rfc2260.txt
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Network Working Group T. Bates
Request for Comments: 2260 Cisco Systems
Category: Informational Y. Rekhter
Cisco Systems
January 1998
Scalable Support for Multi-homed Multi-provider Connectivity
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
2. Abstract
This document describes addressing and routing strategies for multi-
homed enterprises attached to multiple Internet Service Providers
(ISPs) that are intended to reduce the routing overhead due to these
enterprises in the global Internet routing system.
3. Motivations
An enterprise may acquire its Internet connectivity from more than
one Internet Service Provider (ISP) for some of the following
reasons. Maintaining connectivity via more than one ISP could be
viewed as a way to make connectivity to the Internet more reliable.
This way when connectivity through one of the ISPs fails,
connectivity via the other ISP(s) would enable the enterprise to
preserve its connectivity to the Internet. In addition to providing
more reliable connectivity, maintaining connectivity via more than
one ISP could also allow the enterprise to distribute load among
multiple connections. For enterprises that span wide geographical
area this could also enable better (more optimal) routing.
The above considerations, combined with the decreasing prices for the
Internet connectivity, motivate more and more enterprises to become
multi-homed to multiple ISPs. At the same time, the routing overhead
that such enterprises impose on the Internet routing system becomes
more and more significant. Scaling the Internet, and being able to
support a growing number of such enterprises demands mechanism(s) to
contain this overhead. This document assumes that an approach where
routers in the "default-free" zone of the Internet would be required
Bates & Rekhter Informational [Page 1]
RFC 2260 Multihoming January 1998
to maintain a route for every multi-homed enterprise that is
connected to multiple ISPs does not provide an adequate scaling.
Moreover, given the nature of the Internet, this document assumes
that any approach to handle routing for such enterprises should
minimize the amount of coordination among ISPs, and especially the
ISPs that are not directly connected to these enterprises.
There is a difference of opinions on whether the driving factors
behind multi-homing to multiple ISPs could be adequately addressed by
multi-homing just to a single ISP, which would in turn eliminate the
negative impact of multi-homing on the Internet routing system.
Discussion of this topic is beyond the scope of this document.
The focus of this document is on the routing and addressing
strategies that could reduce the routing overhead due to multi-homed
enterprises connected to multiple ISPs in the Internet routing
system.
The strategies described in this document are equally applicable to
both IPv4 and IPv6.
4. Address allocation and assignment
A multi-homed enterprise connected to a set of ISPs would be
allocated a block of addresses (address prefix) by each of these ISPs
(an enterprise connected to N ISPs would get N different blocks).
The address allocation from the ISPs to the enterprise would be based
on the "address-lending" policy [RFC2008]. The allocated addresses
then would be used for address assignment within the enterprise.
One possible address assignment plan that the enterprise could employ
is to use the topological proximity of a node (host) to a particular
ISP (to the interconnect between the enterprise and the ISP) as a
criteria for selecting which of the address prefixes to use for
address assignment to the node. A particular node (host) may be
assigned address(es) out of a single prefix, or may have addresses
from different prefixes.
5. Routing information exchange
The issue of routing information exchange between an enterprise and
its ISPs is decomposed into the following components:
a) reachability information that an enterprise border router
advertises to a border router within an ISP
b) reachability information that a border router within an ISP
advertises to an enterprise border router
Bates & Rekhter Informational [Page 2]
RFC 2260 Multihoming January 1998
The primary focus of this document is on (a); (b) is covered only as
needed by this document.
5.1. Advertising reachability information by enterprise border routers
When an enterprise border router connected to a particular ISP
determines that the connectivity between the enterprise and the
Internet is up through all of its ISPs, the router advertises (to the
border router of that ISP) reachability to only the address prefix
that the ISP allocated to the enterprise. This way in a steady state
routes injected by the enterprise into its ISPs are aggregated by
these ISPs, and are not propagated into the "default-free" zone of
the Internet.
