bgp update...

the update tells you the shit... you can just read it and expect to know it... you have to be it...

the update message (type 2)

origin, code 1, well known transitive

as_path, code 2, well known transitive

next_hop, code 3, well known transitive

and nlri follows...

zoom in...

The mandatory category refers to an attribute that MUST be present in
   both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
   message.  Attributes classified as optional for the purpose of the
   protocol extension mechanism may be purely discretionary,
   discretionary, required, or disallowed in certain contexts.

        attribute           EBGP                    IBGP
         ORIGIN             mandatory               mandatory
         AS_PATH            mandatory               mandatory
         NEXT_HOP           mandatory               mandatory
         MULTI_EXIT_DISC    discretionary           discretionary
         LOCAL_PREF         see Section 5.1.5       required
         ATOMIC_AGGREGATE   see Section 5.1.6 and 9.1.4
         AGGREGATOR         discretionary           discretionary

ORIGIN is a well-known mandatory attribute.  The ORIGIN attribute is
   generated by the speaker that originates the associated routing
   information.  Its value SHOULD NOT be changed by any other speaker.

AS_PATH is a well-known mandatory attribute.  This attribute
   identifies the autonomous systems through which routing information
   carried in this UPDATE message has passed.  The components of this
   list can be AS_SETs or AS_SEQUENCEs.

 The NEXT_HOP is a well-known mandatory attribute that defines the IP
   address of the router that SHOULD be used as the next hop to the
   destinations listed in the UPDATE message.

what doyle says 

Well-known attributes are either mandatory, meaning that they must be included in all BGP
Update messages, or they are discretionary , meaning that they may or may not be sent in a
specific Update message.
If an optional attribute is transitive , a BGP process should accept the path in which it is
included, even if it doesn’t support the attribute, and it should pass the path on to its peers.

If an optional attribute is nontransitive , a BGP process that does not recognize the attribute
can quietly ignore the Update in which it is included and not advertise the path to its other
peers.
 

ORIGIN is a well-known mandatory attribute that specifies the origin of the routing update.
When BGP has multiple routes, it uses the ORIGIN as one factor in determining the
preferred route.

AS_PATH is a well-known mandatory attribute that uses a sequence of AS numbers to
describe the inter-AS path, or route, to the destination specified by the NLRI. When a BGP
speaker originates a route—when it advertises NLRI about a destination within its own
AS—it adds its AS number to the AS_PATH. As subsequent BGP speakers advertise the
route to external peers, they prepend their own AS numbers to the AS_PATH

The NEXT_HOP Attribute
As the name implies, this well-known mandatory attribute describes the IP address of the
next-hop router on the path to the advertised destination. The IP address described by the
BGP NEXT_HOP attribute is not always the address of a neighboring router.  

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