HOW MPLS WORK

[PietroPoliseno27977]

Hi everyone it is not clear to me how mpls works. I understand that the packet that enters the first router A is assigned a label immediately after having made a lookup in the routing table. After that, when router A goes from router B to the packet, the label is removed and in the meantime it performs a lookup of the labels to be assigned to the packet, and so on for any other router that must pass through the packet. My question is how do routers know which label to assign for the packet to follow the correct path to the destination network?

[Harold Ritter · Accepted solution]

> My question is how do routers know which label to assign for the packet to follow the correct path to the destination network?

The router knows where to forward the packets and what label should be used via a control plane protocol (LDP, RSVP. BGP-LU or (OSPF or ISIS in the case of Segment Routing)) or via a static MPLS mapping.

Regards,

[PietroPoliseno27977]

Ok it's all clear and thank you for the very detailed answer. One last question. So calculating the route a priori and assigning a label to each route, the performance improves because you avoid each time executing the query in the routing table but follow the path already established a priori in the right mpls table?

[Martin L · Accepted solution]

MPLS Operation is similar to Router building routing table (RIB), CEF creating Forwarding info base table (FIB) and adjacency table.  MPLS uses Label Information Base table (LIB) and Label FIB (LFIB).  LIB stores all labels known by router while LFIB stores label information only for currently used and the "best route".  To make decision about which label is "the best",  MPLS router will rely on routing protocol and Routing table (RIB).  For each route in RIB, router assigns next available label number, call "local" label and advertises it via LDP.  For each route learned via LDP, router records "remote" label, interface it came in, ID of LDP advertising router (loopback interface), and also adds its own new local label.  Note that remote label will became outgoing label later.  Each MPLS FIB table will have 2 labels (local and outgoing), prefix id and outgoing interface. Your local label is my outgoing label and vice versa. 

Regards, ML
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