JMF-Ch2: MPLS Fundamentals Flashcards
What concerns does MPLS address?
- Scalability of the IP Control plane
- Necessity of integrating with (and eventually replacing) contemporary layer-2 technologies like ATM and Frame Relay
- The need for fine-grained traffic engineering capabilities much more accurate than any approach based on conventional IP Routing
How does MPLS address the traffic engineering & routing scalability concerns?
- Prepend each packet with a small, fixed-length header with a numerical value
- Forward the packet using only the label value
- The possibility of setting arbitrary LSPs to solve complex traffic engineering problems
What are some other MPLS applications?
- Pseudowires for transport of arbitrary layer 2 frames
- Virtual Private Networks
- VPLS, EVP, L2VPN, L3VPN, Multicast technologies
- P2MP LSP for multicast distribution
Describe the MPLS Header
- a 32-bit header that is prepended to each packet as it enters the MPLS domain, and is typically removed at the end of the label-switched-path.
- ‘label’ is commonly used to refer to the entire header
What are the components of the MPLS Header?
- 20-bit Label: used to associate a packet with an LSP
- 3-bit Traffic Class: Formerly ‘Experimental bits’, used for CoS
- 1-bit: Bottom-of-Stack bit: Some applications require multiple stacked labels. This is set to 1 if this is the last label before the payload
- 8-bit Time to Live (TTL): Decremented at every hop
- By default, the IP TTL is copied into the MPLS TTL
What is the significance of labels?
By default, labels only have local significance. At every hop the label will typically change
Labels are generally global to a router, and not tied to the ingress interface. Filters are applied based upon the label & prefix filters, not on interface based filters
What, if any, MPLS labels are reserved?
Labels 0-15 are reserved
4-6,8-12, & 15 are unassigned at this time.
What are explicit & implicit null labels?
- Explicit Null (Label 0): Always assigned an action of ‘decapsulate’ (pop)
- Implicit Null (Label 3): Never in actual MPLS Frames, only within MPLS signaling. Used by the egress router to request the previous router remove the MPLS header. i.e. ‘penultimate-hop popping’ which is the Junos OS Default
What is an LSR?
- Label-switched router
- Any router which participates in MPLS forwarding, including both ingress & egress nodes.
What are the MPLS Label Operations?
- Push: add an MPLS label to a packet (Ingress LSR)
- Swap: replaces the incoming label value with another (Transit LSr)
- Pop: Removes an MPLS label from a packet (Egress/penultimate LSR)
What is an LSP?
- Label-Switched Path
- A unidirectional path through the network defined in terms of label operations
- Typically assigned by a label distribution protocol, but can be configured statically
What is an iLSR?
- Ingress Label-Switching Router
- Performs the initial label operation of ‘push’, and encapsulates non-MPLS traffic by adding one or more labels prior to forwarding it down a label-switched path
What is a tLSR?
- Transit Label-Switching Router
- Typically performs swap operations
- Forwarding decisions exclusively based on the label value
What is the LIB?
- Label Information Base
- Contains the actual label switching table
- Maps incoming labels to forwarding actions
What is penultimate-hop popping?
- MPLS header is removed by the 2nd to lats router in an LSP
- Facilitates forwarding on the last hop and is enabled by default for most MPLS applications
- Avoids having to both remove the MPLS header, and do an additional route lookup
What is an eLSR?
- Egress Label-Switching Router, or tail-end
- Takes a forwarding decision based on the packet payload
What are two scenarios where penultimate-hop popping must be disabled?
- Class of Service applications
- Applications that need to preserve the ‘identity’ of the LSP even at the last hop. ex. Multicast in a VPN, Circuit-cross-connect (CCC)
Where is the LIB stored?
in the mpls.0 table
What needs to be configured to allow an interface to process MPLS frames?
enable family mpls at the logical unit level
set interfaces ge-1/0/0 unit 0 family mpls
What configuration steps must be performed to configure the MPLS protocol?
Specify which interfaces will be running MPLS
What command would verify the MPLS interface configuration?
‘show mpls interface’
What are possible reasons why an MPLS interface might be missing from ‘show mpls interface’?
- family mpls not configured at the logical unit level
- interface has not been listed at the [edit protocols mpls] level
What routing tables are directly affected by MPLS?
- mpls.0
- mpls label-switching table
- maps incoming labels to forwarding actions
- inet.3
- table of LSP egress points
- contains signaled LSPs at ingress node
What command allows for verification of the LSP state?
‘show mpls static-lsp ingress’
