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