AFI(Address Family Identifier) vs SAFI(Subsequent Address Family Identifier)
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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.
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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.
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