document gives an overview of Clos network topology and its
properties. After that, the arguments for selecting BGP as the
single routing protocols are presented. Finally, the document goes
over design detail and specific BGP policy features.
4. Network design requirements
This section describes and summarizes network design requirement for
a large-scale data center.
4.1. Traffic patterns
The primary requirement when building an interconnection network for
large number of servers is accommodating application bandwidth and
latency requirements. For long period of time, it was common to see
traffic flowing mainly to and from the data center. There were no
intense (highly meshed flows) traffic patterns between the machines
within the same tier. As a result, traditional "tree" topology was
sufficient to accommodate data flow, even with high oversubscription
ratios in network equipment. If more bandwidth was required, it was
added by "scaling up" the network elements, e.g. by adding more line-
cards or replacing existing devices with higher capacity switches.
In contrast, large-scale data centers often host applications that
generate large amount of server to server traffic, also known as
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"east-west" traffic. Examples of such applications could be compute
clusters such as Hadoop or live virtual machine migration in "cloud"
data-centers. Scaling up traditional tree topology to match those
bandwidth demands becomes either too expensive or impossible due to
physical limitation.
4.2. CAPEX minimization
Cost of networking component alone (CAPEX) constitutes about 10-15%
of total data center cost [GREENBERG2009]. Still, absolute numbers
are significant, and hence the need to constantly drive cost of
networking elements down. This is normally accomplished in two ways:
o Unifying network elements, preferably using the same hardware type
or even the same device. This allows for bulk purchases with
discounted pricing.
o Driving costs down by introducing diversity of networking vendors
that may supply equipment for data center network
In order to allow for vendor diversity, it is important to minimize
the feature requirements for network equipment software. In
addition, the above strategy means that network equipment vendor
choice may change often, or that the network may have to be multi-
vendor and interoperability becomes critical.
4.3. OPEX minimization
Operating large scale infrastructure could be expensive, provide that
larger amount of elements will statistically fail more often.
Therefore, it is important to operate on the simplest software and
feature set possible.
An important aspect of OPEX minimization is reducing size of failure
domains in the network. Ethernet data-plane is known to be
susceptible to massive impact due to broadcast or unicast storms.
The use of fully routed designs reduces the size of data-plane
failure domains, but at the time introduces the problem of
distributed control-plane failures. This requirement calls for
simpler control-plane protocols that are expected to have less
chances of network meltdown.
4.4. Traffic Engineering
In any data center, application load-balancing is critical function
performed by network devices. Traditionally, load-balancers are
deployed as dedicated devices in traffic forwarding path. A common
problem is scaling load-balancers under growing traffic demand.
Preferable solution would be able to scale load-balancing layer
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horizontally, by adding more of the uniform nodes and distributing
incoming traffic across them.
In situation like this, an ideal choice would be using network
infrastructure to distribute traffic across a group of load-
balancers. A combination of features such Anycast prefix
advertisement [RFC4786] along with Equal Cost Multipath (ECMP)
functionality could be used to accomplish this. To allow for more
granular load-distribution, it is beneficial for the network to
support the ability to perform controlled per-hop traffic
engineering.
5. Requirement List
This section summarizes the requirements in a list, based on the
analysis made before
o REQ1: Select a network topology where capacity could be scaled
"horizontally" by adding more links and network switches of the
same type, without requiring upgrading the network elements
themselves.
o REQ2: Define a narrow set of software features/protocols supported
by multitude of networking equipment vendors.
o REQ3: Among the network protocols, select those having simpler
implementation in terms of minimal programming code complexity.
o REQ4: The selected network routing protocol should support per-hop
change of forwarding behavior.
6. Network topology
This section outlines the most common choice for horizontally
scalable topology in large scale data centers.
