---
title: IP Interconnect Fabrics in Calico
redirect_from: latest/reference/private-cloud/l3-interconnect-fabric
lead_text: 'Where large-scale IP networks and hardware collide'
---


Calico provides an end-to-end IP network that interconnects the
endpoints [^1] in a scale-out or cloud environment. To do that, it needs
an *interconnect fabric* to provide the physical networking layer on
which Calico operates [^2].

While Calico is designed to work with any underlying interconnect fabric
that can support IP traffic, the fabric that has the least
considerations attached to its implementation is an Ethernet fabric as
discussed in our earlier [technical note]({{site.baseurl}}/{{page.version}}/reference/private-cloud/l2-interconnect-fabric).

In most cases, the Ethernet fabric is the appropriate choice, but there
are infrastructures where L3 (an IP fabric) has already been deployed,
or will be deployed, and it makes sense for Calico to operate in those
environments.

However, since Calico is, itself, a routed infrastructure, there are
more engineering, architecture, and operations considerations that have
to be weighed when running Calico with an IP routed interconnection
fabric. We will briefly outline those in the rest of this post. That
said, Calico operates equally well with Ethernet or IP interconnect
fabrics.

## In this post
{: .no_toc}

* TOC
{:toc}

## Background

### Basic Calico architecture overview

A description of the Calico architecture can be found in our
[architectural overview]({{site.baseurl}}/{{page.version}}/reference/architecture).
However, a brief discussion of the routing and data paths is useful for
the discussion.

In a Calico network, each compute server acts as a router for all of the
endpoints that are hosted on that compute server. We call that function
a vRouter. The data path is provided by the Linux kernel, the control
plane by a BGP protocol server, and management plane by Calico's
on-server agent, *Felix*.

Each endpoint can only communicate through its local vRouter, and the
first and last *hop* in any Calico packet flow is an IP router hop
through a vRouter. Each vRouter announces all of the endpoints it is
attached to to all the other vRouters and other routers on the
infrastructure fabric, using BGP, usually with BGP route reflectors to
increase scale. A discussion of why we use BGP can be found in the [Why
BGP?](https://www.projectcalico.org/why-bgp/) blog post.

Access control lists (ACLs) enforce security (and other) policy as
directed by whatever cloud orchestrator is in use. There are other
components in the Calico architecture, but they are irrelevant to the
interconnect network fabric discussion.

### Overview of current common IP scale-out fabric architectures

There are two approaches to building an IP fabric for a scale-out
infrastructure. However, all of them, to date, have assumed that the
edge router in the infrastructure is the top of rack (TOR) switch. In
the Calico model, that function is pushed to the compute server itself.

Furthermore, in most current virtualized environments, the actual
endpoint is not addressed by the fabric. If it is a VM, it is usually
encapsulated in an overlay, and if it is a container, it may be
encapsulated in an overlay, or NATed by some form of proxy, such as is
done in the [weave](http://www.weave.works/) project network model, or
the router in standard [docker](http://www.docker.io/) networking.

The two approaches are outlined below, in this technical note, we will
cover the second option, as it is more common in the scale-out world. If
there is interest in the first approach, please contact Project Calico,
and we can discuss, and if there is enough interest, maybe we will do
another technical note on that approach. If you know of other approaches
in use, we would be happy to host a guest technical note.

1.  The routing infrastructure is based on some form of IGP. Due to the
    limitations in scale of IGP networks (see the [why BGP
    post](https://www.projectcalico.org/why-bgp/) for discussion of this
    topic), the project Calico team does not believe that using an IGP
    to distribute endpoint reachability information will adequately
    scale in a Calico environment. However, it is possible to use a
    combination of IGP and BGP in the interconnect fabric, where an IGP
    communicates the path to the *next-hop* router (in Calico, this is
    often the destination compute server) and BGP is used to distribute
    the actual next-hop for a given endpoint. This is a valid model,
    and, in fact is the most common approach in a widely distributed IP
    network (say a carrier's backbone network). The design of these
    networks is somewhat complex though, and will not be addressed
    further in this technical note. [^3]
2.  The other model, and the one that this note concerns itself with, is
    one where the routing infrastructure is based entirely on BGP. In
    this model, the IP network is "tight enough" or has a small enough
    diameter that BGP can be used to distribute endpoint routes, and the
    paths to the next-hops for those routes is known to all of the
    routers in the network (in a Calico network this includes the
    compute servers). This is the network model that this note
    will address.

