153
Figure 54 De-encapsulation process of an IPsec packet
5. The router forwards an IPsec packet received on the inbound interface to the forwarding
module.
6. Identifying that the destination address of the packet is the tunnel interface and the protocol is
AH or ESP, the forwarding module forwards the packet to the IPsec tunnel interface for
de-encapsulation.
7. The IPsec tunnel interface de-encapsulates the packet, and then delivers the resulting clear
text packet back to the forwarding module.
8. The forwarding module looks up the routing table, and then forwards the clear text packet out of
the physical outbound interface associated with the tunnel interface.
IPsec for IPv6 routing protocols
You can use IPsec to protect routing information and defend against attacks for these IPv6 routing
protocols: OSPFv3, IPv6 BGP, and RIPng. IPsec enables these IPv6 routing protocols to
encapsulate outbound protocol packets and de-encapsulate inbound protocol packets with the AH or
ESP protocol. If an inbound protocol packet is not IPsec protected, or fails to be de-encapsulated, for
example, due to decryption or authentication failure, the routing protocol discards that packet.
You must manually configure SA parameters in an IPsec policy for IPv6 routing protocols. The IKE
key exchange mechanism is applicable only to one-to-one communications. IPsec cannot implement
automatic key exchange for one-to-many communications on a broadcast network, where routers
must use the same SA parameters (SPI and key) to process packets for a routing protocol.
IPsec RRI
IPsec Reverse Route Inject (RRI) enables an IPsec tunnel gateway to automatically add static routes
destined for protected private networks or peer IPsec tunnel gateways to a routing table. In an MPLS
L3VPN network, IPsec RRI can add static routes to VPN instances' routing tables.
IPsec RRI is applicable to gateways, for example, a headquarters gateway that must provide many
IPsec tunnels. It frees you from the tedious work of manually configuring and maintaining static
routes for IPsec tunnels. For example, if you enable RRI on Device A in Figure 55,
Device A can
automatically create a static route to branch network 192.168.2.0/24 for the IPsec protected traffic
from the headquarters to the branch. You do not have to manually add the route by configuring the ip
route-static 192.168.2.0 255.255.255.0 2.2.2.2 command.
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