Bgp As Number Substitution - HP 10500 Series Configuration Manual

Figure 41 Application of OSPF in VPN
With the standard BGP/OSPF interaction, PE 2 advertises the BGP VPN routes to CE 21 and CE
22 in Type 5 LSAs (ASE LSAs). However, CE 11, CE 21, and CE 22 belong to the same OSPF
domain, and route advertisements between them should use Type 3 LSAs (inter-area routes).
With the extended BGP/OSPF interaction, PEs advertise routes from one site to another site in
Type 3 LSAs. The process requires that extended BGP community attributes include the information
for identifying the OSPF attributes.
Each OSPF domain must have a domain ID. HP recommends that you configure the same domain
ID or adopt the default ID for all OSPF processes of the same VPN, so the system can know that
VPN routes with the same domain ID are from the same VPN.
Routing loop detection:
•
If a CE and a PE are connected through the OSPF backbone area, when a PE advertises BGP VPN
routes learned from MPLS/BGP to the VPN site through LSAs, the LSAs might be received by
another PE, resulting in a routing loop.
To avoid routing loops, when creating Type 3 LSAs, the PE always sets the flag bit DN for BGP
VPN routes learned from MPLS/BGP, regardless of whether the PE and the CE are connected
through the OSPF backbone. When performing route calculation, the OSPF process of the PE
ignores the Type 3 LSAs whose DN bit is set.
If the PE needs to advertise routes from other OSPF domains to a CE, it must indicate that it is the
ASBR, and advertise the routes in Type 5 LSAs.

BGP AS number substitution

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 to ensure correct transmission of routing
information.
The BGP AS number substitution function allows physically dispersed CEs to use the same AS number.
The function is a BGP outbound policy and affects routes to be advertised.
With the BGP AS number substitution function, when a PE advertises a route to a CE, if an AS number
identical to that of the CE exists in the AS_PATH of the route, the PE replaces it with its own AS number.
After you enable the BGP AS number substitution function, the PE performs BGP AS number substitution
for all routes and re-advertises them to connected CEs in the peer group.
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