### BGP-only interconnect fabrics

There are multiple methods to build a BGP-only interconnect fabric. We
will focus on three models, each with two widely viable variations.
There are other options, and we will briefly touch on why we didn't
include some of them in the [Other Options appendix](#other-options).

The two methods are:

1.  A BGP fabric where each of the TOR switches (and their subsidiary
    compute servers) are a unique [Autonomous
    System (AS)](https://en.wikipedia.org/wiki/Autonomous_System_(Internet))
    and they are interconnected via either an Ethernet switching plane
    provided by the spine switches in a
    [leaf/spine](http://bradhedlund.com/2012/10/24/video-a-basic-introduction-to-the-leafspine-data-center-networking-fabric-design/)
    architecture, or via a set of spine switches, each of which is also
    a unique AS. We'll refer to this as the *AS per rack* model. This
    model is detailed in [this IETF working group draft](https://tools.ietf.org/html/draft-ietf-rtgwg-bgp-routing-large-dc).
2.  A BGP fabric where each of the compute servers is a unique AS, and
    the TOR switches make up a transit AS. We'll refer to this as the
    *AS per server* model.

Each of these models can either have an Ethernet or IP spine. In the
case of an Ethernet spine, each spine switch provides an isolated
Ethernet connection *plane* as in the Calico Ethernet interconnect
fabric model and each TOR switch is connected to each spine switch.

Another model is where each spine switch is a unique AS, and each TOR
switch BGP peers with each spine switch. In both cases, the TOR switches
use ECMP to load-balance traffic between all available spine switches.

### Some BGP network design considerations

Contrary to popular opinion, BGP is actually a fairly simple protocol.
For example, the BGP configuration on a Calico compute server is
approximately sixty lines long, not counting comments. The perceived
complexity is due to the things that you can *do* with BGP. Many uses of
BGP involve complex policy rules, where the behavior of BGP can be
modified to meet technical (or business, financial, political, *etc.*)
requirements. A default Calico network does not venture into those
areas, [^4] and therefore is fairly straight forward.

That said, there are a few design rules for BGP that need to be kept in
mind when designing an IP fabric that will interconnect nodes in a
Calico network. These BGP design requirements *can* be worked around, if
necessary, but doing so takes the designer out of the standard BGP
*envelope* and should only be done by an implementer who is *very*
comfortable with advanced BGP design.

These considerations are:

AS continuity

:   or *AS puddling* Any router in an AS *must* be able to communicate
    with any other router in that same AS without transiting another AS.

Next hop behavior

:   By default BGP routers do not change the *next hop* of a route if it
    is peering with another router in its same AS. The inverse is also
    true, a BGP router will set itself as the *next hop* of a route if
    it is peering with a router in another AS.

Route reflection

:   All BGP routers in a given AS must *peer* with all the other routers
    in that AS. This is referred to a *complete BGP mesh*. This can
    become problematic as the number of routers in the AS scales up. The
    use of *route reflectors* reduce the need for the complete BGP mesh.
    However, route reflectors also have scaling considerations.

Endpoints

:   In a Calico network, each endpoint is a route. Hardware networking
    platforms are constrained by the number of routes they can learn.
    This is usually in range of 10,000's or 100,000's of routes. Route
    aggregation can help, but that is usually dependent on the
    capabilities of the scheduler used by the orchestration software
    (*e.g.* OpenStack).

A deeper discussion of these considerations can be found in the IP
Fabric Design Considerations\_ appendix.

The designs discussed below address these considerations.

