VPNv4 routes, route distinguishers and route targets.
Advanced · 25 min read
You'll learn to explain what a VPNv4 route is, say what route distinguishers and route targets do, and read a VPNv4 table.
Why BGP carries more than IPv4
Standard BGP carries IPv4 unicast. Multiprotocol BGP (MP-BGP) adds the ability to carry other kinds of address information through address families. One of the most important is VPNv4, used by service providers to offer private networks to many customers over a shared MPLS core.
How a customer route crosses the provider
On small screens, swipe sideways or focus the diagram and use the arrow keys.
The provider core never needs customer routes.
This is a basics lesson. It explains the vocabulary and reads a table. Building a complete MPLS core needs a course of its own.
How a VPNv4 route is built
On small screens, swipe sideways or focus the diagram and use the arrow keys.
The route distinguisher makes overlapping customer addresses unique. The route target decides who may see the route.
The two numbers that matter
The route distinguisher (RD) is prepended to a customer prefix so overlapping addresses stay unique. The route target (RT) is an extended community attached to the route. A VRF exports routes with an RT and imports routes that carry the RT it is configured to import. That is how a provider decides which sites see which routes.
Route distinguisher and route target
On small screens, swipe sideways or focus the diagram and use the arrow keys.
RD makes a prefix unique. RT decides which VRFs may import it.
A provider edge router. The vpnv4 session to the other edge router carries all customers. The VRF address family holds the customer's own eBGP session. Extended communities must be sent, or the route targets are lost.
Illustrative Cisco IOS XE output written for this lesson, not a router capture
PE1#show bgp vpnv4 unicast all
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 64500:1 (default for vrf CUST-A)
*> 10.1.0.0/16 192.0.2.10 0 0 64510 i
*>i 10.2.0.0/16 10.255.0.9 0 100 0 64511 i
Each route is grouped under its route distinguisher. The first was learned from the local customer, the second from the remote provider edge router over the vpnv4 session. These are customer prefixes, not core routes.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
The customer never sees the provider's addressing, and the P router never holds customer routes. Only the PEs know about the VPN, and they exchange its routes with MP-BGP.
Customer edge routers (CE) connect to provider edge routers (PE). Provider core routers (P) only forward labelled packets and need no customer routes. Only PE routers run VRFs and MP-BGP.
Roles in the provider network
On small screens, swipe sideways or focus the diagram and use the arrow keys.
Keeping customer routes at the PE keeps the core small.
Check your understanding
Predict · scenario 1
Two customers both use 10.1.0.0/16. How does BGP keep them apart?
Predict · scenario 2
What decides which VRF imports a route?
Predict · scenario 3
Route targets are not reaching the remote edge router. What is a likely cause?
FAQ
Why is a route distinguisher needed?
Two customers may use the same private addresses. Adding a distinguisher makes each VPNv4 prefix unique so BGP can carry both.
Does the route distinguisher decide who sees a route?
No. Route targets do. The distinguisher only makes the prefix unique.
Where do labels come from?
MPLS. The provider core forwards using labels, and BGP distributes the VPN label that tells the egress router which VRF the packet belongs to.