L3Vpn Ibgp Pe-Ce Reference; Mpls Vpn Carrier Supporting Carrier - Cisco NCS 5500 Series Configuration Manual

Information about Implementing BGP
• Overwriting of next-hop calculation for multipath prefixes is not allowed. The next-hop-unchanged
Note
multipath command disables overwriting of next-hop calculation for multipath prefixes.
• The ability to ignore as-path onwards while computing multipath is added. The bgp multipath
as-path ignore onwards command ignores as-path onwards while computing multipath.

L3VPN iBGP PE-CE Reference

When BGP is used as the provider edge (PE) or the customer edge (CE) routing protocol, the peering sessions
are configured as external peering between the VPN provider autonomous system (AS) and the customer
network autonomous system. The L3VPN iBGP PE-CE feature enables the PE and CE devices to exchange
Border Gateway Protocol (BGP) routing information by peering as internal Border Gateway Protocol (iBGP)
instead of the widely-used external BGP peering between the PE and the CE. This mechanism applies at each
PE device where a VRF-based CE is configured as iBGP. This eliminates the need for service providers (SPs)
to configure autonomous system override for the CE. With this feature enabled, there is no need to configure
the virtual private network (VPN) sites using different autonomous systems.
The neighbor internal-vpn-client command enables PE devices to make an entire VPN cloud act as an
internal VPN client to the CE devices. These CE devices are connected internally to the VPN cloud through
the iBGP PE-CE connection inside the VRF. After this connection is established, the PE device encapsulates
the CE-learned path into an attribute called ATTR_SET and carries it in the iBGP-sourced path throughout
the VPN core to the remote PE device. At the remote PE device, this attribute is assigned with individual
attributes and the source CE path is extracted and sent to the remote CE devices.
ATTR_SET is an optional transitive attribute that carries the CE path attributes received. The ATTR_SET
attribute is encoded inside the BGP update message as follows:
Origin AS is the AS of the VPN customer for which the ATTR_SET is generated. The minimum length of
ATTR_SET is four bytes and the maximum is the maximum supported for a path attribute after taking into
consideration the mandatory fields and attributes in the BGP update message. It is recommended that the
maximum length is limited to 3500 bytes. ATTR_SET must not contain the following attributes: MP_REACH,
MP_UNREACH, NEW_AS_PATH, NEW_AGGR, NEXT_HOP and ATTR_SET itself (ATTR_SET inside
ATTR_SET). If these attributes are found inside the ATTR_SET, the ATTR_SET is considered invalid and
the corresponding error handling mechanism is invoked.

MPLS VPN Carrier Supporting Carrier

Carrier supporting carrier (CSC) is a term used to describe a situation in which one service provider allows
another service provider to use a segment of its backbone network. The service provider that provides the
segment of the backbone network to the other provider is called the backbone carrier. The service provider
that uses the segment of the backbone network is called the customer carrier.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
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+------------------------------+
| Attr Flags (O|T) Code = 128
+------------------------------+
| Attr. Length (1 or 2 octets) |
+------------------------------+
| Origin AS (4 octets)
+------------------------------+
| Path attributes (variable)
+------------------------------+
|
|
|
Implementing BGP