Computer Networks CS 3623 15 Multiprotocol Label Switching
Computer Networks (CS 3623) #15 | Multiprotocol Label Switching
Routing tables & IP forwarding
MPLS Datagram • relies on IP addresses and IP routing protocols to do its job Virtual Circuit • Forward packets by examining relatively short, fixed-length labels, and these labels have local scope
MPLS: What is it good for? • Destination-based forwarding • Explicit routing • Virtual Private Network and Tunnel
#1 DESTINATION-BASED FORWARDING
MPLS: Label allocation • Label Distribution Protocol
Label Edge Router (LER) • R 1 = Label Edge Router (LER) • Performs a complete IP lookup on arriving IP packets, and then applies labels to them as a result of the lookup
Forwarding algorithm • MPLS: Label forwarding (exact match) – [vs] IP destination address lookup (the longest match) • Label is associated with a forwarding equivalence class (FEC) – can be formed using almost any criteria • MPLS Routing = standard IP routing algorithm
Label Header • on a frame encapsulated packet – Ethernet, token ring, PPP etc. • ATM:
Consequences • devices that previously didn’t know how to forward IP packets can be used to forward IP traffic in an MPLS network
#2 EXPLICIT ROUTING
2 FEC: load balancing • normal IP routing = ? ? ? • Use Resource Reservation Protocol (RSVP)
Applications • Traffic engineering – ensuring that sufficient resources are available in a network to meet the demands • Fast reroute – precalculation of the backup path and the explicit routing – No need to wait for routing protocol to find another path
To find a route • explicit routes need not be calculated by a network operator automatically • The most common = Constrained Shortest Path First (CSPF) – like the link-state algorithms, but which also takes constraints into account – Exp: find a path from R 1 to R 7 in that could carry an offered load of 100 Mbps
#3 VIRTUAL PRIVATE NETWORK AND TUNNEL
An ATM circuit is emulated by a tunnel
MPLS tunnel • the tunnel header = MPLS header
Example of layer 3 VPN
- Slides: 18