When an enterprise border router connected to a particular ISP
detemrines that the connectivity between the enterprise and the
Internet through one or more of its other ISPs is down, the router
starts advertising reachability to the address prefixes that was
allocated by these ISPs to the enterprise. This would result in
injecting additional routing information into the "default-free" zone
of the Internet. However, one could observe that the probability of
all multi-homed enterprises in the Internet concurrently losing
connectivity to the Internet through one or more of their ISPs is
fairly small. Thus on average the number of additional routes in the
"default-free" zone of the Internet due to multi-homed enterprises is
expected to be a small fraction of the total number of such
enterprises.
The approach described above is predicated on the assumption that an
enterprise border router has a mechanism(s) by which it could
determine (a) whether the connectivity to the Internet through some
other border router of that enterprise is up or down, and (b) the
address prefix that was allocated to the enterprise by the ISP
connected to the other border router. One such possible mechanism
could be provided by BGP [RFC1771]. In this case border routers
within the enterprise would have an IBGP peering with each other.
Whenever one border router determines that the intersection between
the set of reachable destinations it receives via its EBGP (from its
directly connected ISP) peerings and the set of reachable
destinations it receives from another border router (in the same
enterprise) via IBGP is empty, the border router would start
advertising to its external peer reachability to the address prefix
that was allocated to the enterprise by the ISP connected to the
other border router. The other border router would advertise (via
IBGP) the address prefix that was allocated to the enterprise by the
ISP connected to that router. This approach is known as "auto route
injection".
Bates & Rekhter Informational [Page 3]
RFC 2260 Multihoming January 1998
As an illustration consider an enterprise connected to two ISPs,
ISP-A and ISP-B. Denote the enterprise border router that connects
the enterprise to ISP-A as BR-A; denote the enterprise border router
that connects the enterprise to ISP-B as BR-B. Denote the address
prefix that ISP-A allocated to the enterprise as Pref-A; denote the
address prefix that ISP-B allocated to the enterprise as Pref-B.
When the set of routes BR-A receives from ISP-A (via EBGP) has a
non-empty intersection with the set of routes BR-A receives from BR-B
(via IBGP), BR-A advertises to ISP-A only the reachability to Pref-A.
When the intersection becomes empty, BR-A would advertise to ISP-A
reachability to both Pref-A and Pref-B. This would continue for as
long as the intersection remains empty. Once the intersection becomes
non-empty, BR-A would stop advertising reachability to Pref-B to
ISP-A (but would still continue to advertise reachability to Pref-A
to ISP-A). Figure 1 below describes this method graphically.
+-------+ +-------+ +-------+ +-------+
( ) ( ) ( ) ( )
( ISP-A ) ( ISP-B ) ( ISP-A ) ( ISP-B )
( ) ( ) ( ) ( )
+-------+ +-------+ +-------+ +-------+
| /\ | /\ | /\ |
| || | || | Pref-A (connection
| Pref-A | Pref-B | Pref-B broken)
| || | || | || |
+-----+ +-----+ +-----+ +-----+
| BR-A|------|BR-B | | BR-A|------|BR-B |
+-----+ IBGP +-----+ +-----+ IBGP +-----+
non-empty intersection empty intersection
Figure 1: Reachability information advertised
Although strictly an implementation detail, calculating the
intersection could potentially be a costly operation for a large set
of routes. An alternate solution to this is to make use of a selected
single (or more) address prefix received from an ISP (the ISP's
backbone route for example) and configure the enterprise border
router to perform auto route injection if the selected prefix is not
present via IBGP. Let's suppose ISP-B has a well known address
prefix, ISP-Pref-B for its backbone. ISP-B advertises this to BR-B
and BR-B in turn advertises this via IBGP to BR-A. If BR-A sees a
withdraw for ISP-Pref-B it advertises Pref-B to ISP-A.
Bates & Rekhter Informational [Page 4]
RFC 2260 Multihoming January 1998
The approach described in this section may produce less than the full
Internet-wide connectivity in the presence of ISPs that filter out
routes based on the length of their address prefixes. One could
observe however, that this would be a problem regardless of how the
enterprise would set up its routing and addressing.
