BGP Route Refresh: Route Refresh Capability for BGP-4
A BGP capability that lets a router ask its peer to resend its routes without resetting the session.
Chen, E., "Route Refresh Capability for BGP-4", RFC 2918, DOI 10.17487/RFC2918, September 2000, <https://www.rfc-editor.org/info/rfc2918>.
@misc{rfc2918,
series = {Request for Comments},
number = 2918,
howpublished = {RFC 2918},
publisher = {RFC Editor},
doi = {10.17487/RFC2918},
url = {https://www.rfc-editor.org/info/rfc2918},
author = {E. Chen},
title = {{Route Refresh Capability for BGP-4}},
pagetotal = 4,
year = 2000,
month = sep,
} RFC text
Contents
Network Working Group E. Chen
Request for Comments: 2918 Redback Networks
Category: Standards Track September 2000
Route Refresh Capability for BGP-4
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document defines a new Border Gateway Protocol (BGP) capability
termed 'Route Refresh Capability', which would allow the dynamic
exchange of route refresh request between BGP speakers and subsequent
re-advertisement of the respective Adj-RIB-Out. One possible
application of this capability is to facilitate non-disruptive
routing policy changes.
1. Introduction
Currently there does not exist a mechanism in BGP-4 [BGP-4] to
dynamically request a re-advertisement of the Adj-RIB-Out from a BGP
peer. When the inbound routing policy for a peer changes, all
prefixes from that peer must be somehow made available and then re-
examined against the new policy. To accomplish this, a commonly used
approach, known as 'soft-reconfiguration', is to store an unmodified
copy of all routes from that peer at all times, even though routing
policies do not change frequently (typically no more than a couple
times a day). Additional memory and CPU are required to maintain
these routes.
This document proposes an alternative solution that avoids the
additional maintenance cost. More specifically, it defines a new BGP
capability termed 'Route Refresh Capability', which would allow the
dynamic exchange of route refresh request between BGP speakers and
subsequent re-advertisement of the respective Adj-RIB-Out.
2. Route Refresh Capability
To advertise the Route Refresh Capability to a peer, a BGP speaker
uses BGP Capabilities Advertisement [BGP-CAP]. This capability is
advertised using the Capability code 2 and Capability length 0.
By advertising the Route Refresh Capability to a peer, a BGP speaker
conveys to the peer that the speaker is capable of receiving and
properly handling the ROUTE-REFRESH message (as defined in Section 3)
from the peer.
3. Route-REFRESH Message
The ROUTE-REFRESH message is a new BGP message type defined as
follows:
Type: 5 - ROUTE-REFRESH
Message Format: One <AFI, SAFI> encoded as
0 7 15 23 31
+-------+-------+-------+-------+
| AFI | Res. | SAFI |
+-------+-------+-------+-------+
The meaning, use and encoding of this <AFI, SAFI> field is the
same as defined in [BGP-MP, sect. 7]. More specifically,
AFI - Address Family Identifier (16 bit).
Res. - Reserved (8 bit) field. Should be set to 0 by the
sender and ignored by the receiver.
SAFI - Subsequent Address Family Identifier (8 bit).
4. Operation
A BGP speaker that is willing to receive the ROUTE-REFRESH message
from its peer should advertise the Route Refresh Capability to the
peer using BGP Capabilities advertisement [BGP-CAP].
A BGP speaker may send a ROUTE-REFRESH message to its peer only if it
has received the Route Refresh Capability from its peer. The <AFI,
SAFI> carried in such a message should be one of the <AFI, SAFI> that
the peer has advertised to the speaker at the session establishment
time via capability advertisement.
If a BGP speaker receives from its peer a ROUTE-REFRESH message with
the <AFI, SAFI> that the speaker didn't advertise to the peer at the
session establishment time via capability advertisement, the speaker
shall ignore such a message. Otherwise, the BGP speaker shall re-
advertise to that peer the Adj-RIB-Out of the <AFI, SAFI> carried in
the message, based on its outbound route filtering policy.
5. Security Considerations
This extension to BGP does not change the underlying security issues.
6. Acknowledgments
The concept of Route Refresh proposed is similar to the one used in
IDRP.
The author would like to thank Yakov Rekhter, Ravi Chandra, Srihari
Ramachandra and Bruce Cole for their review and comments.
7. References
[BGP-4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-
4)", RFC 1771, March 1995.
[BGP-MP] Bates, T., Chandra, R., Katz, D. and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.
[BGP-CAP] Chandra, R. and J. Scudder, "Capabilities Advertisement
with BGP-4", RFC 2842, May 2000.
8. Author's Address
Enke Chen
Redback Networks Inc.
350 Holger Way
San Jose, CA 95134
EMail: enke@redback.com
9. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1The Problem with Policy Changes
BGP-4 had no mechanism to dynamically ask a peer to re-advertise its Adj-RIB-Out. When the inbound routing policy for a peer changes, all prefixes from that peer have to be re-examined against the new policy.
The commonly used approach, soft-reconfiguration, stores an unmodified copy of every route from the peer at all times, even though policies change infrequently (the RFC says typically no more than a couple of times a day). That costs additional memory and CPU. RFC 2918 offers an alternative without that cost.
Instead of keeping a photocopy of every map just in case your rules change, you can ask for a fresh copy when you need one.
§2The Route Refresh Capability
A speaker advertises the capability using BGP Capabilities Advertisement, with Capability code 2 and Capability length 0. Advertising it tells the peer that the speaker can receive and properly handle the ROUTE-REFRESH message.
Before asking, each router says I know how to answer this kind of request.
§3The ROUTE-REFRESH Message
ROUTE-REFRESH is a new BGP message type, Type 5. Its body is one <AFI, SAFI> pair: a 16-bit Address Family Identifier, an 8-bit Reserved field and an 8-bit Subsequent Address Family Identifier. The meaning and encoding follow the Multiprotocol Extensions for BGP-4 (RFC 2858).
The Reserved field should be set to 0 by the sender and ignored by the receiver.
The request says which kind of map you want, and nothing else.
§4Operation
A speaker may send ROUTE-REFRESH only if it has received the capability from its peer, and the <AFI, SAFI> should be one that the peer advertised at session establishment through capability advertisement.
If a speaker receives a message with an <AFI, SAFI> it did not advertise, it ignores the message. Otherwise it re-advertises the Adj-RIB-Out for that <AFI, SAFI> to the peer, based on its outbound route filtering policy. The security section says the extension does not change the underlying security issues.
You may only ask if the other router said it can answer, and only for map types it agreed to share.