JNCIS-SP > JMF-Ch3: Label Distribution Protocols > Flashcards

JMF-Ch3: Label Distribution Protocols Flashcards

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1
Q

What is MPLS?

A

Multi-Protocol Label Switching
A routing technique in that directs data from one node to the next based on short path labels rather than long network addresses, thus avoiding complex lookups in a routing table and speeding traffic flows

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2
Q

What are the three different Label Distribution Protocols (LDPs)?

A
  • RSVP
  • LDP
  • Seamless MPLS
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3
Q

Describe LDP

A

Offloads the forwarding plane of core routers by building a full mesh of label switched paths from any node to any other node.

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4
Q

Describe RSVP

A
  • Originally a bandwidth reservation protocol
  • Uses IP as its network layer (protocol number 46)
  • Extremely extensible
  • Signaling protocol, not a routing protocol
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5
Q

What are the RSVP Message type groups?

A
  • LSP Setup
  • LSP Tear-down
  • Error Handling
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6
Q

What are the RSVP message types for LSP setup?

A
  • Path: request for an LSP to be created or periodic refresh
  • Resv: Reserve resources for LSP
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7
Q

What are the RSVP messages used for LSP Tear-down?

A
  • PathTear: Remove path (and corresponding reservation) state
  • ResvTear: Remove reservation state
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8
Q

What are the RSVP Message types for Error handling?

A
  • PathErr: Error message sent upstream to sender
  • ResvErr: Error message sent downstream
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9
Q

What are some objects included in an RSVP PATH message?

A
  • Session: Uniquely identifies the LSP
  • Session attributes: priority, preemption, link/node protection
  • Sender T_Spec: requested bandwidth reservation
  • RSVP hop: the previous hop the PATH message traversed
  • Label Request: request for label binding
  • Record route: lists nodes traversed by the LSP
  • Explicit Route Object: used to specify the path the LSP should take
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10
Q

What are the steps for creating an LSP?

A
  1. RSVP PATH message is sent downstream toward the egress router
    1. Read by every router along the path
  2. The egress LSPR sends a RSVP RESV message to the egress LSR including the allocated label
    1. All transit labels receive a new local label and installs the entry into the label information base
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11
Q

What objects are included in the RSVP RESV message?

A
  • SESSION: Uniquely identifies the LSP being established
  • LABEL: label allocated by the previous hop
  • STYLE: specifies the reservation style (Fixed-filter, Shared Explicit, Wildcard)
  • RECORD_ROUTE: Records the actual path of the LSPs in terms of Nodes & interfaces traversed
  • HOP: Indicates the previous hop’s IP address
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12
Q

What signal condition would trigger the RSVP PathErr message?

A

When an LSR cannot forward the PATH message such as when MPLS is not enabled on the outgoing interface, or when reserving bandwidth if there is not enough free on the outgoing interface

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13
Q

What signal condition would trigger the RSVP ResvErr message?

A
  • When an RESV message is received, but the LSR cannot complete the reservation.
  • Can happen during MPLS label allocation failure
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14
Q

Which LSR receives RSVP PathErr messages?

A

The ingress LSR (Upstream)

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15
Q

Which LSR receives the RSVP ResvErr Message?

A

The egress LSR (downstream)

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16
Q

Which LSR creates the RSVP PATH message?

A

ingress LSR

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17
Q

Which LSR creates the RSVP RESV message?

A

the egress LSR

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18
Q

What is an Explicit Route Object (ERO)?

A
  • Allows the ingress LSR to control the path through which an LSP is established independent of the IGP best path.
  • Used for traffic-engineering
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19
Q

When using an ERO, which router receives the PATH message?

A
  • The first hop listed in the ERO
  • e.g. Ingress LSR R1 sends RSVP with destination of R7. This will be routed according to the best IGP path to the destination.
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20
Q

What does ERO loose constraint processing do?

A

When the first constraint cannot be fulfilled, the loose qualifier creates a PATH state, and forwards the message onto the next constraint to be fulfilled using the best IGP route

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21
Q

What action will occur when a router receives a PATH message with an ERO constraint that it can verify?

A

The fulfilled ERO constraint is removed, and the PATH message is forwarded with the remaining ERO constraints

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22
Q

When the ERO constraint is set to strict, what will occur when the router cannot verify the constraint?

A

the LSR will send a PathErr message of ‘Wrong Delivery’ back upstream

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23
Q

What is the purpose of the Record Route object (RRO)?

A
  • Provides end-to-end path visibility
  • Each nodes adds to the RRO the address of the interface on which the PATH message was received.
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24
Q

How can RRO help to avoid routing loops?

A

If a node receives a PATH message with one of its own local addresses already listed within the RRO, the LSR sends back a PathErr message indicating a Routing Loop detected and aborts the LSP setup.

