Juniper JUNOSE 11.2.X BGP AND MPLS, Руководство по настройке

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The size of the Internet routing table. Placing a full default-free Internet routing table
in the VRF routing table is not feasible because it does not scale. The PE router would
have to support more than 100,000,000 routes, because the full default-free Internet
routing table is currently about 120,000 routes and the router must support up to 1,000
VRFs.
Solutions
The following methods enable advertising of Internet routes to VPN sites and thus enable
traffic flow from the VPNs to the Internet:
• Configure default routes instead of a full default-free Internet routing table in the VRF.
The default routes must point to a shared IP interface that you create on top of the
layer 2 interface that points to the Internet gateway.
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Configure a single full default-free Internet routing table in the context of the parent
VR and share this one table among all VRFs with the fallback global feature. Fallback
global enables an additional lookup in the IP routing table of the parent VR in the event
that the IP route lookup in the child VRF fails.
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When reachability to a small number of networks in the Internet is required, then
configure a global import map to import only the specific route to these networks into
the VRF.
You can create multiple IP interfaces on top of a single layer 2 interface. One of those
interfaces is the primary IP interface for receiving and sending IP packets. The other
interfaces are shared IP interfaces that are used only to send traffic.
Configuring a Default Route to a Shared Interface
For the first solution you create a default route in the VRF that points to a shared IP
interface. You must manually create the shared IP interface on top of the layer 2 interface
that points to the Internet gateway. See Figure 101 on page 463.
The main disadvantage of this approach is that if multiple Internet gateways are available,
BGP cannot select the egress gateway that is optimal for each destination prefix. Because
BGP has only a default route in the VRF, it has to point that single default route to a single
uplink interface. All the Internet-bound traffic must flow out of that interface.
You cannot configure traffic for one prefix to flow out of one uplink interface and traffic
to another prefix to flow out of another uplink interface. That behavior requires a full
default-free Internet routing table in the VRF, which is a complication that you want to
avoid.
Copyright © 2010, Juniper Networks, Inc. 462
JunosE 11.2.x BGP and MPLS Configuration Guide