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In the figure, RTC has IBGP peering sessions
with RTD, RTE, and RTH, and has similar policies toward each. Instead of
formulating and applying similar policies for each neighbor individually,
you can define a peer group that contains the policies and then place
RTC's IBGP peers in the peer group (see the first running configuration in
the figure).
The configuration shown in the figure
defines a peer group called INTERNALMAP that contains several policies.
These policies are applied to all internal neighbors: RTD, RTE, and RTH.
Note that an additional policy, filter-list
3, has been applied to incoming
updates from 172.16.12.1 (RTH). Filter list 3 will override any incoming
policies set by the peer group INTERNALMAP for neighbor RTH. Remember that
outbound update policies cannot be overridden, although additional
policies can be defined to specific peers.
As a final example, look at the peer group
configuration used by RTC for EBGP peers (see the second running
configuration in the figure).
A major difference between IBGP and EBGP
peer groups is that the neighbor
remote-as statement is typically
placed outside the EBGP peer group configuration. This is usually
necessary because external peers do not often reside in the same AS
(although they certainly can). Also note that the configuration shown
defines filter list 3, which can be used to override configuration options
for incoming updates from the neighbor at IP address 172.16.20.3 (RTF).
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