background image
BPG Common Header
must be supported by all implementations of BGP. These attributes are as
Autonomous System Path The AS_PATH (Type Code 2) is a well-
known mandatory attribute. The AS_PATH attribute is composed of a
variable-length series of AS path segments. Each AS path segment con-
tains a path type, a length, and a value. The path segment type is a one-
octet-long field with the values shown in Table 7.3.
The AS_PATH's fields above are modified only by eBGP speakers that
advertise the route outside the local AS. These eBGP speakers prepend
their own AS numbers to the end of the path vector in each of the fields.
When a BGP speaker originates a route, it should include its own ASN in
UPDATE messages sent to other ASes. The field is empty for an AS_PATH
attribute advertised to iBGP speakers belonging to its own ASN. This
allows iBGP to avoid data loops by implementing a rule that specifies that
each iBGP router must ignore any route learned from an iBGP peer.
The AS_PATH attribute makes BGP a path-vector protocol. BGP mes-
sages carry the sequence of AS numbers indicating the complete path a
message has traversed.
Next-hop The NEXT_HOP (Type Code 3) attribute is a well-known
mandatory attribute that indicates the IP address of the next-hop destina-
tion router. The next hop for all destinations is listed in the NLRI field of
the UPDATE message. The BGP speaker should never advertise the
address of a peer as the NEXT_HOP of a route the current speaker is orig-
inating to that peer. Likewise, the speaker should not install a route that
has itself as the next hop unless the NEXT_HOP_SELF configuration
option is used.
An iBGP speaker can advertise any internal BGP router as the next hop as
long as the IP address of the iBGP border router is on the same subnet as
the local and remote BGP speakers. This means that one router can handle
all the announcements on the same subnet.
A BGP speaker can also advertise any external border router as the next
hop if the IP address of the proposed next-hop router is learned from one
of the advertising router's peers, and if the connected interface for the
router is on the same subnet as both the local and remote BGP speakers.
Copyright ©2001 SYBEX , Inc., Alameda, CA