Information Technology Reference
In-Depth Information
Figure 10-8 Automatic Route Filtering on PEs
AS 100
VPNa
R1
VPNa
R6
R3
R8
172.16.0.0/16
VPNb
R5
R9
R7
VPNb
R2
R4
172.16.0.0/16
Example 10-15 shows debug output on R2. The prefix 100:200:172.16.0.0/16 from R5 has
an RT of 100:200, whereas R2 is configured to import only 100:100 for VPNa. Thus, the
prefix is denied automatically.
Example 10-15 Output of debug ip bgp update in on R2
*Dec 13 19:11:00.511: BGP(2): 192.168.100.5 rcvd UPDATE w/ attr: nexthop
172.168.100.5, origin ?, localpref 100, metric 0
*Dec 13 19:11:00.511: BGP(2): 192.168.100.5 rcvd 100:200:172.16.0.0/16 -- DENIED
due to: extended community not supported;
Changes made to the VRF configuration cause route refresh requests to be sent out.
NOTE
AS_PATH Manipulation
When BGP is used between a PE and a CE, AS_PATH manipulation might be necessary to
provide full connectivity among the VPN sites. This section discusses two ways to change
the AS_PATH check on a PE: AS override and allow-AS.
AS Override
When BGP is used between PE and CE routers, the customer VPN might want to reuse
the AS number in different sites. Figure 10-9 shows such a scenario. When the prefix
172.16.0.0/16 is advertised from PE2 to CE2, CE2 detects an AS_PATH loop, and the prefix
is denied.
 
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