What Layer is MPLS?. This basic question needs to be clarified for the Network Engineers. MPLS - Multi-Protocol Label Switching first was invented for fast packet processing. As MPLS Label is 20 bits, and IP address is 32 bits, processing MPLS Label was considered faster back in old times. Today, as of 2022 MPLS is used for many different purposes, one of the most common reasons to have MPLS in the Networks is VPN. MPLS VPNs, MPLS Layer 2 VPN, and MPLS Layer 3 VPN are two of the most common VPN mechanisms in large Enterprise and Service Provider Networks.
The label is used for two reasons in MPLS VPNs. A tunnel label or transport label is used for the reachability between the edge devices, PE devices in MPLS networks. And another label, which is the VPN label is used to differentiate the customers in MPLS VPN. Both of these labels are placed between MAC Header and IP Header in IP Packets. Thus, as you can see from the below picture as well, MPLS is commonly referred to as Layer 2.5.
Figure - MPLS is Layer 2.5
As you can see, from the above picture, it is Layer 2.5. Also, the Label is 20 bits. EXP is Experimental bits, which are 3 bits, used for Quality of Service Purpose. 8 bit is for TTL, Time to Live, and 1 bit is for identifying whether there is more label in the label stack or not. We have many other blog posts on MPLS on the website, from beginner to very expert level, we recommend you to check our MPLS Training as well.
Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021
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