All MRT format messages have a common header which includes a
timestamp, Type, Subtype, and length field. The header is followed
by a message field. The basic MRT format is illustrated below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Header Field Descriptions:
Timestamp:
Time in seconds since 1 January 1970 00:00:00 UTC
Type:
A 2-octet field that indicates the Type of information
contained in the message field. Types 1 through 5 are used for
MRT control information while Types 6 and higher are used for
routing information.
Subtype:
A 2-octet message Subtype field
Length:
A 4-octet message length field. The length does not include
the header.
4. MRT Control Types
The MRT format defines five Control Type messages. These messages
are using to relay the current state of MRT message source. The
message field MAY contain an OPTIONAL ASCII text string for
diagnostic purposes. These control messages are unidirectional in
nature and there is no form of an acknowledgment or response from the
receiver to the sender. The Subtype field is unused for these Types
and SHOULD be set to 0.
The MRT Control Types are defined below:
0 NULL
1 START
2 DIE
3 I_AM_DEAD
4 PEER_DOWN
4.1. NULL Type
The NULL Type message causes no operation, A sender may wish to send
these for synchronization or keep-alive purposes.
4.2. START Type
The START Type indicates a sender is about to begin sending MRT
messages
4.3. DIE Type
A DIE Type signals that the receiver should shut down.
4.4. I_AM_DEAD Type
A I_AM_DEAD indicates that the sender is shutting down.
4.5. PEER_DOWN Type
A PEER_DOWN is sent when the sender's peer is down. In practice, a
sender will likely have multiple peers. It is RECOMMENDED that the
sender use the Message field to convey the IP address of the peer
represented in US-ASCII.
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5. MRT Routing Information Types
The following Types are currently defined for the MRT format. Types
5-12 were defined in the initial MRT Toolkit package. The BGP4MP
Type, number 16, was initially defined in the Zebra routing software
package. The ISIS Type was initially defined in the Sprint Labs
Python Routing Toolkit (PyRT).
5 BGP *DEPRECATED*
6 RIP
7 IDRP *DEPRECATED*
8 RIPNG
9 BGP4PLUS *DEPRECATED*
10 BGP4PLUS_01 *DEPRECATED*
11 OSPF
12 TABLE_DUMP
13 TABLE_DUMP_V2
16 BGP4MP
17 BGP4MP_ET
32 ISIS
33 ISIS_ET
48 OSPFv3
49 OSPFv3_ET
5.1. BGP Type
The BGP Type indicates the Message field contains BGP routing
information. The BGP routing protocol is defined in RFC 4271
[RFC4271]. The information in the message is dependent on the
Subtype value. The BGP Type and all associated Subtypes are
considered to be DEPRECATED by the BGP4MP Type.
The following BGP Subtypes are defined for the MRT BGP Type.
0 BGP_NULL
1 BGP_UPDATE
2 BGP_PREF_UPDATE
3 BGP_STATE_CHANGE
4 BGP_SYNC
5 BGP_OPEN
6 BGP_NOTIFY
7 BGP_KEEPALIVE
5.1.1. BGP_NULL Subtype
The BGP_NULL Subtype is a reserved Subtype.
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5.1.2. BGP_UPDATE Subtype
The BGP_UPDATE Subtype is used to encode BGP UPDATE messages. The
format of the MRT Message field for this Subtype is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP UPDATE Contents (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The BGP UPDATE contents include the entire BGP UPDATE message which
follows the BGP Message Header. The BGP Message Header itself is not
included.
5.1.3. BGP_PREF_UPDATE Subtype
The BGP_PREF_UPDATE Subtype is not defined.
5.1.4. BGP_STATE_CHANGE Subtype
The BGP_STATE_CHANGE Subtype is used to record changes in the BGP
finite state machine. These FSM states and their numeric encodings
are defined in RFC 4271 [RFC4271], Appendix 1. Both the old state
value and the new state value are encoded as 2-octet numbers. The
format of the MRT Message field is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Old State | New State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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5.1.5. BGP_SYNC Subtype
The BGP_SYNC Subtype is used to indicate a File Name where BGP Table
Dump messages should be recorded. The View # corresponds to the View
# provided in the TABLE_DUMP Type messages. The following format
applies to this Subtype:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| View # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| File Name... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The File Name is terminated with a NULL (0) character.
