Mpls Forwarding; Automatic Route Distinguisher Assignment - Cisco CRS Configuration Manual
Ios xr virtual private network
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Implementing MPLS Layer 3 VPNs
• An eBGP session with the CE router
• A Routing Information Protocol (RIP) exchange with the CE router
• Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), and RIP as
Interior Gateway Protocols (IGPs)
The IP prefix is a member of the IPv4 address family. After the PE router learns the IP prefix, the PE converts
it into the VPN-IPv4 prefix by combining it with a 64-bit route distinguisher. The generated prefix is a member
of the VPN-IPv4 address family. It uniquely identifies the customer address, even if the customer site is using
globally nonunique (unregistered private) IP addresses. The route distinguisher used to generate the VPN-IPv4
prefix is specified by the rd command associated with the VRF on the PE router.
BGP distributes reachability information for VPN-IPv4 prefixes for each VPN. BGP communication takes
place at two levels:
• Within the IP domain, known as an autonomous system.
• Between autonomous systems.
PE to PE or PE to route reflector (RR) sessions are iBGP sessions, and PE to CE sessions are eBGP sessions.
PE to CE eBGP sessions can be directly or indirectly connected (eBGP multihop).
BGP propagates reachability information for VPN-IPv4 prefixes among PE routers by the BGP protocol
extensions (see RFC 2283, Multiprotocol Extensions for BGP-4), which define support for address families
other than IPv4. Using the extensions ensures that the routes for a given VPN are learned only by other
members of that VPN, enabling members of the VPN to communicate with each other.
MPLS Forwarding
Based on routing information stored in the VRF IP routing table and the VRF FIB table, packets are forwarded
to their destination using MPLS.
A PE router binds a label to each customer prefix learned from a CE router and includes the label in the
network reachability information for the prefix that it advertises to other PE routers. When a PE router forwards
a packet received from a CE router across the provider network, it labels the packet with the label learned
from the destination PE router. When the destination PE router receives the labeled packet, it pops the label
and uses it to direct the packet to the correct CE router. Label forwarding across the provider backbone is
based on either dynamic label switching or traffic engineered paths. A customer data packet carries two levels
of labels when traversing the backbone:
• The top label directs the packet to the correct PE router.
• The second label indicates how that PE router should forward the packet to the CE router.
More labels can be stacked if other features are enabled. For example, if traffic engineering (TE) tunnels with
fast reroute (FRR) are enabled, the total number of labels imposed in the PE is four (Layer 3 VPN, Label
Distribution Protocol (LDP), TE, and FRR).
Automatic Route Distinguisher Assignment
To take advantage of iBGP load balancing, every network VRF must be assigned a unique route distinguisher.
VRF is require a route distinguisher for BGP to distinguish between potentially identical prefixes received
from different VPNs.
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
How MPLS L3VPN Works
177
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