Discussion

have to set up an IKE SA and a firewall filter to direct traffic into the tunnel. Again,
this recipe is for M-series and T-series routers with ES PICs.
The IKE negotiation happens in two phases, Phase 1 and Phase 2. In Phase 1, the IKE
SA is negotiated based on the IKE policy and IKE proposal, and the result is the creation
of an IKE SA. This negotiated IKE SA is then used to secure the Phase 2
exchange, which uses the IPSec proposal and IPSec policy to create an IPSec SA pair
(one SA for inbound and a second for outbound traffic). The IKE policy is used for
the negotiation during Phase 1, and IPSec policy is used for the negotiation in Phase 2.
This means that the IKE and IPSec proposals can use different algorithms.
For IKE, this recipe defines a negotiation proposal, here called site1-site2-ikeproposal,
that creates an IKE SA based on the stated authentication and encryption
algorithms. The set dh-group command configures this proposal to use 768-bit Diffie-
Hellman prime modulus group when establishing the IKE session keys. By default,
the IKE SA is valid for 3,600 seconds (1 hour). When it expires, a new one is negotiated.
The IKE SA references an IKE policy (here, policy 10.0.97.62) that defines the
preshared key to use for negotiation. The policy is identified by the IP address of the
security gateway at the remote end of the tunnel, which is the tunnel destination
addresses configured on ES interface es-3/0/0.
For IPSec, also define a proposal (here, site1-site2-ipsec-proposal) and a policy
(site1-site2-ipsec-policy). The proposal uses the same parameters as in the manual
SA in Recipe 3.1. By default, the negotiated SA is valid for 28,800 seconds (8
hours). When it expires, a new one is negotiated. On the ES PIC, anti-replay is disabled
by default. (On the AS PIC, it is enabled by default with a default window size
of 64 bits.) This recipe enables anti-replay with a window size of 64 bits.
The lifetime of both the IKE and IPSec proposals is configurable by using the set
lifetime-seconds command. Here, you change the IKE proposal lifetime to 2 hours
and the IPSec proposal lifetime to 10 hours:
[edit security ike]
aviva@router1# set proposal site1-site2-ike-proposal lifetime-seconds 7200
[edit security ipsec]
aviva@router1# set proposal site1-site2-ipsec-proposal lifetime-seconds 36000
The IPSec policy, site1-site2-ipsec-policy, defines which proposals IKE should
consider and in which order (if you configure more than one). This recipe also
enables perfect forward secrecy (PFS) security for keys, which means that the shared
key material can be used to drive the IPSec SA keys only once. With PFS, if an IKE
SA is present (Phase 1 of the negotiation), then during the IPSec SA negotiation
(Phase 2 of the negotiation), a Diffie-Hellman exchange is required for every rekeying
to generate the shared key material. Without PFS, a Diffie-Hellman exchange is
done only during the initial keying but is not done again during the rekeying operation.
PFS is considered to be more secure because it gets fresh keying material every
time an IPsec SA is renegotiated.
Configuring IPSec Dynamic SAs | 113
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