BGP-4 Multiprotocol: Multiprotocol Extensions for BGP-4
Routing protocol extensions for multiple address families.
Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 2283, DOI 10.17487/RFC2283, February 1998, <https://www.rfc-editor.org/info/rfc2283>.
@misc{rfc2283,
series = {Request for Comments},
number = 2283,
howpublished = {RFC 2283},
publisher = {RFC Editor},
doi = {10.17487/RFC2283},
url = {https://www.rfc-editor.org/info/rfc2283},
author = {T. Bates and R. Chandra and D. Katz and Y. Rekhter},
title = {{Multiprotocol Extensions for BGP-4}},
pagetotal = 9,
year = 1998,
month = feb,
} RFC text
Contents
Network Working Group T. Bates
Request for Comments: 2283 Cisco Systems
Category: Standards Track R. Chandra
Cisco Systems
D. Katz
Juniper Networks
Y. Rekhter
Cisco Systems
February 1998
Multiprotocol Extensions 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 (1998). All Rights Reserved.
2. Abstract
Currently BGP-4 [BGP-4] is capable of carrying routing information
only for IPv4 [IPv4]. This document defines extensions to BGP-4 to
enable it to carry routing information for multiple Network Layer
protocols (e.g., IPv6, IPX, etc...). The extensions are backward
compatible - a router that supports the extensions can interoperate
with a router that doesn't support the extensions.
3. Overview
The only three pieces of information carried by BGP-4 that are IPv4
specific are (a) the NEXT_HOP attribute (expressed as an IPv4
address), (b) AGGREGATOR (contains an IPv4 address), and (c) NLRI
(expressed as IPv4 address prefixes). This document assumes that any
BGP speaker (including the one that supports multiprotocol
capabilities defined in this document) has to have an IPv4 address
(which will be used, among other things, in the AGGREGATOR
attribute). Therefore, to enable BGP-4 to support routing for
multiple Network Layer protocols the only two things that have to be
added to BGP-4 are (a) the ability to associate a particular Network
Layer protocol with the next hop information, and (b) the ability to
associated a particular Network Layer protocol with NLRI. To identify
individual Network Layer protocols this document uses Address Family,
as defined in [RFC1700].
One could further observe that the next hop information (the
information provided by the NEXT_HOP attribute) is meaningful (and
necessary) only in conjunction with the advertisements of reachable
destinations - in conjunction with the advertisements of unreachable
destinations (withdrawing routes from service) the next hop
information is meaningless. This suggests that the advertisement of
reachable destinations should be grouped with the advertisement of
the next hop to be used for these destinations, and that the
advertisement of reachable destinations should be segregated from the
advertisement of unreachable destinations.
To provide backward compatibility, as well as to simplify
introduction of the multiprotocol capabilities into BGP-4 this
document uses two new attributes, Multiprotocol Reachable NLRI
(MP_REACH_NLRI), and Multiprotocol Unreachable NLRI
(MP_UNREACH_NLRI). The first one (MP_REACH_NLRI) is used to carry the
set of reachable destinations together with the next hop information
to be used for forwarding to these destinations. The second one
(MP_UNREACH_NLRI) is used to carry the set of unreachable
destinations. Both of these attributes are optional and non-
transitive. This way a BGP speaker that doesn't support the
multiprotocol capabilities will just ignore the information carried
in these attributes, and will not pass it to other BGP speakers.
4. Multiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code 14):
This is an optional non-transitive attribute that can be used for the
following purposes:
(a) to advertise a feasible route to a peer
(b) to permit a router to advertise the Network Layer address of
the router that should be used as the next hop to the destinations
listed in the Network Layer Reachability Information field of the
MP_NLRI attribute.
