Hi
P3 receives the labeled packet and will pop the label, forwarding the IP packet to PE2. This is called penultimate hop popping and is performed to save PE2 the trouble of looking at the MPLS label.
why P3 is POP the lable , why P2 not doing
Thanks
Hello Sims
The reason we want to do this is to save PE2 from doing one more lookup. It is a way of reducing CPU and memory usage on devices and making the process more efficient. Take a look at this post for more details:
I hope this has been helpful!
Laz
Q-Why CEF is mandatory in MPLS , Can u give me a high level over view. ??
Q-P router contains only the label of PE routers loopback …Right ??
Hello Narad
As stated by Rene in the following post, the Forwarding Information Base (FIB), which is an integral part of the function of MPLS, is the CEF table. CEF is the fundamental switching path for MPLS. Without CEF, MPLS forwarding does not occur. For this reason, CEF is mandatory for MPLS to function.
The labels found within the LDP bindings table correspond to the LSR ID. The LSR ID is by default the highest IP address on a loopback interface. So in this case, yes, the labels will correspond to the Loopback interface IPs of the routers.
I hope this has been helpful!
Laz
I want to build a combined P/PE on a single device. Maybe i’m looking wrong but i can’t find it.
Maybe somebody over here knows were i can find the combined P/PE config settings.
thank you
Hello Michel
I assume you are referring to the Provider (P) routers and the Provider Edge (PE) routers used in MPLS, correct?
Keep in mind that the only difference between a P and PE router is the fact that the PE router has the added configuration of VRFs and customer connections. Otherwise the rest of the configuration is the same.
So really, it’s not so much the configuration of the router that makes it a P or PE, but the location it has in the topology. Imagine you have an MPLS network of just two routers connected to each other. They would both be PE routers since they connect customers, but they would also perform label distribution like a P router. Even so, they are called PE routers. Such a topology would have no P routers at all.
So really, there is no such thing as a combined P/PE router. Either you’re a PE router because you’re connected to customers, or you’re a P router because you’re not connected to customers. There’s no combining the two. Does that make sense?
I hope this has been helpful!
Laz
Yes, this was very helpful. Thank you.
Gr,
Michel
Hi Rene
Your sessions are easy to understand, Can you please throw some light on BGP LU - BGP LU ( Labeled unicast) ?
Hello Tejas
BGP Labeled Unicast or BGP-LU is a technology that combines BGP with MPLS for efficient network routing.
In a typical IP network, the destination IP address is used to make routing decisions by each individual router along the path. In MPLS, however, it is the first router that makes a decision on the destination address and assigns a label to the packet. This label is then used by all subsequent routers to quickly and efficiently route the packet to its destination without having to independently determine the best route.
Now typically in an MPLS topology, these labels are exchanged using the Lable Distribution Protocol or LDP. More about that here:
BGP-LU can be viewed as an alternative to the use of LDP for the exchange of labels. BGP-LU uses BGP UPDATES to exchange these MPLS labels between routers, so it can also be used in a large-scale network where BGP is typically used, like ISP networks that compose sections of the internet. This allows for efficient, scalable routing over large distances.
Within the BGP UPDATE header, there is a field called Subsequent Address Family Identifier (SAFI). This identifier indicates what kind of information is carried by the update. A SAFI of 4 indicates that an MPLS label is included in the update using BGP-LU.
BGP-LU is often used in Service Provider networks for interconnecting different parts of the network or for providing MPLS-based services to customers. It also provides a way for network service providers to create VPNs and other services on top of their existing IP networks.
When would you use BGP-LU instead of LDP? Primarily when your MPLS topology spans multiple ASes. Typically, using BGP-LU will be able to exchange labels across AS borders much more easily than LDP would.
It’s important to remember that BGP-LU is not a separate protocol but a mechanism or feature that combines the capabilities of BGP and MPLS. The actual protocols used for communication remain BGP for routing information and MPLS for label information.
BGP-LU is defined in RFC 3107, and more about it in the context of Cisco can be found here:
I hope this has been helpful!
Laz
Hello!
I am still pretty early when it comes to MPLS and labels and one thing caught me off guard a little… So apparently, BGP prefixes are not installed into the LFIB and labelled?
Following the Introduction to MPLS topology without the GRE tunnel and with MPLS enabled.
Why is the BGP prefix not in the MPLS forwarding table? And if it’s not there, how does PE1 know what labels should traffic destined for 5.5.5.5/32 use?
I’ve read somewhere that only IGP prefixes are actually installed into the forwarding table and labelled? Why so? I’ve read it’s because MP-BGP can be used for L3 VPNs but that confused me a little…
Thank you!
Hello David
This does sound strange indeed. However, this is how MPLS is intended to work. The LDP labels are used to denote specific paths within the MPLS core, so you will only see prefixes from within the core in the forwarding table. Indeed, the label that is added corresponds to the next hop IP rather than the actual destination network prefix.
A PE receives a packet from a CE, and if the destination is a remote site reachable via another PE router, the BGP table for that VRF will have an entry indicating the next hop (which is typically the other PE router). The corresponding label is added to the packet.
In the example you shared, the 5.5.5.5 network based on the BGP table, is reachable with a next hop of 3.3.3.3 (PE2). The label added to the packet that denotes the particlar path in the MPLS infrastructure is 17, which is the one added to the next hop address of 3.3.3.3.
I hope this has been helpful!
Laz
Hi Lazarus,
if we consider BGP-LU an alternative to LDP label exchange, can we have an MPLS environment using ONLY BGP-LU? (No, IGP, no LDP no RSVP, just BGP and LU).
Thanks!
Hello Alberto
Yes, it is possible to have an MPLS environment using only BGP-LU. However, it’s important to note that while you can technically operate an MPLS network without an IGP, LDP, and RSVP, it’s not necessarily ideal for all scenarios. BGP-LU alone may not be sufficient for networks requiring traffic engineering, fast reroute, or other advanced MPLS features. Those typically require protocols like RSVP for signaling and IGP for intra-domain routing information.
So while it’s technically possible, the decision to use only BGP-LU should be based on your specific network requirements and design considerations.
I hope this has been helpful!
Laz
Thanks a lot Laz
This question is just for studying purposes, trying to implement on a lab.
Regards
Alberto
