213.11.1 213.11.2 3.3.3.3RTDLOCAL PREF= 150LOCAL PREF = 200LOC...
128.213.11.1
128.213.11.2
3.3.3.3
RTD
Local Pref= 150
Local Pref = 200
Local preference is an indication to the AS about which path is preferred
to exit the AS in order to reach a certain network. A path with a higher
local preference is more preferred. The default value for local
preference is 100.
Unlike the weight attribute which is only relevant to the local router,
local preference is an attribute that is exchanged among routers in the
same AS.
Local preference is set via the “bgp default local-preference <value>”
command or with route-maps as will be demonstrated in the following
example:
The bgp default local-preference
<value> command will set the local pref-
erence on the updates out of the router going to peers in the same AS.
In the above diagram, AS256 is receiving updates about 170.10.0.0 from
two different sides of the organization. Local preference will help us
determine which way to exit AS256 in order to reach that network. Let us
assume that RTD is the preferred exit point. The following configuration
will set the local preference for updates coming from AS300 to 200 and
those coming from AS100 to 150.
RTC#
router bgp 256
neighbor 1.1.1.1 remote-as 100
neighbor 128.213.11.2 remote-as 256
bgp default local-preference 150
RTD#
neighbor 3.3.3.4 remote-as 300
neighbor 128.213.11.1 remote-as 256
bgp default local-preference 200
In the above configuration RTC will set the local preference of all
updates to 150. The same RTD will set the local preference of all updates
to 200. Since local preference is exchanged within AS256, both RTC and
RTD will realize that network 170.10.0.0 has a higher local preference
when coming from AS300 rather than when coming from AS100. All traffic in
AS256 addressed to that network will be sent to RTD as an exit point.
More flexibility is provided by using route maps. In the above example,
all updates received by RTD will be tagged with local preference 200 when
they reach RTD. This means that updates coming from AS34 will also be
tagged with the local preference of 200. This might not be needed. This
is why we can use route maps to specify what specific updates need to be
tagged with a specific local preference as shown below:
neighbor 3.3.3.4 setlocalin in
ip as-path 7 permit ^300$
route-map setlocalin permit 10
match as-path 7
set local-preference 400