User's Manual

453
Command Purpose
no synchronization
Cancel the synchronization between BGP and
IGP.
While canceling synchronization, you should use command “clear ip bgp” to clear BGP dialogue.
For an example about BGP synchronization, please refer to the section in the bottom of this chapter “an
example of BGP path filtration by the neighbors”.
Normally, you do not expect to redistribute all routes to your IGP. A common design is to redistribute one or
two routes, and make them the external routes in IGRP, or force the BGP session to generate an AS default
route. When BGP redistribute routes to IGP, only the routes acquired through EBGP will be redistributed. Under
most situations, you do not want to allocate your IGP to BGP, just use configuration command “network ” to
list the network in AS, then your network will be broadcasted. The networks listed in this form are called local
network, and enables BGP to have attribute “Origin” of IGP. They must appear in the main IP routing table,
and are effective; for example, they are direct-connected routes, static routes or routes known through IGP.
BGP routing process periodically scans the main IP routing table to check the existence of a local network,
and accordingly updates BGP routing table if you really want BGP to execute redistribution, you must be very
careful, because these may be the routes in IGP that are injected by other routers through BGP, this may bring
force a kind of situation that BGP potentially injects the information into IGP, and then send back the information
to BGP. Vice versa.
6. Configure BGP route value
BGP route value is a number set to BGP route in order to control the route selection process, value is local for
the router. The value ranges from 0 to 65535. BGP route generated locally has a default value of 32768, the
route got from the neighbour values 0. The administrator can implement routing policy through the change of
route value.
Use the following router configuration command to configure BGP route weight:
Command Purpose
neighbor
{ip-address | peer-group-name}
weight
weight
Designate a value to each route from one
neighbour.
Besides, you can change the route weight through route-map.
7. Configure BGP route filter based on the neighbour
There are 4 methods in BGP implementation of router software to filter BGP routes of the designated
neighbours:
Use Aspath list filter together with global configuration command “ip aspath-list” and command “neighbour
filter-list”.
Command Purpose
ip aspath-list
aspaths-list-name {
permit | deny
}
as-regular-expression
Define an accessing list relative to BGP.
router bgp
autonomous-system
Enter into router configuration mode.
neighbor
{ip-address | peer-group-name}
filter-
list
aspath-list-name {
in | out
}
Establish a BGP filter.
Use access list together with global configuration command “ip access-list” and command “neighbour distribute-list”.