Bgp As Number Substitution And Soo Attribute - HP MSR2000 Configuration Manual

As shown in
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 1 1, 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 1 1, 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.
OSPF sham link
As shown in
• 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 57 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.

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
function to ensure correct transmission of routing information.
Figure 56, Site 1 is connected to two PEs. When a PE advertises VPN routes learned from
Figure 57, two routes exist between Site 1 and Site 2 of VPN 1:
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