5.1.6. BGP_OPEN Subtype
The BGP_OPEN Subtype is used to encode BGP OPEN messages. The format
of the MRT Message field for this Subtype is the same as the
BGP_UPDATE, however, the last field contains the contents of the BGP
OPEN message.
5.1.7. BGP_NOTIFY Subtype
The BGP_NOTIFY Subtype is used to encode BGP NOTIFICATION messages.
The format of the MRT Message field for this Subtype is the same as
the BGP_UPDATE, however, the last field contains the contents of the
BGP NOTIFICATION message.
5.1.8. BGP_KEEPALIVE Subtype
The BGP_KEEPALIVE Subtype is used to encode BGP KEEPALIVE messages.
The format of the MRT Message field for this Subtype is the same as
the BGP_UPDATE, however, the last field contains no information.
5.2. RIP Type
The RIP Type is used to export RIP protocol packets as defined in RFC
1058 [RFC1058]. The Subtype field is currently reserved for this
Type and SHOULD be set to 0.
The format of the MRT Message field for the RIP Type is as follows:
Message:
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A variable length message. The contents of this field are
context dependent on the Type and Subtype fields.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIP Message Contents (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.3. IDRP Type
The IDRP Type is used to export Inter-Domain-Routing Protocol (IDRP)
protocol information as defined in the ISO/IEC 10747 standard. The
Subtype field is unused. This Type is deprecated due to lack of
deployment of IDRP.
5.4. RIPNG Type
The RIPNG Type is used to export RIPNG protocol packets as defined in
RFC 2080 [RFC2080]. The Subtype field is currently reserved for this
Type and SHOULD be set to 0.
The format of the MRT Message field for the RIPNG Type is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIPNG Message Contents (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.5. BGP4PLUS and BGP4PLUS_01 Types
The BGP4PLUS and BGP4PLUS_01 Types were defined to support IPv6 BGP
routing information. The BGP4PLUS Type was specified based on the
initial Internet Draft for Multiprotocol Extensions to BGP-4. The
BGP4PLUS_01 Type was specified to correspond to the -01 revision of
this Internet Draft. The two Types share the same definitions in
terms of their MRT format specifications.
The Subtype field definitions are shared with the BGP Type, however,
the address fields in the BGP_UPDATE, BGP_OPEN, BGP_NOTIFY,
BGP_KEEPALIVE, and BGP_STATE_CHANGE Subtype messages are extended to
16 octets for IPv6 addresses. As with the BGP Type, the BGP4PLUS and
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BGP4PLUS_01 Types are deprecated as they superseded by the BGP4MP
Type.
5.6. OSPF Type
This Type supports the OSPF Protocol as defined in RFC 2328
[RFC2328]. The Subtype field may contain two possible values:
0 OSPF_STATE_CHANGE
1 OSPF_LSA_UPDATE
The format of the MRT Message field for the OSPF Type is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OSPF Message Contents (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.7. TABLE_DUMP Type
The TABLE_DUMP Type is used to encode the contents of a BGP Routing
Information Base (RIB). Each RIB entry is encoded in a distinct
sequential MRT record. The Subtype field is used to encode whether
the RIB entry contains IPv4 or IPv6 addresses. There are currently
two possible values for the Subtype as shown below.
1 AFI_IPv4
2 AFI_IPv6
The format of the TABLE_DUMP Type is illustrated below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| View # | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originated Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer AS | Attribute Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Attribute... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The View field is normally 0 and is intended for cases where an
implementation may have multiple RIB views (such as a route server).
The Sequence field is a simple incremental counter for each RIB
entry. A typical RIB dump will exceed the 16-bit bounds of this
counter and implementation should simply wrap back to zero and
continue incrementing the counter in such cases.