6.1. Clos topology overview
A common choice for horizontally scalable topology is folded Clos
topology (sometimes called "fat-tree"). This topology features odd
number of stages (dimensions) and commonly made of the same uniform
elements, e.g. switches of the same port count. Therefore, the
choice of Clos topology satisfies both REQ1 and REQ2. See Figure 2
below for an example of folded 3-stage Clos topology:
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+-------+
| |----------------------------+
| |------------------+ |
| |--------+ | |
+-------+ | | |
+-------+ | | |
| |--------+---------+-------+ |
| |--------+-------+ | | |
| |------+ | | | | |
+-------+ | | | | | |
+-------+ | | | | | |
| |------+-+-------+-+-----+ | |
| |------+-+-----+ | | | | |
| |----+ | | | | | | | |
+-------+ | | | | | | ---------> M links
Tier1 | | | | | | | | |
+-------+ +-------+ +-------+
| | | | | |
| | | | | | Tier2
| | | | | |
+-------+ +-------+ +-------+
| | | | | | | | |
| | | | | | ---------> N Links
| | | | | | | | |
O O O O O O O O O Servers
Figure 2: 3-Stage Folded Clos topology
In the networking industry, a topology like this is sometimes
referred to as "Leaf and Spine", where Spine is the name for the
middle stage of the Clos topology (Tier 1) and Leaf is the name of
input/output stage (Tier 2). However, for consistency, the document
will be using "Tier n" notation.
6.2. Clos topology properties
The following are some key properties of the Clos topology:
o Topology is fully non-blocking (or more accurately - non-
interfering) if M >= N and oversubscribed by a factor of N/M
otherwise. Here M and N is the uplink and downlink port count
respectively, for Tier 2 switch, as shown on Figure 2
o Implementing Clos topology requires a routing protocol supporting
ECMP with the fan-out of M or more
o Every Tier 1 device has exactly one path to every end host
(server) in this topology
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o Traffic flowing from server to server is naturally load-balanced
over all available paths using simple ECMP behavior
6.3. Scaling Clos topology
Clos topology could be scaled either by increasing network switch
radix or adding more stages, e.g. moving to a 5-stage Clos, as
illustrated on Figure 3 below:
Tier1
+-----+
| |
+--| |--+
| +-----+ |
Tier2 | | Tier2
+-----+ | +-----+ | +-----+
+-------------| DEV |--+--| |--+--| |-------------+
| +-----| C |--+ | | +--| |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+-----| DEV |--+ | | +--| |-----+-----+ |
| | | +---| D |--+--| |--+--| |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| DEV | | DEV | +--| |--+ | | | |
| A | | B | Tier3 | | Tier3 | | | |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
O O O O <- Servers -> O O O O
Figure 3: 5-Stage Clos topology
The topology on Figure 3 is built from switches with radix 4 and
provides full bisection bandwidth to all connected servers. We'll be
referring to the collection of directly connected Tier 2 and Tier 3
switches as "cluster" in this document. For example, devices A, B,
C, and D on Figure 3 form a cluster.
In practice, Tier 3 level of the network (typically top of rack
switches, or ToRs) often introduces oversubscription to allow for
packaging more servers in data center. The main reason to
oversubscribe only at a single layer of the network is to simplify
application development that would need to account for two bandwidth
pools: within the same access switch (e.g. rack) and outside of the
local switch. Oversubscription, however, does not affect routing
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design and hence not considered in more details in this document.
7. Routing design
This section discusses the motivation for choosing BGP as the routing
protocol and BGP configuration for routing in Clos topology.
7.1. Choosing the routing protocol
The set of requirement provide above calls for a single routing
protocol (REQ2) in the data center to reduce complexity and
interdependencies. While it's common to rely on an IGP in this
situation, the document proposes to use BGP only. The advantages of
using BGP are argued below.
o BGP has less complexity in protocol design - internal data
structures and state-machines are simpler when compared to a link-
state IGP. For example, as opposed to implementing adjacency
formation and maintenance, flow-control, etc. BGP simply relies
on TCP as the underlying transport. This also simplified protocol
testing and fulfills REQ1 and REQ2.
o BGP information flooding overhead is less when compared to link-
state IGPs. Indeed, since every BGP router normally re-calculates
and propagates best-paths only, a network failure is masked as
soon as BGP speaker finds an alternate path. On contrary, event
propagation scope of a link-state IGP is single area/domain,
regardless of the failure type. Furthermore, all well-known link-
state IGPs feature periodic refresh updates, while BGP does not
expire routing state.
o BGP supports third-party (recursively resolved) next-hops, which
allows for injecting custom routing paths into any device in the
network, using eBGP multi-hop peering session. This satisfied
REQ4 stated above. Some IGPs, such as OSPF, support similar
functionality using special concepts such as "Forwarding Address",
but do not satisfy other requirement, such as protocol simplicity.
o BGP is easier to troubleshoot, mostly because of simplified
protocol mechanics and database structures that directly map to
forwarding tables structure. For example, it is straightforward
to dump contents of LocRIB and compare it to the router's RIB and
FIB. As another example, BGP routing updates translate directly
into NLRI information, as compared to LSA/LSP information that
describes network topology. Thus BGP fully satisfies REQ3.
