
Configuring Global BGP Parameters
308628-15.0 Rev 00
3-25
Disabling BayRS Local Preference Calculation and Route
Selection
A BGP speaker calculates a local preference value for each route that it receives
from an external peer and passes this value as the local_pref attribute in routing
updates that it announces to its internal BGP (IBGP) peers. A BGP speaker that
receives a routing update from an IBGP peer uses the local_pref attribute in its
best-route selection process.
By default, BGP calculates a local-preference value by using the algorithm
described in “
Calculating BGP-4 Local Preference Values” on page 3-25. The
best-route selection process is described in “
Best-Route Selection” on page 3-27.
Different implementations of BGP use different methods of arriving at a local
preference value. However, within an AS, all BGP speakers should use the same
method to determine local preference and to select the best routes.
If you are configuring BGP in a network that also includes routers from vendors
other than Nortel Networks, you can disable BayRS local preference calculation
and route selection. Instead, BGP assigns a value of 100 to externally received
routes and uses the rules listed in Table 3-4 on page 3-27
for best-route selection.
Calculating BGP-4 Local Preference Values
BGP-4 update messages contain a local preference value that an AS border router
can assign to a route when advertising it to IBGP peers. The calculation of the
local preference attribute is specific to each implementation. A higher value
indicates that the route is more preferred.
The router uses the following equation to calculate a value for the local preference
attribute:
local_preference
= 8191 -
origin_value
-
AS_path_weight
origin_value
is 0 for routes with an origin path attribute of IGP or 4096 for other
routes.
Note:
For a comparison of
the Nortel Networks and Cisco route selection
processes, see “Comparing BGP Route Selection Processes” on page B-11.
Komentáře k této Příručce