196
777H
Figure 60
, Site 1 is connected to two PEs. When a PE advertises VPN routes learned
from MP-BGP to Site 1 through OSPF, the routes might be received by the other PE. This results in a
routing loop.
OSPF VPN extension uses the following tags to avoid routing loops:
•
DN bit (for Type 3 LSAs)
—When a PE redistributes BGP routes into OSPF and creates Type 3
LSAs, it sets the DN bit for the LSAs. When receiving the Type 3 LSAs advertised by CE 11, the
other PE ignores the LSAs whose DN bit is set to avoid routing loops.
•
Route tag (for Type 5 or 7 LSAs)
—The two PEs use the same route tag. When a PE
redistributes BGP routes into OSPF and creates Type 5 or 7 LSAs, it adds the route tag to the
LSAs. When receiving the Type 5 or 7 LSAs advertised by CE 11, the other PE compares the
route tag in the LSAs against the local route tag. If they are the same, the PE ignores the LSAs
to avoid routing loops.
459B
OSPF sham link
As shown in
778H
Figure 61
, two routes exist between Site 1 and Site 2 of VPN 1:
•
A route over MPLS backbone
—It is an inter-area route if PE 1 and PE 2 have the same
domain ID, or is an external route if PE 1 and PE 2 are configured with no domain ID or with
different domain IDs.
•
A direct route between CEs
—It is an intra-area route that is called a backdoor link.
VPN traffic is always forwarded through the backdoor link because it has a higher priority than the
inter-area route. To forward VPN traffic over the inter-area route, you can establish a sham link
between the two PEs to change the inter-area route to an intra-area route.
Figure 61 Network diagram for sham link
A sham link is considered a virtual point-to-point link within a VPN and is advertised in a Type 1 LSA.
It is identified by the source IP address and destination IP address that are the local PE address and
the remote PE address in the VPN address space. Typically, the source and destination addresses
are loopback interface addresses with a 32-bit mask.
To add a route to the destination IP address of a sham link to a VPN instance, the remote PE must
advertise the source IP address of the sham link as a VPN-IPv4 address through MP-BGP. To avoid
routing loops, a PE does not advertise the sham link's destination address.
231B
BGP AS number substitution and SoO attribute
BGP detects routing loops by examining AS numbers. If EBGP runs between PE and CE, you must
assign different AS numbers to geographically different sites or configure the BGP AS number
substitution feature to ensure correct transmission of routing information.