(c) to allow a given router to report some or all of the
Subnetwork Points of Attachment (SNPAs) that exist within the
local system
The attribute contains one or more triples <Address Family
Information, Next Hop Information, Network Layer Reachability
Information>, where each triple is encoded as shown below:
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Length of Next Hop Network Address (1 octet) |
+---------------------------------------------------------+
| Network Address of Next Hop (variable) |
+---------------------------------------------------------+
| Number of SNPAs (1 octet) |
+---------------------------------------------------------+
| Length of first SNPA(1 octet) |
+---------------------------------------------------------+
| First SNPA (variable) |
+---------------------------------------------------------+
| Length of second SNPA (1 octet) |
+---------------------------------------------------------+
| Second SNPA (variable) |
+---------------------------------------------------------+
| ... |
+---------------------------------------------------------+
| Length of Last SNPA (1 octet) |
+---------------------------------------------------------+
| Last SNPA (variable) |
+---------------------------------------------------------+
| Network Layer Reachability Information (variable) |
+---------------------------------------------------------+
The use and meaning of these fields are as follows:
Address Family Identifier:
This field carries the identity of the Network Layer protocol
associated with the Network Address that follows. Presently
defined values for this field are specified in RFC1700 (see the
Address Family Numbers section).
Subsequent Address Family Identifier:
This field provides additional information about the type of
the Network Layer Reachability Information carried in the
attribute.
Length of Next Hop Network Address:
A 1 octet field whose value expresses the length of the
"Network Address of Next Hop" field as measured in octets
Network Address of Next Hop:
A variable length field that contains the Network Address of
the next router on the path to the destination system
Number of SNPAs:
A 1 octet field which contains the number of distinct SNPAs to
be listed in the following fields. The value 0 may be used to
indicate that no SNPAs are listed in this attribute.
Length of Nth SNPA:
A 1 octet field whose value expresses the length of the "Nth
SNPA of Next Hop" field as measured in semi-octets
Nth SNPA of Next Hop:
A variable length field that contains an SNPA of the router
whose Network Address is contained in the "Network Address of
Next Hop" field. The field length is an integral number of
octets in length, namely the rounded-up integer value of one
half the SNPA length expressed in semi-octets; if the SNPA
contains an odd number of semi-octets, a value in this field
will be padded with a trailing all-zero semi-octet.
Network Layer Reachability Information:
A variable length field that lists NLRI for the feasible routes
that are being advertised in this attribute. When the
Subsequent Address Family Identifier field is set to one of the
values defined in this document, each NLRI is encoded as
specified in the "NLRI encoding" section of this document.
The next hop information carried in the MP_REACH_NLRI path attribute
defines the Network Layer address of the border router that should be
used as the next hop to the destinations listed in the MP_NLRI
attribute in the UPDATE message. When advertising a MP_REACH_NLRI
attribute to an external peer, a router may use one of its own
interface addresses in the next hop component of the attribute,
provided the external peer to which the route is being advertised
shares a common subnet with the next hop address. This is known as a
"first party" next hop. A BGP speaker can advertise to an external
peer an interface of any internal peer router in the next hop
component, provided the external peer to which the route is being
advertised shares a common subnet with the next hop address. This is
known as a "third party" next hop information. A BGP speaker can
advertise any external peer router in the next hop component,
provided that the Network Layer address of this border router was
learned from an external peer, and the external peer to which the
route is being advertised shares a common subnet with the next hop
address. This is a second form of "third party" next hop
information.
Normally the next hop information is chosen such that the shortest
available path will be taken. A BGP speaker must be able to support
disabling advertisement of third party next hop information to handle
imperfectly bridged media or for reasons of policy.
A BGP speaker must never advertise an address of a peer to that peer
as a next hop, for a route that the speaker is originating. A BGP
speaker must never install a route with itself as the next hop.
When a BGP speaker advertises the route to an internal peer, the
advertising speaker should not modify the next hop information
associated with the route. When a BGP speaker receives the route via
an internal link, it may forward packets to the next hop address if
the address contained in the attribute is on a common subnet with the
local and remote BGP speakers.
