Information Technology Reference
In-Depth Information
The configuration for Router A in Figure 9-6 is as follows:
router bgp 100
neighbor 1.2.1.1 remote-as 100
neighbor 1.3.1.1 remote-as 100
The configuration for Router B in Figure 9-6 is as follows:
router bgp 100
neighbor 2.2.2.1 remote-as 300
neighbor 1.1.1.1 remote-as 100
neighbor 1.3.1.1 remote-as 100
bgp default local-preference 300
The configuration for Router C in Figure 9-6 is as follows:
router bgp 100
neighbor 3.1.1.1 remote-as 200
neighbor 1.1.1.1 remote-as 100
neighbor 1.2.1.1 remote-as 100
bgp default local-preference 400
Origin Attribute
Origin is a well-known, mandatory attribute that defines the source of the path information.
Do not confuse defining the origin with comparing if the route is eBGP or iBGP. The origin
attribute is received from the source BGP speaker. The three types of origin attributes follow:
•
IGP—Indicated by an
i
in the BGP table; present when the route is learned by way of the
network
statement
•
EGP—Indicated by an
e
in the BGP table; learned from exterior gateway protocol (EGP)
•
Incomplete—Indicated by a
?
in the BGP table; learned from redistribution of the route
In terms of choosing a route based on origin, BGP prefers routes that have been verified by the
IGP over routes that have been learned from EGP peers, and BGP prefers routes learned from
eBGP peers over incomplete paths.
AS Path Length Attribute
The AS Path is a well-known, mandatory attribute that contains a list of AS numbers in the path
to the destination. Each AS prepends its own AS number to the AS Path. The AS Path describes
all the autonomous systems that a packet must travel to reach the destination IP network; it
ensures that the path is loop free. When the AS Path attribute selects a path, the route with the
least number of AS hops is preferred. In the case of a tie, other attributes, such as MED, decide.
Example 9-9 shows the AS Path for network 200.50.32.0/19. To reach the destination, a packet
must pass autonomous systems 3561, 7004, and 7418. The command
show ip bgp 200.50.32.0
displays the AS Path information.
Search WWH ::
Custom Search