H3C Technologies H3C SR8800 - Manual (Page 214)

198
BGP path attributes
• ORIGIN
ORIGIN is a well-known mandatory attribute, which defines the origin of routing information (how
a route became a BGP route). It involves the following types:
{
IGP—Has the highest priority. Routes added to the BGP routing table using the network
command have the IGP attribute.
{
EGP—Has the second highest priority. Routes obtained via EGP have the EGP attribute.
{
Incomplete—Has the lowest priority. The source of routes with this attribute is unknown, which
does not mean such routes are unreachable. The routes redistributed from other routing
protocols have the incomplete attribute.
• AS_PATH
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. When a route is
advertised from the local AS to another AS, each passed AS number is added into the AS_PATH
attribute, so the receiver can determine ASs to route the message back. The number of the AS
closest to the receiver’s AS is leftmost, as shown in
Figure 76 :
Figure 76 AS_PATH attribute
In general, a BGP router does not receive routes containing the local AS number to avoid routing
loops.
NOTE:
•
The current implementation supports using the peer allow-as-loop command to receive routes
containing the local AS number in order to meet special requirements.
• The SR8800 check the AS_PATH attribute of EBGP routes rather than IBGP routes, and reject any EBGP
route whose AS_PATH attribute contains the local AS number.
The AS_PATH attribute can be used for route selection and filtering. BGP gives priority to the route
with the shortest AS_PATH length if other factors are the same. As shown in
Figure 76 , the BGP
8.0.0.0
AS 10
D = 8.0.0.0
(10) D = 8.0.0.0
(10)
AS 20 AS 40
D = 8.0.0.0
(20,10)
AS 30 AS 50
D = 8.0.0.0
(30,20,10)
D = 8.0.0.0
(40,10)