The Prefix field contains the IP address of a particular routing RIB
entry. The size of this field is dependent on the value of the
Subtype for this message. For AFI_IPv4, this field is 4 octets, for
AFI_IPv6, it is 16 octets in length. The Prefix Length field
indicates the length in bits of the prefix mask for the preceding
Prefix field.
The Status octet is not used in the TABLE_DUMP Type and SHOULD be set
to 1.
The Originated Time contains the 4-octet time at which this prefix
was heard. The value represents the time in seconds since 1 January
1970 00:00:00 UTC.
The Peer IP field is the IP address of the peer which provided the
update for this RIB entry. As with the Prefix field, the size of
this field is dependent on the Subtype. AFI_IPv4 indicates a 4 octet
field and an IPv4 address, while a Subtype of AFI_IPv6 requires a 16
octet field and an IPv6 address. The Peer AS field contains the AS
number of the peer.
Attribute length is the length of Attribute field and is 2-octets.
The Attribute field contains the attribute information for the RIB
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entry.
5.8. TABLE_DUMP_V2 Type
The TABLE_DUMP_V2 type updates the TABLE_DUMP type to include 32-bit
ASN support and full support for BGP Multiprotocol extensions. It
also improves upon the space efficiency of the TABLE_DUMP type by
employing an index table for peers and permitting a single MRT record
per NLRI entry. The following subtypes are used with the
TABLE_DUMP_V2 type.
1 INDEX_TABLE
2 RIB_IPV4_UNICAST
3 RIB_IPV4_MULTICAST
4 RIB_IPV6_UNICAST
5 RIB_IPV6_MULTICAST
6 RIB_GENERIC
An initial INDEX_TABLE MRT record provides the BGP ID of the
collector, an optional view name, and a list of indexed peers.
Following the INDEX_TABLE MRT record, a series of MRT records are
used to encode RIB table entries. The header of the INDEX_TABLE
Subtype is shown below. The View Name is optional and if not
present, the View Name Length MUST be set to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Collector BGP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| View Name Length | View Name (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Peer Type field is a bit field which encodes the type of the AS
and IP address as follows:
Bit 0 - unset for IPv4 Peer IP address, set of IPv6
Bit 1 - unset when Peer AS field is 16 bits, set when it's 32 bits
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer BGP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer AS (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Peer Type, Peer BGP ID, Peer IP, and Peer AS fields are repeated
as indicated by the Peer Count field. The position of the Peer in
the INDEX_TABLE is used as an index in the subsequent TABLE_DUMP_V2
MRT records. The index number begins with 0.
The records which follow the INDEX_TABLE record constitute the RIB
entries and include a header which specify a sequence number, NLRI,
and a count of the number of RIB entries which follow.
The RIB_IPV4_UNICAST, RIB_IPV4_MULTICAST, RIB_IPV6_UNICAST, and
RIB_IPV6_MULTICAST headers are shown below. The Prefix Length and
Prefix fields are encoded in the same manner as the BGP NLRI encoding
for IPV4 and IPV6 prefixes. Namely, 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Entry Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RIB_GENERIC header is shown below. It includes Address Family
Identifier (AFI), Subsequent AFI and a single NLRI entry. The NLRI
information is specific to the AFI and SAFI values. An
implementation which does not recognize particular AFI and SAFI
values SHOULD discard the remainder of the MRT record.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family Identifier |Subsequent AFI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Layer Reachability Information (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Entry Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RIB entry headers are followed by a series of RIB entries which
are repeated Entry Count times. These entries share a common format
as shown below. They include a Peer Index from the INDEX_TABLE MRT
record, an originated time for the RIB entry, and the BGP path
attribute length and attributes encoded as provided in a BGP Update
message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originated Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Attributes... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There is one exception to the encoding of BGP attributes for the BGP
MP_REACH_NLRI attribute (BGP Type Code 14) [RFC 4760]. Since the
AFI, SAFI, and NLRI information is already encoded in the
MULTIPROTOCOL header, only the Next Hop Address Length and Next Hop
Address fields are included. The Reserved field is omitted. The
attribute length is also adjusted to reflect only the length of the
Next Hop Address Length and Next Hop Address fields.