### The *AS Per Rack* model

This model is the closest to the model suggested by the [IETF's Routing
Area Working Group draft on BGP use in data
centers](https://tools.ietf.org/html/draft-ietf-rtgwg-bgp-routing-large-dc).

As mentioned earlier, there are two versions of this model, one with an
set of Ethernet planes interconnecting the ToR switches, and the other
where the core planes are also routers. The following diagrams may be
useful for the discussion.

![]({{site.baseurl}}/images/l3-fabric-diagrams-as-rack-l2-spine.png)

The diagram above shows the *AS per rack model* where the ToR switches are
physically meshed via a set of Ethernet switching planes.

![]({{site.baseurl}}/images/l3-fabric-diagrams-as-rack-l3-spine.png)

The diagram above shows the *AS per rack model* where the ToR switches are
physically meshed via a set of discrete BGP spine routers, each in
their own AS.

In this approach, every ToR-ToR or ToR-Spine (in the case of an AS per
spine) link is an eBGP peering which means that there is no
route-reflection possible (using standard BGP route reflectors) *north*
of the ToR switches.

If the L2 spine option is used, the result of this is that each ToR must
either peer with every other ToR switch in the cluster (which could be
hundreds of peers).

If the AS per spine option is used, then each ToR only has to peer with
each spine (there are usually somewhere between two and sixteen spine
switches in a pod). However, the spine switches must peer with all ToR
switches (again, that would be hundreds, but most spine switches have
more control plane capacity than the average ToR, so this might be more
scalable in many circumstances).

Within the rack, the configuration is the same for both variants, and is
somewhat different than the configuration north of the ToR.

Every router within the rack, which, in the case of Calico is every
compute server, shares the same AS as the ToR that they are connected
to. That connection is in the form of an Ethernet switching layer. Each
router in the rack must be directly connected to enable the AS to remain
contiguous. The ToR's *router* function is then connected to that
Ethernet switching layer as well. The actual configuration of this is
dependent on the ToR in use, but usually it means that the ports that
are connected to the compute servers are treated as *subnet* or
*segment* ports, and then the ToR's *router* function has a single
interface into that subnet.

This configuration allows each compute server to connect to each other
compute server in the rack without going through the ToR router, but it
will, of course, go through the ToR switching function. The compute
servers and the ToR router could all be directly meshed, or a route
reflector could be used within the rack, either hosted on the ToR
itself, or as a virtual function hosted on one or more compute servers
within the rack.

The ToR, as the eBGP router redistributes all of the routes from other
ToRs as well as routes external to the data center to the compute
servers that are in its AS, and announces all of the routes from within
the AS (rack) to the other ToRs and the larger world. This means that
each compute server will see the ToR as the next hop for all external
routes, and the individual compute servers are the next hop for all
routes internal to the rack.

### The *AS per Compute Server* model

This model takes the concept of an AS per rack to its logical
conclusion. In the earlier referenced [IETF
draft](https://tools.ietf.org/html/draft-ietf-rtgwg-bgp-routing-large-dc)
the assumption in the overall model is that the ToR is first tier
aggregating and routing element. In Calico, the ToR, if it is an L3
router, is actually the second tier. Remember, in Calico, the compute
server is always the first/last router for an endpoint, and is also the
first/last point of aggregation.

Therefore, if we follow the architecture of the draft, the compute
server, not the ToR should be the AS boundary. The differences can be
seen in the following two diagrams.

![]({{site.baseurl}}/images/l3-fabric-diagrams-as-server-l2-spine.png)

The diagram above shows the *AS per compute server model* where the ToR
switches are physically meshed via a set of Ethernet switching planes.

![]({{site.baseurl}}/images/l3-fabric-diagrams-as-server-l3-spine.png)

The diagram above shows the *AS per compute server model* where the ToR
switches are physically connected to a set of independent routing
planes.

As can be seen in these diagrams, there are still the same two variants
as in the *AS per rack* model, one where the spine switches provide a
set of independent Ethernet planes to interconnect the ToR switches, and
the other where that is done by a set of independent routers.

