Cisco Ccie Fundamentals: Network Design

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When more overhead can be tolerated, frame sequencing is important, but extremely reliable delivery is not needed, and CCIE Workbooks packets can be sent over serial, Token Ring, Ethernet, and FDDI networks using FST encapsulation. FST is similar to TCP in that it provides packet sequencing.
However, unlike TCP, FST does not provide packet-delivery acknowledgment.
For extremely reliable delivery in environments in which moderate overhead can be tolerated, you can choose to encapsulate SRB frames in TCP/IP packets. This solution is not only reliable, it can also take advantage of routing features that include handling via routing protocols, packet ?ltering,
and multipath routing.
Generic Routing Encapsulation (GRE)
Cisco's Generic Routing Encapsulation (GRE) multiprotocol carrier protocol encapsulates IP,CLNP, IPX, AppleTalk, DECnet Phase IV, XNS, VINES, and Apollo packets inside IP tunnels.With GRE tunneling, a Cisco router at each site encapsulates protocol-speci?c packets in an IP header,creating a virtual point-to-point link to Cisco routers at other ...
... ends of an IP cloud, where the IP header is stripped off. By connecting multiprotocol subnetworks in a single-protocol backbone environment, IP tunneling allows network expansion across a single-protocol backbone environment. GRE tunneling involves ccie routing and switching labthree types of protocols:
Passenger—The protocol is encapsulated (IP, CLNP, IPX, AppleTalk, DECnet Phase IV, XNS,
VINES and Apollo).
Carrier—GRE protocol provides carrier services.
Transport—IP carries the encapsulated protocol.
GRE tunneling allows desktop protocols to take advantage of the enhanced route selection capabilities of IP. Many local-area network (LAN) protocols, including AppleTalk and Novell IPX,are optimized for local use. They have limited route selection metrics and hop count limitations. In contrast, IP routing protocols allow more exible route selection and scale better over large internetworks. Figure 2-10 illustrates GRE tunneling across a single IP backbone between sites.
Regardless of how many routers and paths may be associated with the IP cloud, the tunnel is seen as a single hop.Tunneling is CPU intensive, and as such, should be turned on cautiously. Routing updates, SAP updates, and other administrative trafc may be sent over each tunnel interface. It is easy to saturate
a physical link with routing information if several tunnels are congured over it. Performance depends on the passenger protocol, broadcasts, routing updates, and bandwidth of the physical interfaces. It is also diffcult to debug the physical link if problems occur. This problem can be mitigated in several ways. In IPX environments, route lters and SAP lters cut down on the size of the updates that ccie security lab over tunnels. In AppleTalk networks, keeping zones small and using route lters can limit excess bandwidth requirements.