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Single-Protocol IP Backbone 311
Tunneling can put a lot of CPU overhead on the router, and that's not usually a good idea.
Suppose five sites are connected, as shown in Figure 9-11. Router A is in San Diego and
currently is running EIGRP on its LAN and WAN interfaces. Router B is located in San Jose
and is running EIGRP and OSPF on Serial Interface 1. Router C is running only OSPF. Router
D is running OSPF and EIGRP. Router E is running only EIGRP. The administrators have said
that they need the EIGRP updates to travel from Router A to Router E, but the EIGRP routes
cannot be seen in Router C. The administrators do not want the router to be overloaded by
running two different routing processes on Router C. The overload can be avoided by using a
tunnel--specifically, by using the command
tunnel mode ipip
--and configuring EIGRP in the
tunnel.
The EIGRP updates will traverse through the tunnel, and neighbor adjacencies will form
through the tunnel.
Figure 9-11
IP-in-IP Tunneling
IBM compatible
IBM compatible
Router E
Router D
Router C
Router B
Router A
EIGRP 100
Political issues
OSPF only
EIGRP 100
IP tunnel, tunneling EIGRP
Router C knows nothing
about EIGRP
87200333.book Page 311 Wednesday, August 22, 2001 2:53 PM