The real difference in this model, is that the compute servers as well
as the ToR switches are all independent autonomous systems. To make this
work at scale, the use of four byte AS numbers as discussed in
[RFC 4893](http://www.faqs.org/rfcs/rfc4893.html "RFC 4893"). Without
using four byte AS numbering, the total number of ToRs and compute
servers in a calico fabric would be limited to the approximately five
thousand available private AS [^5] numbers. If four byte AS numbers are
used, there are approximately ninety-two million private AS numbers
available. This should be sufficient for any given Calico fabric.

The other difference in this model *vs.* the AS per rack model, is that
there are no route reflectors used, as all BGP peerings are eBGP. In
this case, each compute server in a given rack peers with its ToR switch
which is also acting as an eBGP router. For two servers within the same
rack to communicate, they will be routed through the ToR. Therefore,
each server will have one peering to each ToR it is connected to, and
each ToR will have a peering with each compute server that it is
connected to (normally, all the compute servers in the rack).

The inter-ToR connectivity considerations are the same in scale and
scope as in the AS per rack model.

### The *Downward Default* model

The final model is a bit different. Whereas, in the previous models, all
of the routers in the infrastructure carry full routing tables, and
leave their AS paths intact, this model [^6] removes the AS numbers at
each stage of the routing path. This is to prevent routes from other
nodes in the network from not being installed due to it coming from the
*local* AS (since they share the source and dest of the route share the
same AS).

The following diagram will show the AS relationships in this model.

![]({{site.baseurl}}/images/l3-fabric-downward-default.png)

In the diagram above, we are showing that all Calico nodes share the same
AS number, as do all ToR switches. However, those ASs are different
(*A1* is not the same network as *A2*, even though the both share the
same AS number *A* ).

While the use of a single AS for all ToR switches, and another for all
compute servers simplifies deployment (standardized configuration), the
real benefit comes in the offloading of the routing tables in the ToR
switches.

In this model, each router announces all of its routes to its upstream
peer (the Calico routers to their ToR, the ToRs to the spine switches).
However, in return, the upstream router only announces a default route.
In this case, a given Calico router only has routes for the endpoints
that are locally hosted on it, as well as the default from the ToR.
Since the ToR is the only route for the Calico network the rest of the
network, this matches reality. The same happens between the ToR switches
and the spine. This means that the ToR only has to install the routes
that are for endpoints that are hosted on its downstream Calico nodes.
Even if we were to host 200 endpoints per Calico node, and stuff 80
Calico nodes in each rack, that would still limit the routing table on
the ToR to a maximum of 16,000 entries (well within the capabilities of
even the most modest of switches).

Since the default is originated by the Spine (originally) there is no
chance for a downward announced route to originate from the recipient's
AS, preventing the *AS puddling* problem.

There is one (minor) drawback to this model, in that all traffic that is
destined for an invalid destination (the destination IP does not exist)
will be forwarded to the spine switches before they are dropped.

It should also be noted that the spine switches do need to carry all of
the Calico network routes, just as they do in the routed spines in the
previous examples. In short, this model imposes no more load on the
spines than they already would have, and substantially reduces the
amount of routing table space used on the ToR switches. It also reduces
the number of routes in the Calico nodes, but, as we have discussed
before, that is not a concern in most deployments as the amount of
memory consumed by a full routing table in Calico is a fraction of the
total memory available on a modern compute server.

## Recommendation

The Project Calico team recommends the use of the [AS per rack](#the-as-per-rack-model) model if
the resultant routing table size can be accommodated by the ToR and
spine switches, remembering to account for projected growth.

If there is concern about the route table size in the ToR switches, the
team recommends the [Downward Default](#the-downward-default-model) model.

If there are concerns about both the spine and ToR switch route table
capacity, or there is a desire to run a very simple L2 fabric to connect
the Calico nodes, then the user should consider the Ethernet fabric as
detailed in [this post]({{site.baseurl}}/{{page.version}}/reference/private-cloud/l2-interconnect-fabric).

If a Calico user is interested in the AS per compute server, the Project
Calico team would be very interested in discussing the deployment of
that model.

