User's Manual

Configuring GRE, NAT, RIPSO, and BFE Services
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308625-14.00 Rev 00
Requirements for GRE Tunnels Encapsulating IP
Before configuring a tunnel encapsulating IP, you should be aware of a limitation
inherent in the use of all tunnels, including GRE tunnels. A tunnel is a virtual
point-to-point connection between two routers that are actually several hops apart.
This point-to-point connection can hide the real distance between the routers from
portions of the network, leading to unintended, suboptimal routing decisions and
in some cases, to routing loops.
In particular, if a router at one end of a tunnel determines that the best route to the
remote physical end point of the tunnel is through the tunnel itself, a loop, internal
to the router, occurs and prevents the tunnel from operating. You must configure
one of the following at each end of the tunnel to prevent routing loops:
• Announce policy
• Accept policy
• Static route
The best choice depends on the network topology to which it is applied.
Announce Policies
An announce policy governs the advertisement of routing information. When
preparing a routing advertisement, IP consults its announce policies to determine
whether or not to advertise the route. For GRE tunneling, you can configure an
announce policy for each routing protocol (RIP, OSPF, BGP) configured on the
logical tunnel interface to block the advertisement of a range of network addresses
that contains the tunnel’s local physical interface address. For information about
configuring RIP and OSPF announce policies, see Configuring IP, ARP, RARP,
RIP, and OSPF Services. For information about configuring BGP announce
policies, see Configuring IP Exterior Gateway Protocols (BGP and EGP).
Note:
If you are using GRE tunneling to encapsulate the IPX or OSI protocol,
skip this section. The requirements discussed below do not apply to tunnels
encapsulating IPX or OSI.
Note:
When configuring a tunnel with IP encapsulation, you must implement
an announce or accept policy or a static route at each end of the tunnel for the
tunnel to operate correctly.