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. Plea 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 Rerved.
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, ). 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 (expresd as an IPv4
address), (b) AGGREGATOR (contains an IPv4 address), and (c) NLRI
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(expresd 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 ud, 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 Bates, et. al. Standards Track [Page 1]
individual Network Layer protocols this document us Address Family, as defined in [RFC1700].
One could further obrve that the next hop information (the
information provided by the NEXT_HOP attribute) is meaningful (and
necessary) only in conjunction with the advertiments of reachable
destinations - in conjunction with the advertiments of unreachable destinations (withdrawing routes from rvice) the next hop
information is meaningless. This suggests that the advertiment of
reachable destinations should be grouped with the advertiment of
the next hop to be ud for the destinations, and that the
advertiment of reachable destinations should be gregated from the advertiment of unreachable destinations.
To provide backward compatibility, as well as to simplify
introduction of the multiprotocol capabilities into BGP-4 this
document us two new attributes, Multiprotocol Reachable NLRI
(MP_REACH_NLRI), and Multiprotocol Unreachable NLRI
(MP_UNREACH_NLRI). The first one (MP_REACH_NLRI) is ud to carry the t of reachable de
stinations together with the next hop information to be ud for forwarding to the destinations. The cond one
(MP_UNREACH_NLRI) is ud to carry the t of unreachable
destinations. Both of the 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 the 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 ud for the following purpos:
(a) to adverti a feasible route to a peer
(b) to permit a router to adverti the Network Layer address of
the router that should be ud 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:
Bates, et. al. Standards Track [Page 2]
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subquent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Length of Next Hop Network Address (1 octet) |
+---------------------------------------------------------+
| Network Address of Next Hop (variable) |
扑灭英文+---------------------------------------------------------+
| Number of SNPAs (1 octet) |
+---------------------------------------------------------+
| Length of first SNPA(1 octet) |
+---------------------------------------------------------+
| First SNPA (variable) |
+---------------------------------------------------------+
| Length of cond SNPA (1 octet) |
+---------------------------------------------------------+
| Second SNPA (variable) |
+---------------------------------------------------------+
| ... |
+---------------------------------------------------------+
| Length of Last SNPA (1 octet) |
+---------------------------------------------------------+
| Last SNPA (variable) |
+---------------------------------------------------------+
| Network Layer Reachability Information (variable) |
a level学校+---------------------------------------------------------+
The u and meaning of the fields are as follows:口头表达能力训练
Address Family Identifier:
This field carries the identity of the Network Layer protocol
associated with the Network Address that follows. Prently
defined values for this field are specified in RFC1700 (e the Address Family Numbers ction).
英语口语学习Subquent 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 who value express the length of the
"Network Address of Next Hop" field as measured in octets Bates, et. al. Standards Track [Page 3]
Network Address of Next Hop:
quite的用法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 ud to indicate that no SNPAs are listed in this attribute.
Length of Nth SNPA:sonatas
A 1 octet field who value express the length of the "Nth
SNPA of Next Hop" field as measured in mi-octets
Nth SNPA of Next Hop:
A variable length field that contains an SNPA of the router
who 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 expresd in mi-octets; if the SNPA
contains an odd number of mi-octets, a value in this field
will be padded with a trailing all-zero mi-octet.
麦克马斯特大学
Network Layer Reachability Information:
A variable length field that lists NLRI for the feasible routes that are being advertid in this attribute. When the
Subquent Address Family Identifier field is t to one of the values defined in this document, each NLRI is encoded as
specified in the "NLRI encoding" ction 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 ud 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 u one of its own
interface address in the next hop component of the attribute,
provided the external peer to which the route is being advertid
shares a common subnet with the next hop address. This is known as a "first party" next hop. A BGP speaker can adverti 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
advertid shares a common subnet with the next hop address. This is known as a "third party" next hop information. A BGP speaker can
adverti any external peer router in the next hop component,
Bates, et. al. Standards Track [Page 4]
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 advertid shares a common subnet with the next hop
address. This is a cond form of "third party" next hop
autopilot
information.
Normally the next hop information is chon such that the shortest
available path will be taken. A BGP speaker must be able to support disabling advertiment of third party next hop information to handle imperfectly bridged media or for reasons of policy.
A BGP speaker must never adverti 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 itlf as the next hop.
When a BGP speaker advertis 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 nt the message. If that is not the ca, the local
system shall nd the NOTIFICATION message with Error Code UPDATE
Message Error, and the Error Subcode t to Malformed AS_PATH.
5. Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):
This is an optional non-transitive attribute that can be ud for the purpo of withdrawing multiple unfeasible routes from rvice.
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) |
+---------------------------------------------------------+
| Subquent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Withdrawn Routes (variable) |
upper+---------------------------------------------------------+
Bates, et. al. Standards Track [Page 5]
The u and the meaning of the fields are as follows:
Address Family Identifier:
This field carries the identity of the Network Layer protocol
associated with the NLRI that follows. Prently defined values for this field are specified in RFC1700 (e the Address Family Numbers ction).
Subquent 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 rvice. When the Subquent Address
Family Identifier field is t to one of the values defined in this document, each NLRI is encoded as specified in the "NLRI
encoding" ction 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>, who fields are described
below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The u and the meaning of the 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) address (with prefix,
itlf, of zero octets).
Bates, et. al. Standards Track [Page 6]