An UPDATE message that carries the MP_REACH_NLRI must also carry the
ORIGIN and the AS_PATH attributes (both in EBGP and in IBGP
exchanges). Moreover, in IBGP exchanges such a message must also
carry the LOCAL_PREF attribute. If such a message is received from an
external peer, the local system shall check whether the leftmost AS
in the AS_PATH attribute is equal to the autonomous system number of
the peer than sent the message. If that is not the case, the local
system shall send the NOTIFICATION message with Error Code UPDATE
Message Error, and the Error Subcode set to Malformed AS_PATH.
5. Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):
This is an optional non-transitive attribute that can be used for the
purpose of withdrawing multiple unfeasible routes from service.
The attribute contains one or more triples <Address Family
Information, Unfeasible Routes Length, Withdrawn Routes>, where each
triple is encoded as shown below:
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Withdrawn Routes (variable) |
+---------------------------------------------------------+
The use and the meaning of these fields are as follows:
Address Family Identifier:
This field carries the identity of the Network Layer protocol
associated with the NLRI that follows. Presently defined values
for this field are specified in RFC1700 (see the Address Family
Numbers section).
Subsequent Address Family Identifier:
This field provides additional information about the type of
the Network Layer Reachability Information carried in the
attribute.
Withdrawn Routes:
A variable length field that lists NLRI for the routes that are
being withdrawn from service. When the Subsequent Address
Family Identifier field is set to one of the values defined in
this document, each NLRI is encoded as specified in the "NLRI
encoding" section of this document.
An UPDATE message that contains the MP_UNREACH_NLRI is not required
to carry any other path attributes.
6. NLRI encoding
The Network Layer Reachability information is encoded as one or more
2-tuples of the form <length, prefix>, whose fields are described
below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the address
prefix. A length of zero indicates a prefix that matches all
(as specified by the address family) addresses (with prefix,
itself, of zero octets).
b) Prefix:
The Prefix field contains address prefixes followed by enough
trailing bits to make the end of the field fall on an octet
boundary. Note that the value of trailing bits is irrelevant.
7. Subsequent Address Family Identifier
This document defines the following values for the Subsequent Address
Family Identifier field carried in the MP_REACH_NLRI and
MP_UNREACH_NLRI attributes:
1 - Network Layer Reachability Information used for unicast
forwarding
2 - Network Layer Reachability Information used for multicast
forwarding
3 - Network Layer Reachability Information used for both unicast
and multicast forwarding
This document reserves values 128-255 for vendor-specific
applications.
This document reserves value 0.
Subsequent Address Family Identifiers (other than those reserved for
vendor specific use) are assigned only by the IETF consensus process
and IESG approval.
8. Security Considerations
This extension to BGP does not change the underlying security issues.
9. Acknowledgements
The authors would like to thank members of the IDR Working Group for
their review and comments.
10. References
[BGP-4] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC1700] Reynolds, J., and J. Postel, "Assigned Numbers," STD 2,
RFC 1700, October 1994. (see also
http://www.iana.org/iana/assignments.html)
11. Author Information
Tony Bates
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: tbates@cisco.com
Ravi Chandra
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: rchandra@cisco.com
Dave Katz
Juniper Networks, Inc.
3260 Jay St.
Santa Clara, CA 95054
EMail: dkatz@jnx.com
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: yakov@cisco.com
12. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
AI ELI5
On this page
§1The Limitation
BGP-4 (Border Gateway Protocol version 4, defined in RFC 1771) is the routing protocol the Internet uses to exchange reachability information between autonomous systems. However, it was designed exclusively for IPv4: the original 32-bit Internet address format.
As IPv6 and other protocols emerged, BGP-4 routers could not advertise routes for them. A router had no way to tell its peers "I can reach this IPv6 address" or "I can reach this IPX network." The protocol needed to grow without breaking existing deployments.
BGP is how routers tell each other where to find IP addresses. But it only knew IPv4 addresses. When IPv6 and other protocols arrived, it was stuck.
