The prefix is the destination network and is denoted using the network address. In the context of MPLS, but also when referring to routing protocols and routing tables, the word prefix is almost always used to describe the destination network. You can find prefixes in the routing table, the BGP table, the MPLS forwarding table, and you can also find them in the routing updates sent by dynamic routing protocols.
For example, in the lesson, 192.168.2.0/24 is a prefix that is shared among routers on the network using MPLS.
Not really sure, but ins’t an error on the example 2.2 RT (Route Target)
CE advertise prefix PE1 tag the route with the rt I will tent to say this is the export isn’t it ? Then PE2 receives the prefix then import into the vrf and advertise to CE2 instead of exporting, could you please clarify?
The terms import and export have to do with the specific commands placed within the VRF used for the particular customer connected to that PE router. The route-target export command is used to export routes from the CE into the PE router and the MPLS domain. The route-target import command is used to import routes into the CE from the PE.
Now because often the same RT is used for both import and export, this is hard to see. Take a look at the following lesson:
In it, different RTs are used for import and export. You will notice that PE1 uses the 1:1 route target to export prefixes from CE1, and when PE2 receives these, they are indeed appended to RT 1:1 (and not to 2:2 which is the RT for the import command).
The VPNv4 Address is something that contains the RD (64 bits) and the IPv4 Prefix (32 bits). This refers to a particular prefix from a customer. The VPNv4 addresses are advertised between PE routers. Now it is confusing how this VPNv4 address contains information about the prefix length since it is 96 bits in length, there doesn’t seem to be any room for the prefix length information. If you take a look at the BGP prefix within the NLRI in a Wireshark capture, you’ll see that the Prefix Length is one of the parameters there.
For example, take a look at this NLRI information:
The prefix length includes the size of the label as well, which is 24 bits.
So for the first BGP prefix, we have 220.127.116.11/32 with a prefix length of 120:
label = 24 bits
RD = 64 bits
IPv4 prefix = 32
total = 120
For the second BGP prefix, we have 18.104.22.168/32 with a prefix length of 112:
label = 24 bits
RD = 64 bits
IPv4 prefix = 24
total = 112
Why is the IPv4 prefix 24? Well, it’s 22.214.171.124, which means the last 8 bits are zeros, which means the prefix ends there.
The RD for a certain VRF does not need to be the same across all of the MPLS domain. The RT will be used to import and export prefixes between VRFs on different PEs.
I believe that this Cisco community thread quite clearly explains it:
Specifically, it states:
MP-BGP exchanges the VPNv4 (RD:IPv4) prefixes between the PEs (P routers are not VPNv4 aware) not IPv4 routes - When a PE receives a VPNv4 route it discards the RD attached to it and attaches its local RD (according to the VRF that the route is going to be injected into according to the attached RT and the local VRFs import RT) - Since for the local router routing table database the VRF is identified via the RD prepended to the IPv4 route to construct the VPNv4 routes.
This is not a question, but a comment. I wanted to say this before I get tied up on other things and forget. This site is such a great wealth of knowledge. I find myself coming back here more and more for my Technical questions and needs. Your way of Teaching is so concise, clear, straight forward, and easy to understand and to grasp imo.
I have CBT Nuggets experience, IPTV pro, INE, Udemy, Global Knowledge, and few others in the past and present. But this site for me is my favorite. Keep up the great work Rene and others. I am learning at an accelerated rate and understanding what it is I am applying. This site is a big part of that. Thanks for being responsive as well! Thanks for the great content guys.
It’s posts like this that make it all worth it. Thanks so much for your kind words, @ReneMolenaar does his best to ensure the content is useful, relevant, and easy to understand, and we do our best to respond as quickly and comprehensively as possible. We do it because we like what we do.
We’ll do our utmost to maintain this QoS if you will () of the service.
We’re happy that the site is a helpful tool in your certification journey, as well as in the development of your skills and knowledge…
MPLS VPN is a technology that uses a combination of various features such as LDP, VRFs, and BGP. In the MP-BGP (Multi Protocol BGP) section of the following lesson, Rene goes on to explain that iBGP is used between PE routers in order for them to redistrubute and advertise each customer’s VRF routing table between PE routers. Specifically, MP-BGP is used to do so.