Cabletron Systems SmartSwitch Router, Руководство пользователя

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Chapter 10: BGP Configuration Guide
156 SmartSwitch Router User Reference Manual
In router SSR12’s CLI configuration file, the import preference is set to 160:
Using the formula for local preference [Local_Pref = 254 - (global protocol preference for
this route) + metric], the Local_Pref value put out by router SSR12 is 254 - 160+100 = 194.
For router SSR13, the import preference is set to 150. The Local_Pref value put out by
router SSR12 is 254 - 160+100 = 204.
Notes on Using the Local_Pref Attribute
• All routers in the same network that are running GateD and participating in IBGP
should use the setpref metric, and the setpref metric should be set to the same value.
For example, in Figure 13 , routers SSR12, SSR13, and SSR14 have the following line in
their CLI configuration files:
• The value of the setpref metric should be consistent with the import policy in the
network.
The metric value should be set high enough to avoid conflicts between BGP routes and
IGP or static routes. For example, if the import policy sets GateD preferences ranging
from 170 to 200, a setpref metric of 170 would make sense. You should set the metric
high enough to avoid conflicts between BGP routes and IGP or static routes.
Multi-Exit Discriminator Attribute Example
Multi-Exit Discriminator (MED) is a BGP attribute that affects the route selection process.
MED is used on external links to discriminate among multiple exit or entry points to the
same neighboring AS. All other factors being equal, the exit or entry point with a lower
metric should be preferred. If received over external links, the MED attribute may be
propagated over internal links to other BGP speakers within the same AS. The MED
attribute is never propagated to other BGP speakers in neighboring autonomous systems.
Figure 14 shows a sample BGP configuration where the MED attribute has been used.
#
# Set the set-pref metric for the IBGP peer group
#
bgp set peer-group as901 set-pref 100
ip-router policy create bgp-import-source as900 autonomous-system 64900
preference 160
ip-router policy create bgp-import-source as900 autonomous-system 64900
preference 150
bgp set peer-group as901 set-pref 100