Motorola BSR 2000, Configuration And Management Manual

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Release 1.0 Configuring BGP
526360-001-00 Rev. B MGBI 12-33
1-255
is the administrative distance for routes external to the AS. The default is
20.
1-255
is the distance for routes internal to the AS. The default is 200.
1-255
is the administrative distance for local routes. The default is 200.
Example
In the following example, internal routes are preferable to those learned through the
IGP. The administrative distance values are set accordingly: 20 for routes external and
internal to AS 100 and 200 for local routes.
MOT(config)#router bgp 100
MOT(config-bgp)#network 160.20.0.0
MOT(config-bgp)#neighbor 156.30.10.1 remote-as 100
MOT(config-bgp)#neighbor 131.65.1.2 remote-as 200
MOT(config-bgp)#distance bgp 20 20 200
Disabling Route Summarization
Route summarization condenses routing information. Without summarization, each
router in a network must retain a route to every subnet in the network. With
summarization, routers can reduce some sets of routes to a single advertisement,
reducing both the load on the router and the perceived complexity of the network. The
importance of route summarization increases with network size.
The reduction in route propagation and routing information overhead is significant.
For example, without summarization, each router in a network with 1,000 subnets
must contain 1,000 routes. With summarization in a Class B network with eight bits of
subnet address space, each router must know all of the routes for each subnet in its
network number. This is 250 routes, assuming that 1,000 subnets fall into four major
networks of 250 routes each. In addition, the router must know one route for each of
the other three networks for a total of 253 routes. This represents a nearly 75 percent
reduction in the size of the routing table.
In Figure 12-11 , Router Albany maintains one route for all destination networks
beginning with B, and Router Chicago maintains one route for all destination
networks beginning with A. This is the essence of route summarization. Router New
York tracks all routes because it exists on the boundary between A and B.
To disable automatic network summarization of routes, use the no auto-summary
command in Router Configuration mode:
MOT(config-bgp)#
no auto-summary