Internet Draft
Network Working Group Tony Bates
Internet Draft Cisco Systems
Expiration Date: January 1998 Ravi Chandra
Cisco Systems
Dave Katz
Juniper Networks
Yakov Rekhter
Cisco Systems
Multiprotocol Extensions for BGP-4
draft-bates-bgp4-multiprotocol-03.txt
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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ftp.isi.edu (US West Coast).
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.
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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.
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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 , 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 Length (2 octets)|
+---------------------------------------------------------+
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| 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. Presently defined values for this field are:
1 - Network Layer Reachability Information used for unicast
forwarding
2 - Unicast Network Layer Reachability Information used for
multicast forwarding
3 - Unicast Network Layer Reachability Information used for
both unicast and multicast forwarding
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:
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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 Length:
This 2-octets unsigned integer indicates the total length of
the Network Layer Reachability Information field in octets.
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 (see above), 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
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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.
When an UPDATE message carries the MP_REACH_NLRI attribute, the
attribute shall be placed after all other attributes in the message.
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 , where each
triple is encoded as shown below:
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Unfeasible Routes Length (2 octets) |
+---------------------------------------------------------+
| Withdrawn Routes (variable) |
+---------------------------------------------------------+
The use and the meaning of these fields are as follows:
Address Family Identifier:
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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. Presently defined values for this field are:
1 - Network Layer Reachability Information used for unicast
forwarding
2 - Unicast Network Layer Reachability Information used for
multicast forwarding
3 - Unicast Network Layer Reachability Information used for
both unicast and multicast forwarding
Unfeasible Routes Length:
This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field in octets.
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 (see above), 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.
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6. NLRI encoding
The Network Layer Reachability information is encoded as one or more
2-tuples of the form , 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. IPv6 Specific Issues
A BGP speaker shall advertise to its peer in the Network Address of
Next Hop field the global IPv6 address of the next hop, followed by
the Link Local IPv6 address of the next hop if and only if the peer
shares a common subnet with the entity identified by the global IPv6
address carried in the Network Address of Next Hop field (the value
of the Length of Next Hop Network Address field shall be set to 32).
In all other cases a BGP speaker shall advertise to its peer in the
Network Address field only the global IPv6 address of the next hop
(the value of the Length of Network Address of Next Hop field shall
be set to 16). As a consequence, a BGP speaker that advertises a
route to an internal peer may modify the Network Address of Next Hop
field by removing the Link Local IPv6 address of the next hop.
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8. Security Considerations
Security issues are not discussed in this document.
9. Acknowledgements
To be supplied.
10. References
[BGP-4]
[IPv4]
[IPv6]
[RFC1700]
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
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