5.9. BGP4MP Type
This Type was initially defined in the Zebra software package for the
BGP protocol with multiprotocol extension support as defined by RFC
4760 [RFC4760]. It supersedes the BGP, BGP4PLUS, BGP4PLUS_01 Types.
The BGP4MP Type has four Subtypes which are defined as follows:
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0 BGP4MP_STATE_CHANGE
1 BGP4MP_MESSAGE
2 BGP4MP_ENTRY *DEPRECATED*
3 BGP4MP_SNAPSHOT
4 BGP4MP_STATE_CHANGE_AS4
5 BGP4MP_MESSAGE_AS4
5.9.1. BGP4MP_STATE_CHANGE Subtype
This record is used to encode state changes in the BGP finite state
machine. As with the BGP_STATE_CHANGE Subtype, the BGP FSM states
are encoded in the Old State and New State fields to indicate the
previous and current state. The format is illustrated below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source AS number | Destination AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Index | Address Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Old State | New State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
While BGP4MP_STATE_CHANGE message is similar to the BGP_STATE_CHANGE
message, it also includes interface index and Address Family fields.
As with the BGP_STATE_CHANGE message, the FSM states and their
numeric encodings are defined in RFC 4271 [RFC4271], Appendix 1. The
interface index provides the interface number of the peering session.
The index value is OPTIONAL and MAY be zero if unknown or
unsupported. The Address Family indicates what types of addresses
are in the the address fields. At present, the following AFI Types
are supported:
1 AFI_IPv4
2 AFI_IPv6
5.9.2. BGP4MP_MESSAGE Subtype
This Subtype is used to encode BGP Messages. It is similar to the
BGP_UPDATE Subtype, except that is can be used to encode any Type of
message (not just BGP UPDATES). In order to determine the BGP
message Type, the entire BGP message, including the BGP header, is
included in the BGP Message field. The BGP4MP_MESSAGE fields are
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shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source AS number | Destination AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Index | Address Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Message... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The interface index provides the interface number of the peering
session. The index value is OPTIONAL and MAY be zero if unknown or
unsupported. The Address Family indicates what types of addresses
are in the the subsequent address fields. At present, the following
AFI Types are supported:
1 AFI_IPv4
2 AFI_IPv6
Note that the Address Family value only applies to the IP addresses
contained in the MRT header. The BGP4MP_MESSAGE Subtype is otherwise
transparent to the contents of the actual message which may contain
any valid AFI/SAFI values. Only one BGP message may be encoded in
the BGP4MP_MESSAGE Subtype.
5.9.3. BGP4MP_ENTRY Subtype
This Subtype is similar to the TABLE_DUMP Type and is used to record
RIB table entries. It extends the TABLE_DUMP Type to include true
multiprotocol support. However, this type does not support 32-bit AS
numbers and has not been widely implemented. This type is deprecated
in favor of the TABLE_DUMP_V2 which includes 32-bit AS number support
and a more compact format.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source AS number | Destination AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Index | Address Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| View # | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time last change |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | SAFI | Next-Hop-Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Attribute... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.9.4. BGP4MP_SNAPSHOT Subtype
This Subtype is used to indicate a filename containing BGP4MP_ENTRY
records. It is similar to the BGP_SYNC message Subtype and shares
the same fields.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| View # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| File Name... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.9.5. BGP4MP_MESSAGE_AS4 Subtype
This Subtype updates the BGP4MP_MESSAGE Subtype to support 32BIT
Autonomous System numbers. The BGP4MP_MESSAGE_AS4 fields are shown
below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination AS number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Index | Address Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Message... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.10. BGP4MP_ET
This Type was initially defined in the Sprint Labs Python Routing
Toolkit (PyRT). It extends the header field of the BGP4MP Type to
include a 32-bit microsecond timestamp field. The Subtypes and other
field definitions remain as defined for the BGP4MP Type. The 32-bit
microsecond timestamp immediately follows the length field in the
BGP4MP Type and precedes all other fields in the message. The header
modification is illustrated below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| microsecond timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.11. ISIS Type
This Type was initially defined in the Sprint Labs Python Routing and
supports the IS-IS routing protocol as defined in RFC 1195 [RFC1195].