## Appendix

### Other Options

The way the physical and logical connectivity is laid out in this note,
and the [Ethernet fabric note]({{site.baseurl}}/{{page.version}}/reference/private-cloud/l2-interconnect-fabric),
The next hop router for a given route is always directly connected to
the router receiving that route. This makes the need for another
protocol to distribute the next hop routes unnecessary.

However, in many (or most) WAN BGP networks, the routers within a given
AS may not be directly adjacent. Therefore, a router may receive a route
with a next hop address that it is not directly adjacent to. In those
cases, an IGP, such as OSPF or IS-IS, is used by the routers within a
given AS to determine the path to the BGP next hop route.

There may be Calico architectures where there are similar models where
the routers within a given AS are not directly adjacent. In those
models, the use of an IGP in Calico may be warranted. The configuration
of those protocols are, however, beyond the scope of this technical
note.

### IP Fabric Design Considerations

#### AS puddling

The first consideration is that an AS must be kept contiguous. This
means that any two nodes in a given AS must be able to communicate
without traversing any other AS. If this rule is not observed, the
effect is often referred to as *AS puddling* and the network will *not*
function correctly.

A corollary of that rule is that any two administrative regions that
share the same AS number, are in the same AS, even if that was not the
desire of the designer. BGP has no way of identifying if an AS is local
or foreign other than the AS number. Therefore re-use of an AS number
for two *networks* that are not directly connected, but only connected
through another *network* or AS number will not work without a lot of
policy changes to the BGP routers.

Another corollary of that rule is that a BGP router will not propagate a
route to a peer if the route has an AS in its path that is the same AS
as the peer. This prevents loops from forming in the network. The effect
of this prevents two routers in the same AS from transiting another
router (either in that AS or not).

#### Next hop behavior

Another consideration is based on the differences between iBGP and eBGP.
BGP operates in two modes, if two routers are BGP peers, but share the
same AS number, then they are considered to be in an *internal* BGP (or
iBGP) peering relationship. If they are members of different AS's, then
they are in an *external* or eBGP relationship.

BGP's original design model was that all BGP routers within a given AS
would know how to get to one another (via static routes, IGP [^7]
routing protocols, or the like), and that routers in different ASs would
not know how to reach one another unless they were directly connected.

Based on that design point, routers in an iBGP peering relationship
assume that they do not transit traffic for other iBGP routers in a
given AS (i.e. A can communicate with C, and therefore will not need to
route through B), and therefore, do not change the *next hop* attribute
in BGP [^8].

A router with an eBGP peering, on the other hand, assumes that its eBGP
peer will not know how to reach the next hop route, and then will
substitute its own address in the next hop field. This is often referred
to as *next hop self*.

In the Calico [Ethernet fabric]({{site.baseurl}}/{{page.version}}/reference/private-cloud/l2-interconnect-fabric)
model, all of the compute servers (the routers in a Calico network) are
directly connected over one or more Ethernet network(s) and therefore
are directly reachable. In this case, a router in the Calico network
does not need to set *next hop self* within the Calico fabric.

The models we present in this technical note insure that all routes that
may traverse a non-Calico router are eBGP routes, and therefore *next
hop self* is automatically set correctly. If a deployment of Calico in
an IP interconnect fabric does not satisfy that constraint, then *next
hop self* must be appropriately configured.

#### Route reflection

As mentioned above, BGP expects that all of the iBGP routers in a
network can see (and speak) directly to one another, this is referred to
as a *BGP full mesh*. In small networks this is not a problem, but it
does become interesting as the number of routers increases. For example,
if you have 99 BGP routers in an AS and wish to add one more, you would
have to configure the peering to that new router on each of the 99
existing routers. Not only is this a problem at configuration time, it
means that each router is maintaining 100 protocol adjacencies, which
can start being a drain on constrained resources in a router. While this
might be *interesting* at 100 routers, it becomes an impossible task
with 1000's or 10,000's of routers (the potential size of a Calico
network).

