
Configuring IP Services
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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 Policy
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 (see “IP Routing Policies and Filters
” on
page 1-14
). For GRE tunneling, you 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. To configure an announce policy for RIP, see
“Configuring RIP Accept and Announce Policies
” on page 6-29. To configure an
announce policy for OSPF, see “Configuring OSPF Accept and Announce
Policies” on page 7-59. To configure an announce policy for BGP, see
“Configuring BGP Accept and Announce Policies
” on page 8-55.
The disadvantage of using an announce policy is that it prevents the advertisement
of other subnets within the blocked range. Depending on the network topology,
this configuration may not be desirable.
Note:
You must implement an announce or accept policy or a static route at
each end of the tunnel for the tunnel to operate correctly.
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