7.2. BGP configuration for Clos topology
This section provides configuration guidelines for a 5-stage Clos
topology. It is easy to reduce it to a 3-Stage Clos configuration,
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and having topology that has more than 5 stages is very uncommon due
to high link density of associated designs.
7.2.1. BGP Autonomous System numbering layout
The diagram below illustrates suggests BGP Autonomous System Number
(BGP ASN) allocation scheme. The following is a list of guiding
principles:
o All BGP peering sessions are external BGP (eBGP) established over
direct point-to-point links interconnecting the network switches.
o 16-bit (two octet) BGP ASNs are used, for the reason of wider
vendor support and better vendor interoperability (e.g. no need to
support BGP capability negotiation).
o Private BGP ASNs from the range 64512-64534 are used for the
reasons of avoiding ASN conflicts and being able to use BGP
private ASN stripping feature (see below).
o A single BGP ASN is allocated to the Clos middle stage ("Tier 1"),
e.g. ASN 64534 on Figure 4
o Unique BGP ASN is allocated per every group of "Tier 2" switches.
All Tier 2 switches in the same group share the BGP ASN.
o Unique BGP ASN is allocated to every Tier 3 switch (e.g. ToR) in
this topology.
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ASN 64534
+---------+
| +-----+ |
| | | |
+-|-| |-|-+
| | +-----+ | |
ASN 64XXX | | | | ASN 64XXX
+---------+ | | | | +---------+
| +-----+ | | | +-----+ | | | +-----+ |
+-----------|-| |-|-+-|-| |-|-+-|-| |-|-----------+
| +---|-| |-|-+ | | | | +-|-| |-|---+ |
| | | +-----+ | | +-----+ | | +-----+ | | |
| | | | | | | | | |
| | | | | | | | | |
| | | +-----+ | | +-----+ | | +-----+ | | |
| +-----+---|-| |-|-+ | | | | +-|-| |-|---+-----+ |
| | | +-|-| |-|-+-|-| |-|-+-|-| |-|-+ | | |
| | | | | +-----+ | | | +-----+ | | | +-----+ | | | | |
| | | | +---------+ | | | | +---------+ | | | |
| | | | | | | | | | | |
+-----+ +-----+ | | +-----+ | | +-----+ +-----+
| ASN | | | +-|-| |-|-+ | | | |
|65YYY| | ... | | | | | | ... | | ... |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| | | | +---------+ | | | |
O O O O <- Servers -> O O O O
Figure 4: BGP ASN layout for 5-stage Clos
7.2.2. Non-unique private BGP ASN's
The use of private BGP ASNs limits to a range of 1022 unique numbers.
It is possible that the number of network switches could exceed this
value, and such situation requires a workaround. One approach could
be re-using the private ASN's assigned to Tier 3 switches across
different clusters. For example, private BGP ASN's 65001, 65003 ...
65032 could be used within every individual cluster to be assigned to
Tier 3 switches.
To avoid Tier 3 route discards on the Tier 3 switches sharing the
same ASN due to AS PATH loop prevention, upstream eBGP sessions on
Tier 3 switches must be configured with so-called "AllowAS In"
feature. This BGP policy feature allows accepting device's own ASN
in incoming BGP path advertisements. Introduction of this feature
does not create the opportunity for permanent routing loops under
misconfiguration since AS PATH is always increments when routes are
propagated from tier to tier.
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Another solution to this problem would be switching over to using
four-octet (32-bit) BGP ASNs. However, there is no explicitly
reserved private ASN range in four-octet numbering, but a work is in
progress to request such an allocation in
[I-D.mitchell-idr-as-private-reservation]. This will also require
vendors to implement specific policy features, such as private AS
removal from AS-PATH attribute.
7.2.3. Prefix advertisement
Clos topology has large number of point-to-point links and associated
prefixes. Advertising all of them into BGP may create FIB sizing
issues, and there are two possible solutions to overcome this:
o Do not advertise any of the point-to-point links into BGP. Since
eBGP peering changes next-hop address at every node, this will not
create any reachability issues for subnets advertised from Tier 3
switches.
o Advertising point-to-point links, but summarizing them on every
advertising device. This requires proper address allocation, for
example allocating a consecutive block of IP addresses per Tier 1
and Tier 2 device to be used for point-to-point interface
numbering.
