Transcript Slides

MPLS
• H/W update
• Brief description of the lab
• What it is?
• Why do we need it?
• Mechanisms and Protocols
Multi-Protocol Label Switching
(MPLS)
• A mechanism for building tunnels
– Label Switched Path (LSP)
• Forwarding based on “labels”
– 20 bits for label
• Packet contains MPLS header
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Label
3 EXP (experimental) bits
8 bits for TTL
1 bit for bottom of stack
• EXAMPLE
• LSPs are unidirectional
Label Operations
• PUSH
– Add a label on the packet
• At the entry point of the tunnel
– Packet may already have a label
• Label stack
• POP
– Remove a label from the packet
– At the exit point of the tunnel
• SWAP
– Exchange the label on the packet with another
one
– During forwarding inside the tunnel
LSP
• Has an “ingress”
– Entrance
• Egress
– Exit
• It goes over some transit nodes
• What traffic does it carry
– Forwarding Equivalence class (FEC)
– Anything that matches some rules that use the IP header
information
• We not have the “EDGE” where information is put
into the LSP
– Edge classifies packet into a FEC and there is no need
to do it again inside the network
MPLS forwarding
• Routers than can do MPLS forwarding are
called Label Switch Routers (LSRs)
– Both a router and an LSR
• Before we had the RIB and FIB
• Now we (also) have the LIB and the LFIB
• Lookup incoming label in LFIB
– Determine where the packet goes and
– What label it should carry
– SWAP labels and send the packet
Label Stack
• Can have tunnel hierachies
– Push another label on a labeled path
– I have a label stack
• Forwarding decisions are always based on
the label at the top of the stack
• Swap operations apply to the label at the top
of the stack
Signaling
• In order to create the LSP somebody has to
setup the labels
– They are put in the LFIB – Label FIB
• MPLS signaling protocols
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Label Distribution Protocol (LDP)
RSVP-TE
Even BGP (for VPNs)
They distribute labels and establish tunnels
Reserved Labels
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0 – IPv4 explicit NULL
1 – Router Altert
2 – IPv6 explicit NULL
3 - Implicit NULL
What are these NULLs?
– Determine what the Penultimate hop (PHP) will
do
• May pop the label completely
• Or may keep an empty label in the packet
PHP
• Penultimate Hop Popping
– The egress scales better
– But does not know that it was an MPLS packet
– Potentially useful information (e.g. EXP bits)
are lost
Label Distribution modes
• Ordered vs. Independent
– Will I wait for the downstream nodes to give
me their label?
• Unsolicited vs. on-demand
– Will I wait for a neighbor to ask me for a label
or I will send my labels to them anyway?
• Label Space
– Per platform
– Per interface
• Labels have link-local scope
– They are unique for the same link only
What is the big deal
• Initially people though MPLS forwarding
will be faster than IP forwarding
– Not an issue anymore
• Now people like MPLS because it can be
used to setup tunnels
– Tunnels for Traffic Engineering
– Tunnels for Recovery from failures
– Tunnels for VPNs
How can I use the tunnels
• Use them if I am their ingress
– Can have a static route pointing to a tunnel next-hop
• Share traffic
– Between IGP
– Between IGP/LSPs
– Between LSPs
• Can share with unequal weights
• Advertise them in IGP and use them as links
– “forwarding adjacencies”
• I already can do things that I could not do with
IGP
– Can control quite well where my traffic goes
• EXAMPLE
Intro to TE
• An ISP has a number of core routers that
attach to PoPs
– ISP usually knows the traffic matrix
• Traffic demands between PoPs
– ISP would like to control exactly how this
traffic moves around in his network
– Can setup a full mesh of LSPs and control
exactly what is going on
• Of course scalability is a issue
RSVP-TE
• Based on Resource Reservation Protocol
– Extended to carry labels (TE)
• It is a signaling protocol
– Sets up network state
– Does not have anything to do with
routing/forwarding
• Basic Idea
– I am telling RSVP the path I want and it will
setup an LSP over this path
– Specify the whole path
• Loose of strict source route
• RSVP calls it Explicit Route object (ERO)
PATH and RESV messages
• Ingress sends a PATH message downstream
– Follows the route we specified until it hits the
egress
• ERO or simply IP forwarding towards the
destination
• Egress sends a RESV message upstream
– Until it hits the ingress
• Intermediate routers install state
• Labels are allocated
– Downstream ordered mode
Soft state
• PATH and RESV messages create state in RSVP
routers
• PATH and RESV messages are send periodically
to refresh this state
• If state is not refreshed in time it expires and it is
cleared
• Simpler
– Do not need to worry about reliable messages etc
– Do not need to check with hellos if my neighbor is alive
– But I have the overhead of state refreshes
A big example
• How messages are sent
• How labels are allocated