§2The Multiprotocol Approach
The RFC identifies exactly three pieces of BGP-4 that are IPv4-specific: the NEXT_HOP attribute (which holds the IPv4 address of the next router), the AGGREGATOR attribute (which also contains IPv4 addresses), and NLRI (Network Layer Reachability Information, which lists the IPv4 address prefixes being advertised). To support multiple network layer protocols, BGP-4 needs only two additions:
- A way to associate a network layer protocol with the next hop information.
- A way to associate a network layer protocol with NLRI.
Each protocol is identified using an Address Family Identifier (from RFC 1700). To maintain backward compatibility, the new information is carried in optional, non-transitive attributes. A router that does not understand multiprotocol extensions simply ignores these new attributes and passes them along unchanged.
Routers only need to learn two new skills: remembering what kind of address a route is for (IPv4 vs. IPv6) and how to say "I don't know this new thing" without breaking.
| Field | BGP-4 Role | Multiprotocol Solution |
|---|---|---|
| NEXT_HOP attribute | IPv4 address of the next router | Wrapped in MP_REACH_NLRI with Address Family |
| NLRI | IPv4 address prefixes | Carried in MP_REACH_NLRI labeled by Address Family |
| AGGREGATOR | IPv4 address of aggregating router | Still IPv4; not changed |
§3The New Attributes
Two new optional, non-transitive attributes carry multiprotocol information:
MP_REACH_NLRI (Type Code 14) advertises reachable routes for a particular network layer protocol. It contains a 2-octet Address Family Identifier, a 1-octet Subsequent Address Family Identifier (providing additional detail such as whether the routes are for unicast, multicast, or both), the network address of the next hop, optional subnetwork attachment information (SNPAs), and the network layer reachability information for destinations reachable via that next hop.
MP_UNREACH_NLRI (Type Code 15) withdraws (removes from service) routes for a particular protocol. It contains the Address Family Identifier, the Subsequent Address Family Identifier, and the withdrawn routes. Unlike MP_REACH_NLRI, it requires no next hop information because withdrawn routes need only be removed.
Because these attributes are optional and non-transitive, a BGP speaker that receives an UPDATE message carrying them but does not understand them simply ignores them and does not forward them to other peers.
The two new message types are labeled with what kind of address they are for, just like a mailbox is labeled with a house number and street.
§4Interoperability and Rules
Routers using multiprotocol extensions follow strict rules to ensure correct operation. For example, a BGP speaker must never advertise its own address as the next hop for a route it originates, and must never install a route with itself as the next hop. When advertising a route to an external peer, the next hop can be the router's own interface (a "first party" next hop), an internal peer's interface (a "third party" next hop), or an external peer's interface (another form of third party next hop).
Any UPDATE message carrying MP_REACH_NLRI must also carry the ORIGIN and AS_PATH attributes, and in internal BGP (IBGP) exchanges must also include LOCAL_PREF. If such a message comes from an external peer, the router checks that the leftmost AS in the AS_PATH matches the peer's autonomous system number; if not, it sends a NOTIFICATION message with an error.
When routers talk to each other about routes, they follow traffic rules so nobody gets confused about where packets should go.
§5Backward Compatibility
The design ensures that multiprotocol-enabled routers can coexist with standard BGP-4 routers. A router that does not support the multiprotocol extensions will ignore MP_REACH_NLRI and MP_UNREACH_NLRI attributes in received UPDATE messages, because they are marked optional and non-transitive. The router continues to function normally, processing only the standard IPv4 NLRI and next hop information.
This one-way compatibility means a deployment can gradually introduce multiprotocol support. New routers can advertise IPv6 routes to each other while still exchanging IPv4 routes with legacy routers. The RFC reserves values 128-255 in the Subsequent Address Family Identifier field for vendor-specific uses, and requires IETF consensus and IESG approval for assigning other new values.
Old routers simply skip the new multiprotocol labels and keep working, so you can upgrade some routers without breaking the old ones.