Conveniently, large scale/Internet scale networks solved this problem
almost 20 years ago by deploying BGP route reflection as described in
[RFC 1966](http://www.faqs.org/rfcs/rfc1966.html "RFC 1966"). This is a
technique supported by almost all BGP routers today. In a large network,
a number of route reflectors [^9] are evenly distributed and each iBGP
router is *peered* with one or more route reflectors (usually 2 or 3).
Each route reflector can handle 10's or 100's of route reflector clients
(in Calico's case, the compute server), depending on the route reflector
being used. Those route reflectors are, in turn, peered with each other.
This means that there are an order of magnitude less route reflectors
that need to be completely meshed, and each route reflector client is
only configured to peer to 2 or 3 route reflectors. This is much easier
to manage.

Other route reflector architectures are possible, but those are beyond
the scope of this document.

#### Endpoints

The final consideration is the number of endpoints in a Calico network.
In the [Ethernet fabric]({{site.baseurl}}/{{page.version}}/reference/private-cloud/l2-interconnect-fabric)
case the number of endpoints is not constrained by the interconnect
fabric, as the interconnect fabric does not *see* the actual endpoints,
it only *sees* the actual vRouters, or compute servers. This is not the
case in an IP fabric, however. IP networks forward by using the
destination IP address in the packet, which, in Calico's case, is the
destination endpoint. That means that the IP fabric nodes (ToR switches
and/or spine switches, for example) must know the routes to each
endpoint in the network. They learn this by participating as route
reflector clients in the BGP mesh, just as the Calico vRouter/compute
server does.

However, unlike a compute server which has a relatively unconstrained
amount of memory, a physical switch is either memory constrained, or
quite expensive. This means that the physical switch has a limit on how
many *routes* it can handle. The current industry standard for modern
commodity switches is in the range of 128,000 routes. This means that,
without other routing *tricks*, such as aggregation, a Calico
installation that uses an IP fabric will be limited to the routing table
size of its constituent network hardware, with a reasonable upper limit
today of 128,000 endpoints.

[^1]: In Calico's terminology, an endpoint is an IP address and
    interface. It could refer to a VM, a container, or even a process
    bound to an IP address running on a bare metal server.

[^2]: This interconnect fabric provides the connectivity between the
    Calico (v)Router (in almost all cases, the compute servers) nodes,
    as well as any other elements in the fabric (*e.g.* bare metal
    servers, border routers, and appliances).

[^3]: If there is interest in a discussion of this approach, please let
    us know. The Project Calico team could either arrange a discussion,
    or if there was enough interest, publish a follow-up tech note.

[^4]: However those tools are available if a given Calico instance needs
    to utilize those policy constructs.

[^5]: The two byte AS space reserves approximately the last five
    thousand AS numbers for private use. There is no technical reason
    why other AS numbers could not be used. However the re-use of global
    scope AS numbers within a private infrastructure is strongly
    discouraged. The chance for routing system failure or incorrect
    routing is substantial, and not restricted to the entity that is
    doing the reuse.

[^6]: We first saw this design in a customer's lab, and thought it
    innovative enough to share (we asked them first, of course). Similar
    *AS Path Stripping* approaches are used in ISP networks, however.

[^7]: An Interior Gateway Protocol is a local routing protocol that does
    not cross an AS boundary. The primary IGPs in use today are OSPF and
    IS-IS. While complex iBGP networks still use IGP routing protocols,
    a data center is normally a fairly simple network, even if it has
    many routers in it. Therefore, in the data center case, the use of
    an IGP can often be disposed of.

[^8]: A Next hop is an attribute of a route announced by a routing
    protocol. In simple terms a route is defined by a *target*, or the
    destination that is to be reached, and a *next hop*, which is the
    next router in the path to reach that target. There are many other
    characteristics in a route, but those are well beyond the scope of
    this post.

[^9]: A route reflector may be a physical router, a software appliance,
    or simply a BGP daemon. It only processes routing messages, and does
    not pass actual data plane traffic. However, some route reflectors
    are co-resident on regular routers that do pass data plane traffic.
    While they may sit on one platform, the functions are distinct.
