Configuring Layer 3 VPNs to Carry IBGP Traffic
An independent AS domain is separate from the primary routing instance domain. An AS
is a set of routers that are under a single technical administration and that generally use
a single IGP and metrics to propagate routing information within the set of routers. An
AS appears to other ASs to have a single, coherent interior routing plan and presents a
consistent picture of what destinations are reachable through it.
Configuring an independent domain allows you to keep the AS paths of the independent
domain from being shared with the AS path and AS path attributes of other domains,
including the master routing instance domain.
If you are using BGP on the router, you must configure an AS number.
When you configure BGP as the routing protocol between a PE router and a CE router in
a Layer 3 VPN, you typically configure external peering sessions between the Layer 3 VPN
service provider and the customer network ASs.
If the customer network has several sites advertising routes through an external BGP
session to the service provider network and if the same AS is used by all the customer
sites, the CE routers reject routes from the other CE routers. They detect a loop in the
BGP AS path attribute.
To prevent the CE routers from rejecting each other’s routes, you could configure the
following:
•
PE routers advertising routes received from remote PE routers can remap the customer
network AS number to its own AS number.
•
AS path loops can be configured.
•
The customer network can be configured with different AS numbers at each site.
These types of configurations can work when there are no BGP routing exchanges between
the customer network and other networks. However, they do have limitations for customer
networks that use BGP internally for purposes other than carrying traffic between the
CE routers and the PE routers. When those routes are advertised outside the customer
network, the service provider ASs are present in the AS path.
To improve the transparency of Layer 3 VPN services for customer networks, you can
configure the routing instance for the Layer 3 VPN to isolate the customer’s network
attributes from the service provider’s network attributes.
When you include the independent-domain statement in the Layer 3 VPN routing instance
configuration, BGP attributes received from the customer network (from the CE router)
are stored in a BGP attribute (ATTRSET) that functions like a stack. When that route is
advertised from the remote PE router to the remote CE router, the original BGP attributes
are restored. This is the default behavior for BGP routes that are advertised to Layer 3
VPNs located in different domains.
This functionality is described in the Internet draft draft-marques-ppvpn-ibgp-version.txt,
RFC 2547bis Networks Using Internal BGP as PE-CE Protocol.
849Copyright © 2017, Juniper Networks, Inc.
Chapter 26: Configuring Layer 3 VPNs
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