H3C Technologies H3C S10500 Series Switches - Manual (Page 208)

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Keepalive
Keepalive messages are sent between peers to maintain connectivity. Its format contains only the
message header.
Route-refresh
A route-refresh message is sent to a peer to request the specified address family routing information.
Figure 75 BGP route-refresh message format
• AFI—Address family identifier
• Res—Reserved; set to 0
• SAFI—Subsequent address family identifier
BGP path attributes
Classification of path attributes
BGP path attributes are a group of parameters encapsulated in the path attributes field of update
messages. They give detailed route attributes information that can be used for route filtering and
selection.
Path attributes fall into the following categories:
• Well-known mandatory—Must be recognized by all BGP routers and be included in every Update
message. Routing information errors occur without this attribute.
• Well-known discretionary—Can be recognized by all BGP routers and optionally included in every
Update message as needed.
• Optional transitive—Transitive attribute between ASs. A BGP router not supporting this attribute can
still receive routes with this attribute and advertise them to other peers.
• Optional non-transitive—If a BGP router does not support this attribute, it will not advertise routes
with this attribute.
The usage of each BGP path attribute is described in the following table.
Table 4 Usage of BGP path attributes
Name Category
ORIGIN Well-known mandatory
AS_PATH Well-known mandatory
NEXT_HOP Well-known mandatory
LOCAL_PREF Well-known discretionary
ATOMIC_AGGREGATE Well-known discretionary
AGGREGATOR Optional transitive
COMMUNITY Optional transitive
MULTI_EXIT_DISC (MED) Optional non-transitive
ORIGINATOR_ID Optional non-transitive