Server facing subnets on Tier 3 switches are announced into BGP
without using summarization on Tier 2 and Tier 1 switches.
Summarizing subnets in the Clos topology will result in route black-
holing under a single link failure (e.g. between Tier 2 and Tier 3
switch) and hence must be avoided. The use of peer links within the
same tier to resolve the black-holing problem is undesirable due to
O(N^2) complexity of the peering mesh and waste of ports on the
switches.
7.2.4. External connectivity
A dedicate cluster (or clusters) in Clos topology could be selected
solely for the purpose of connecting to the Wide Area Network (WAN)
edge devices, which we will call WAN Routers. Tier 3 switches in
such cluster would be replaced with WAN Routers, but eBGP peering
will be used as usual, though WAN routers are likely to belong to a
public ASN.
The Tier 2 devices in such dedicated cluster will be referenced as
"Border Routers" in this document. These devices have to perform a
few special functions:
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o Hide network topology information when advertising paths to WAN
routers, i.e. remove some BGP AS-PATH information. This is
typically done to avoid BGP ASN number collisions across the data
centers. A BGP policy feature called "Remove Private AS" is
commonly used to accomplish this. This feature strips contiguous
sequence of private ASNs found in AS PATH attribute prior to
advertising the path to a neighbor. This assumes that all BGP
ASN's used for intra data center numbering are from private range.
o Originate a default route to the data center devices. This is the
only place where default route could be originated, as route
summarization is highly undesirable for the "scale-out" topology.
Alternatively, Border Routers may simply relay the default route
learned from WAN routers.
7.3. ECMP Considerations
This section goes over Equal Cost Multipath (ECMP) functionality for
Clos topology and covers a few special requirements.
7.3.1. Basic ECMP
ECMP is the key load-sharing mechanism leveraged by Clos topology.
Effectively, every lower-tier switch will use all of its directly
attached upper-tier devices to load-share traffic to the same prefix.
Number of ECMP paths between two input/output switches in Clos
topology equals to the number of the switches in the middle stage
(Tier 1). For example, Figure 5 illustrates the topology where Tier
3 device A has four paths to reach servers X and Y, via Tier 2
devices B and C and then Tier 1 devices 1, 2, 3, and 4 respectively.
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Tier 1
+-----+
| DEV |
+->| 1 |--+
| +-----+ |
Tier 2 | | Tier 2
+-----+ | +-----+ | +-----+
+------------>| DEV |--+->| DEV |--+--| |-------------+
| +-----| B |--+ | 2 | +--| |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+---->| DEV |--+ | DEV | +--| |-----+-----+ |
| | | +---| C |--+->| 3 |--+--| |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| DEV | | | Tier 3 +->| DEV |--+ Tier 3 | | | |
| A | | | | 4 | | | | |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
O O O O <- Servers -> X Y O O
Figure 5: ECMP fan-out tree from A to X and Y
The ECMP requirement implies that BGP implementation must support
multi-path fan-out for up to the maximum number of devices directly
attached at any point in the topology. Normally, this number does
not exceed half of the ports found on a switch in the topology, e.g.
32 for a 64-port switch.
Most implementations declare paths to be equal from ECMP perspective
if they match up to and including step (e) in Section 9.1.2.2 of
[RFC4271]. In the proposed network design there is no underlying
IGP, so all IGP costs are automatically assumed to be zero (or
otherwise the same value across all paths). Loop prevention is
assumed to be handled by BGP best-path selection process.
7.3.2. BGP ECMP over multiple ASN
For the purpose of application load-balancing purposes same prefix
could be advertised from multiple Tier-3 switches. From the
perspective of other devices, such prefix would have BGP paths with
different AS PATH attribute values, though having the same AS PATH
length. BGP implementation must support load-sharing for the paths
having different AS PATH attribute values with equal attribute
length. This feature is sometimes known as "AS PATH multipath relax"
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and effectively allows for ECMP to be done across different
neighboring ASNs.
7.4. BGP convergence properties
This section reviews routing convergence properties of BGP in the
proposed design. A case is made that sub-second convergence is
achievable provided that implementation supports fast BGP peering
session shutdown upon failure of an associated link.
7.4.1. Convergence timing
BGP typically relies on IGP to route around link/node failures inside
an AS, and implements either polling based or event-driven mechanism
to obtain updates on IGP state changes. The proposed routing design
lacks any IGP, so the only mechanism that could be used for fault
detection is BGP keep-alive packet exchange.