5.2. Further improvements
The approach described in the previous section allows to
significantly reduce the routing overhead in the "default-free" zone
of the Internet due to multi-homed enterprises. The approach
described in this section allows to completely eliminate this
overhead.
An enterprise border router would maintain EBGP peering not just with
the directly connected border router of an ISP, but with the border
router(s) in one or more ISPs that have their border routers directly
connected to the other border routers within the enterprise. We
refer to such peering as "non-direct" EBGP.
An ISP that maintains both direct and non-direct EBGP peering with a
particular enterprise would advertise the same set of routes over
both of these peerings. An enterprise border router that maintains
either direct or non-direct peering with an ISP advertises to that
ISP reachability to the address prefix that was allocated by that ISP
to the enterprise. Within the ISP routes received over direct
peering should be preferred over routes received over non-direct
peering. Likewise, within the enterprise routes received over direct
peering should be preferred over routes received over non-direct
peering.
Forwarding along a route received over non-direct peering should be
accomplished via encapsulation [RFC1773].
As an illustration consider an enterprise connected to two ISPs,
ISP-A and ISP-B. Denote the enterprise border router that connects
the enterprise to ISP-A as E-BR-A, and the ISP-A border router that
is connected to E-BR-A as ISP-BR-A; denote the enterprise border
router that connects the enterprise to ISP-B as E-BR-B, and the ISP-B
border router that is connected to E-BR-B as ISP-BR-B. Denote the
address prefix that ISP-A allocated to the enterprise as Pref-A;
denote the address prefix that ISP-B allocated to the enterprise as
Pref-B. E-BR-A maintains direct EBGP peering with ISP-BR-A and
advertises reachability to Pref-A over that peering. E-BR-A also
maintain a non-direct EBGP peering with ISP-BR-B and advertises
reachability to Pref-B over that peering. E-BR-B maintains direct
EBGP peering with ISP-BR-B, and advertises reachability to Pref-B
over that peering. E-BR-B also maintains a non-direct EBGP peering
Bates & Rekhter Informational [Page 5]
RFC 2260 Multihoming January 1998
with ISP-BR-A, and advertises reachability to Pref-A over that
peering.
When connectivity between the enterprise and both of its ISPs (ISP-A
and ISP-B is up, traffic destined to hosts whose addresses were
assigned out of Pref-A would flow through ISP-A to ISP-BR-A to E-BR-
A, and then into the enterprise. Likewise, traffic destined to hosts
whose addresses were assigned out of Pref-B would flow through ISP-B
to ISP-BR-B to E-BR-B, and then into the enterprise. Now consider
what would happen when connectivity between ISP-BR-B and E-BR-B goes
down. In this case traffic to hosts whose addresses were assigned
out of Pref-A would be handled as before. But traffic to hosts whose
addresses were assigned out of Pref-B would flow through ISP-B to
ISP-BR-B, ISP-BR-B would encapsulate this traffic and send it to E-
BR-A, where the traffic will get decapsulated and then be sent into
the enterprise. Figure 2 below describes this approach graphically.
+---------+ +---------+
( ) ( )
( ISP-A ) ( ISP-B )
( ) ( )
+---------+ +---------+
| |
+--------+ +--------+
|ISP-BR-A| |ISP-BR-B|
+--------+ +--------+
| /+/ |
/\ | Pref-B /+/ |
|| | /+/ \./
Pref-A| /+/ non- /.\
|| | /+/ direct |
| /+/ EBGP |
+------+ +-------+
|E-BR-A|-----------|E-BR-B |
+------+ IBGP +-------+
Figure 2: Reachability information advertised via non-direct EBGP
Observe that with this scheme there is no additional routing
information due to multi-homed enterprises that has to be carried in
the "default-free" zone of the Internet. In addition this scheme
doesn't degrade in the presence of ISPs that filter out routes based
on the length of their address prefixes.
Note that the set of routers within an ISP that maintain non-direct
peering with the border routers within an enterprise doesn't have to
be restricted to the ISP's border routers that have direct peering
Bates & Rekhter Informational [Page 6]
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