... of messages exchanged is greatly reduced in aggressive mode. Figure 13- 3 graphically shows IKE phase 1 main mode.Figure 13- 3 IKE Phase 1, Main ModeIn main mode, the first two exchanges negotiate ... 150x01x.book Page 270 Monday, June 18, 2007 8:52 AM Exam Topic ListThis chapter covers the following topics that you need to master for the CCNP ISCW exam: ■ Site-to-Site VPN Overview—Describes how ... requirements.Today, DES, 3DES, and AES are examples of symmetric encryption algorithms:■ DES, with its 56-bit key, has been broken in less than 24 hours using modern computers. ■ 3DES applies three...