a crude method of limiting long-prefix propagation
Joe Abley
jabley-ietf at automagic.org
Fri Mar 23 07:30:17 UTC 2001
On Thu, Mar 22, 2001 at 11:37:44PM -0700, mccreary at pch.net wrote:
> With a small change in semantics, it seems like this scheme is sufficient to
> support restricted announcement of any route to an explicit set of ASes. Just
> change the meaning of EPPC_ALOW:ASN a little so it indicates `advertiseable
> to', so that a participating AS will only advertise this route to a peer if
> the proper community attribute appears in the route.
Yup. I already piped the -00 draft through asp and derived something
very similar. The following draft does not contain the one-AS-hop
limitation, and includes sample config for IOS and JUNOS.
http://www.automagic.org/~jabley/draft-jabley-edge-policy-propagation-control-01.txt
As suggested by Curtis, Ben is plugging the basic idea encompassed in
this mechanism into a new bgp attribute, as an alternative approach to
using community strings.
Joe
Network Working Group J. Abley
Internet-Draft Metromedia Fiber Network Inc.
Expires: September 21, 2001 March 23, 2001
Edge Policy Propagation Control
draft-jabley-edge-policy-propagation-control-01
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
There is a requirement for some multi-homed sites to influence the
path selected by autonomous systems beyond those that are immediately
adjacent.
One of the few generic mechanisms available is to deaggregate and
advertise long component prefixes to the network, since there can be
some confidence that the longest prefix will be used, regardless of
other local policy such as local preference. Most ASes exhibit
liberal route import policy with respect to prefix length, which
facilitates this technique.
Unfortunately, although the deaggregated prefix set may be required
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to be installed in only a few targeted ASes for the aims of the
origin to be achieved, there is no reliable mechanism to limit the
propagation of the prefixes. This contributes to prefix bloat in the
default-free zone, which is a concern.
This draft describes a community-based convention which might be used
to limit propagation of long prefixes to those ASes where they are
required.
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1. Introduction
There is a requirement for some multi-homed sites to influence the
path selected by autonomous systems beyond those that are immediately
adjacent.
One of the few generic mechanisms available is to deaggregate and
advertise long component prefixes to the network, since there can be
some confidence that the longest prefix will be used, regardless of
other local policy such as local preference. Most ASes exhibit
liberal route import policy with respect to prefix length, which
facilitates this technique.
Unfortunately, although the deaggregated prefix set may be required
to be installed in only a few targeted ASes for the aims of the
origin to be achieved, there is no reliable mechanism to limit the
propagation of the prefixes. This contributes to prefix bloat in the
default-free zone, which is a concern.
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2. Terminology
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 [1].
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3. Edge Policy Propagation Convention
3.1 At the Edge
An edge site deaggregates its advertisements according to the
required fine-grain policy. Aggregate prefixes are advertised as
normal; long prefixes are advertised tagged with community attributes
which define their scope:
EPPC_ALLOW:0 -- this prefix should be handled according to the
convention described in this document.
EPPC_ALLOW:A -- it is desirable that this prefix should propagate to
AS A. Multiple communities of the form EPPC_ALLOW:A may be
present to define propagation scope.
EPPC_ALLOW is some 16-bit quantity, well-known amongst the community
of operators who cooperate according to this convention. It should
be chosen from the private-use range of ASNs specified in [2].
3.2 Towards the Other Edge
ASes which support this convention must include additional clauses in
their advertisement policy to all neighbour ASes, as follows:
o When announcing prefixes to AS A: if the community attributes
EPPC_ALLOW:0 and EPPC_ALLOW:A are both present, then delete the
EPPC_ALLOW:A community, and then advertise the prefix.
o If the community attribute EPPC_ALLOW:0 is present, and
EPPC_ALLOW:A is not present, then suppress the advertisement.
An implementation of this policy for cisco and Juniper routers can be
found in Section 5.
3.3 Related Work
A similar approach based on a new BGP attribute is described in the
companion document draft-black-prop-list-00.txt.
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4. Example
Consider the following internetwork:
+------+
+-----+ AS B +-----+
| +---+--+ |
+---+--+ | +--+---+ +------+
| AS A | | | AS D +----+ AS F |
+---+--+ | +--+---+ +------+
| +---+--+ |
+-----+ AS C +-----+
+---+--+
|
+---+--+
| AS E |
+------+
AS A requires a particular set of prefixes to propagate within AS B,
D and F, but not elsewhere.
AS A therefore advertises the set of prefixes with the community
attributes EPPC_ALLOW:0, EPPC_ALLOW:D and EPPC_ALLOW:F.
AS B suppresses the advertisements towards AS C, since the community
attribute APPC_ALLOW:0 is present without APPC_ALLOW:C. AS B
advertises the prefixes towards AS D, removing APPC_ALLOW:D before
sending.
Similarly, AS D suppresses the advertisements towards AS C, and
advertises the prefixes towards AS F after removing APPC_ALLOW:E.
AS F suppresses the advertisements towards all peers, since
APPC_ALLOW:0 is present without any other APPC_ALLOW:*.
The result is that the long prefix routes only propagate to AS B, AS
D and AS F, in accordance with the policy specified by AS A.
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5. Sample Implementations
5.1 Juniper JUNOS
policy-options {
/* EPPC policy towards A. */
policy-statement eppc-to-A {
/* If this route is an EPPC route and is for A, then delete
* EPCC:A and continue.
*/
term is-A {
from community [ comm-eppc-zero comm-eppc-A ];
then {
community delete comm-eppc-A;
next policy;
}
}
/* If this route is an EPPC route, then drop it. */
term is-eppc {
from community comm-eppc-zero;
then reject;
}
/* Otherwise continue as normal. */
then next policy
}
/* The EPPC:0 community */
community comm-eppc-zero members EPPC_ALLOW:0;
/* The EPPC:A community meaning send to AS A */
community comm-eppc-A members EPPC_ALLOW:A;
}
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5.2 cisco IOS
ip community-list EPPC-0 permit EPPC_ALLOW:0
ip community-list EPPC-200 permit EPPC_ALLOW:200
!
route-map AS200 permit 10
match comm-list EPPC-0 EPPC-200
set comm-list EPPC-0 delete
!
route-map AS200 deny 20
match comm-list EPPC-0
!
route-map AS200 permit 30
!
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6. Acknowledgements
Thanks to Andrew Partan for excellent envelope-scribbling and for the
Juniper config fragment.
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Hawkinson, J. and T. Bates, "Guidelines for creation, selection,
and registration of an Autonomous System (AS)", RFC 1930, March
1996.
[3] Huston, G., "Analyzing the Internet's BGP Routing Table",
January 2001.
Author's Address
Joe Abley
Metromedia Fiber Network Inc.
2204 Pembroke Court
Burlington, ON L7P 3X8
Canada
Phone: +1 905 319 9064
EMail: jabley at mfnx.net
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Internet-Draft Edge Policy Propagation Control March 2001
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Abley Expires September 21, 2001 [Page 11]
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