Some one who read my blog asked me a question on mechanism to identify right Security Policy entry, now that there could be multiple policy entries with the same selectors.
In a deployment where there are multiple branch offices, head office security gateway has as many Encapsulation-Less Routerbased VPN (ELR VPN) interfaces as number of branch offices. It is quite possible that each branch office has different security requirements, hence there would be multiple SPD policy entries in head office router. When a branch office router initiates the IKE exchange, head office securty router should identify the right SPD (Security Policy Database) policy entry to negotiate security parameters for data Security associations. In typical policy based VPN and route based VPN, the SPD policy record selection is done based on selector values being negoatiated. But in Encapsulation-less Route based VPN (ELR VPN), selectors are always 0.0.0.0. Hence, this can't be used to select the SPD policy record.
ELR VPN implementation must use some other paramters to select the SPD policy entry. At the same time, implementations should not be creating proprietary extensions to IKE. So, we decided to go with 'remote ID'(ID of the initiated party) as index parameter to select right SPD policy record from SPD policy database.
In above example, each ELR VPN is associated with branch office identities. When IKE exchange is initiated by branch office router, it sends its identitity. Using this identity, head office router selects policy record that corresponds to branch office.
Hope this helps.