Which two options does Cisco PfR use to control the entrance link selection with inbound
optimization? (Choose two.)
A.
Prepend extra AS hops to the BGP prefix.
B.
Advertise more specific BGP prefixes (longer mask).
C.
Add (prepend) one or more communities to the prefix that is advertised by BGP.
D.
Have BGP dampen the prefix.
Explanation:
PfR Entrance Link Selection Control Techniques
The PfR BGP inbound optimization feature introduced the ability to influence inbound traffic. A
network advertises reachability of its inside prefixes to the Internet using eBGP advertisements to
its ISPs. If the same prefix is advertised to more than one ISP, then the network is multihoming.
PfR BGP inbound optimization works best with multihomed networks, but it can also be used with
a network that has multiple connections to the same ISP. To implement BGP inbound
optimization, PfR manipulates eBGP advertisements to influence the best entrance selection for
traffic bound for inside prefixes. The benefit of implementing the best entrance selection is limitedto a network that has more than one ISP connection.
To enforce an entrance link selection, PfR offers the following methods:
BGP Autonomous System Number Prepend When an entrance link goes out-of-policy (OOP) due
to delay, or in images prior to Cisco IOS Releases 15.2(1) T1 and 15.1(2)S, and PfR selects a
best entrance for an inside prefix, extra autonomous system hops are prepended one at a time (up
to a maximum of six) to the inside prefix BGP advertisement over the other entrances. In Cisco
IOS Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of policy
(OOP) due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix,
six extra autonomous system hops are prepended immediately to the inside prefix BGP
advertisement over the other entrances. The extra autonomous system hops on the other
entrances increase the probability that the best entrance will be used for the inside prefix. When
the entrance link is OOP due to unreachable or loss reasons, six extra autonomous system hops
are added immediately to allow the software to quickly move the traffic away from the old entrance
link. This is the default method PfR uses to control an inside prefix, and no user configuration is
required.
BGP Autonomous System Number Community Prepend
When an entrance link goes out-of-policy (OOP) due to delay, or in images prior to Cisco IOS
Releases 15.2
(1)T1 and 15.1(2)S, and PfR selects a best entrance for an inside prefix, a BGP prepend
community is attached one at a time (up to a maximum of six) to the inside prefix BGP
advertisement from the network to another autonomous system such as an ISP. In Cisco IOS
Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of-policy (OOP)
due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix, six BGP
prepend communities are attached to the inside prefix BGP advertisement. The BGP prepend
community will increase the number of autonomous system hops in the advertisement of the
inside prefix from the ISP to its peers. Autonomous system prepend BGP community is the
preferred method to be used for PfR BGP inbound optimization because there is no risk of the
local ISP filtering the extra autonomous system hops. There are some issues, for example, not all
ISPs support the BGP prepend community, ISP policies may ignore or modify the autonomous
system hops, and a transit ISP may filter the autonomous system path. If you use this method of
inbound optimization and a change is made to an autonomous system, you must issue an
outbound reconfiguration using the “clear ip bgp” command.http://www.cisco.com/en/US/docs/ios-xml/ios/pfr/configuration/15-2s/pfr-bgp-inbound.html#GUIDF8A59E24-
1D59-4924-827D-B23B43D9A8E0
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