There is no Type specific header for the ISIS Type. The Subtype code
for this Type is undefined. The ISIS PDU directly follows the MRT
common header fields.
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5.12. ISIS_ET Type
The ISIS_ET Type extends the the ISIS Type to support microsecond
timestamps. As with the BGP4MP_ET Type, a 32-bit microsecond
timestamp field is appended to the MRT common header after the length
field. The ISIS_ET Type is otherwise identical to the ISIS Type.
5.13. OSPFv3 Type
The OSPFv3 Type extends the original OSPF Type to support IPv6
addresses for the OSPFv3 protocol as defined in RFC 2740 [RFC2740].
The format of the MRT Message field for the OSPFv3 Type is as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OSPF Message Contents (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.14. OSPFv3_ET Type
The OSPFv3_ET Type extends the the OSPFv3 Type to support microsecond
timestamps. As with the BGP4MP_ET Type, a 32-bit microsecond
timestamp field is appended to the MRT common header after the length
field. The OSPFv3_ET Type is otherwise identical to the OSPFv3 Type.
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6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the MRT
specification, in accordance with BCP 26, RFC 2434 [RFC2434].
There are two name spaces in MRT that require registration: Type
Codes and Subtype Codes.
MRT is not intended as a general-purpose specification for protocol
information export, and allocations should not be made for purposes
unrelated to routing protocol information export.
The following policies are used here with the meanings defined in BCP
26: "Specification Required", "IETF Consensus".
6.1. Type Codes
Type Codes have a range from 0 to 65535, of which 0-64 have been
allocated. New Type Codes MUST be allocated starting at 65. Type
Codes 65 - 32767 are to be assigned by IETF Consensus. Type Codes
32768 - 65535 are assigned based on Specification Required.
6.2. Subtype Codes
Subtype Codes have a range from 0 to 65535. Subtype definitions are
specific to a particular Type Code definition. New Subtype Code
definition must reference an existing Type Code to which the Subtype
belongs. As Subtype Codes are specific to Type Codes, new numbers
must be unique for the particular Type Code to which the Subtype
applies. Subtype Codes specific to the Type Codes 0 - 32767 are
assigned by IETF Consensus. Suptype Codes specific to Type Codes
32768 - 65535 are assigned based on Specification Required.
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7. Security Considerations
The MRT Format utilizes a structure which can store routing protocol
information data. The fields defined in the MRT specification are of
a descriptive nature and provide information that is useful to
facilitate the analysis of routing data. As such, the fields
currently defined in the MRT specification do not in themselves
create additional security risks, since the fields are not used to
induce any particular behavior by the recipient application.
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8. References
8.1. Normative References
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058,
June 1988.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2080] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
January 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2740] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
RFC 2740, December 1999.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
8.2. Informative References
[MRT PROG GUIDE]
Labovitz, C., "MRT Programmer's Guide", November 1999,
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Authors' Addresses
Larry Blunk
Merit Network
Email: ljb@merit.edu
Manish Karir
Merit Network
Email: mkarir@merit.edu
Craig Labovitz
Arbor Networks
Email: labovit@arbor.net
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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specification can be obtained from the IETF on-line IPR repository at
The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Blunk, et al. Expires September 6, 2007 [Page 27]
------------------------------------
2. 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 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 RFC 1700 (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.
An UPDATE message that carries no NLRI, other than the one encoded in
the MP_REACH_NLRI attribute, should not carry the NEXT_HOP attribute.
If such a message contains the NEXT_HOP attribute, the BGP speaker
that receives the message should ignore this attribute.
3. 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 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 RFC 1700 (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.