Relying purely on BGP keep-alive packets may result in high
convergence delays, on the order of multiple seconds (normally, the
minimum recommended BGP hold time value is 3 seconds). However, many
BGP implementations can shut down local eBGP peering sessions in
response to the "link down" event for the outgoing interface used for
BGP peering. This feature is sometimes called as "fast fall-over".
Since majority of the links in modern data centers are point to point
fiber connections, a physical failure translates into interface going
down within order of milliseconds, and trigger BGP re-convergence.
Furthermore, popular link technologies, such as 10Gbps Ethernet, may
support simple form of OAM for failure signaling such as
[FAULTSIG10GE], which makes failure detection more robust.
Alternatively, as opposed to relying on physical layer for fault
signaling, some platforms may support Bidirectional Forwarding
Detection ([RFC5880]) to allow for sub-second failure detection and
fault signaling to BGP process. This, however, presents additional
requirements to vendor software and possibly hardware, and may
contradict REQ1.
7.4.2. Failure impact scope
BGP is inherently a distance-vector protocol, and as such some of
failures could be masked if the local node can immediately find a
backup path. Worst case is that all devices would have to either
withdraw a prefix completely, or update the ECMP paths in the FIB.
That fault domain cannot be reduced by using summarization, since
using this technique may create route black-holing issues as
mentioned previously.
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7.4.3. Third-party route injection
BGP allows for a third-party BGP speaker (not necessarily directly
attached to the network devices) to inject routes at any point of
network topology. This could be achieved by peering an external
speaker using eBGP multi-hop session with some or even all devices in
the topology. Furthermore, BGP diverse path distribution
[I-D.ietf-grow-diverse-bgp-path-dist] could be used to inject
multiple next-hop for the same prefix and facilitate load-balancing.
Using that technique, it is possible to implement unequal-cost load-
balancing across multiple clusters in the data-center, by associating
the same prefix with next-hops mapping to different clusters.
For example, a third-party BGP speaker may peer with Tier 3 and Tier
1 switches, injecting the same prefix, but using a special set of BGP
next-hops for Tier 1 devices. Those next-hops are assumed to resolve
recursively via BGP, and could be, for example, IP addresses on Tier
3 switches. The resulting forwarding table programming could provide
desired traffic proportion distribution among different clusters.
8. Security Considerations
The design does not introduce any special security concerns others
than normally associated with BGP deployments. For control plane
security, BGP peering sessions could be authenticated using TCP MD5
signature extension header [RFC2385]. Furthermore, BGP TTL security
[I-D.gill-btsh] could be used to reduce the risk of session spoofing
and TCP SYN flooding attacks against the control plane.
9. IANA Considerations
There are no considerations associated with IANA for this document.
10. Acknowledgements
This publication summarizes work of many people who participated in
developing, testing and deploying the proposed design. Their names,
in alphabetical order, are George Chen, Parantap Lahiri, Dave Maltz,
Edet Nkposong, Robert Toomey, and Lihua Yuan. Authors would also
like to thank Jon Mitchell for reviewing and providing valuable
feedback on the document.
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11. Informative References
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, December 2006.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[I-D.ietf-grow-diverse-bgp-path-dist]
Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
Kumaki, "Distribution of diverse BGP paths.",
draft-ietf-grow-diverse-bgp-path-dist-07 (work in
progress), May 2012.
[I-D.mitchell-idr-as-private-reservation]
Mitchell, J., "Autonomous System (AS) Reservation for
Private Use", draft-mitchell-idr-as-private-reservation-00
(work in progress), June 2012.
[I-D.gill-btsh]
Gill, V., Heasley, J., and D. Meyer, "The BGP TTL Security
Hack (BTSH)", draft-gill-btsh-02 (work in progress),
May 2003.
[GREENBERG2009]
Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a
Cloud: Research Problems in Data Center Networks",
January 2009.
[FAULTSIG10GE]
Frazier, H. and S. Muller, "Remote Fault & Break Link
Proposal for 10-Gigabit Ethernet", September 2000.
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Authors' Addresses
Petr Lapukhov
Microsoft Corp.
One Microsfot Way
Redmond, WA 98052
US
Phone: +1 425 7032723 X 32723
Email: petrlapu@microsoft.com
URI: http://microsoft.com/
Ariff Premji
Arista Networks
5470 Great America Parkway
Santa Clara, CA 95054
US
Phone: +1 408-547-5699
Email: ariff@aristanetworks.com
URI: http://aristanetworks.com/
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