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25
What is the minimum required for RSVP Configuration?
* ‘family mpls’ must be configured at the logical unit level * all interfaces must be listed under ‘[edit protocols mpls]’ level
26
What are the mandatory parameters for configuring RSVP?
* LSP Name * LSP Endpoint * This will be installed into the inet.3 table as a /32 prefix which will then be used by MPLS applications such as Traffic Engineering & MPLS VPNs
27
On a Junos device, where in the hierarchy is RSVP configured?
[protocols mpls]
28
What is the function of the ‘no-cspf’ command?
* Disables automatic path computation via IGP TE (Traffic Engineering) extensions * By default the JUNOS OS tries to use ‘Constrained Shortest Path First’ (CSPF) to pre-compute LSP Paths
29
What route object is being configured in the following image?
Specifying an Explicit Route Object
30
Describe the following command: show mpls lsp
* By far the most useful command to check lsp status * output can be restricted using ‘ingress’, ‘egress’, or 'transit', or select a single LSP by name *(note: show mpls lsp extensive is also an option)*
31
Describe the following command: show rsvp sessions
Complements 'show mpls lsp' by providing actual MPLS labels (label-in, label-out), the current path MTU, and traffic protection features active at the current node
32
Describe the following command: show rsvp interface
Shows number of existing reservations, and the bandwidth available to new reservations
33
Describe the following commands: clear rsvp session, clear mpls lsp
These commands tear down and re-signal LSPs
34
What is the preferred way to detect LSP failure?
Bidirectional Forwarding Detection (BFD)
35
What is an RSVP Auto-mesh?
* Creates the new LSPs automatically triggered by a BGP session coming up to a new node, and advertising labeled routes (e.g L3VPN) * Stale LSPs are torn down after a timeout
36
What are the requirements for a basic RSVP Auto-mesh configuration?
* Destination prefix parameters * id * LSP Template (optional)
37
What command can be used to list all LSPs that have been dynamically created by RSVP Auto-mesh?
‘show dynamic-tunnels database'
38
What is the function of the ‘ultimate-hop-popping’ command?
Disables penultimate-hop popping, and overrides ‘explicit null’. Preserving the identify of an LSP even at the egress LSR. Required for some applications such as MPLS-TP, and in-band OAM
39
What command allows you to monitor RSVP Loss/Delay?
'monitor mpls loss-delay rsvp '
40
What function can be configured to provide debugging for RSVP?
traceoptions
41
What is LDP?
* Layer Distribution Protocol, a subset of Layer Distribution Protocols. * Prevents the need for full route lookup on every packet. * Introduces Forwarding Equivalence Class FEC) * Creates a tree of LSPs
42
What are the LDP Message Types?
* Discovery: Locate potential LDP peers (uses UDP, Port 646, multicast 224.0.0.2) * Session: Manage peer-to-peer TCP sessions (higher IP is responsible for establishing TCP session) * Advertisement: Create, change or delete label mappings * Notification: provide advisory information (includes errors, and advisory)
43
Describe LDP Label Assignment
* Downstream peer assigns labels * Scalable, 1 LDP session can maintain all LSPs between two routers) * Follows IGP, does not support explicit routing, Cannot be used for traffic engineering
44
Describe the minimal LDP Configuration
* listing the interfaces on which LDP is requested at [edit protocol ldp] level * configuring family mpls at the logical unit level and listing the interface at the [edit protocols mpls] level
45
What methods are available for authentication LDP Sessions?
* MD5 Authentication * Keychain (hitless key rollover)
46
What are the three types of LDP Policies?
* import: affects label mappings received from neighbors * export: affects label mappings sent to neighbors * egress: allows distribution of additional prefixes
47
What is LDP Tunneling?
Combines RSVP Traffic Engineering with LDP label distribution
48
What are the requirements to setup an LDP Tunnel?
* Two LSPs (i.e. a bidrectional RSVP path) between endpoints * A targeted LDP session between routers' loobacks * Uses two stacked tables, inner LDP (preserved across the core) and outer RSVP (subject to swap at every hop)
49
Describe: 'show ldp interface'
Used to check the which interfaces LDP is running on, as well as the number of neighbors seen behind each interface
50
Describe: 'show ldp neighbor'
lists the LDP neighbors
51
Describe: 'show ldp session'
shows the status and hold-time of each active LDP session together with the advertising mode.
52
Describe: 'show ldp database session \<*session*\>'
returns the actual label mappings sent and received from an LDP neighbor
53
Describe: 'show route protocol ldp table inet.3'
Shows all LDP LSP destinations (LDP egress points) *note: omiting 'inet.3' LDP-Installed entries including tables for transit.*
54
Describe: 'show mpls label usage'
displays the available label space as an absolute value, and as a percentage of the total
55
Describe 'Seamless MPLS' or 'Unified MPLS'
* All user data is forwarded using MPLS from entry to exit from network * no MPLS boundaries exist * Network can be divided into regions * Helps scale when there are 10s of thousands of nodes as IGPs do not generally scale at that level
56
What types of nodes appear in an MPLS network?
* Service Node (SN) * Transport Node (TN) * Border Node (BN) * Service Helper (SH) * Access Node (AN) * End Node (EN)
57
Describe the following MPLS Node: Service Node (SN)
Nodes that apply services to customer packets ex. Layer 2 PE routers, Layer 3 PE routers, SONET Clock Generators (SCGs)
58
Describe the following MPLS Node: Transport Node (TN)
Transport nodes are ones that connect access nodes to service nodes, and service nodes to service nodes. Ideally TNs do not have a customer or service specific configuration
59
Describe the following MPLS Node: Border Node (BN)
Border Nodes enable inter-region packet transport similar to ABRs and ASBRs
60
Describe the following MPLS Node: Access Node (AN)
Access Nodes are where traffic enters the MPLS network. Ex. PE, DSLAM or Cell-Site router.
61
Describe the following MPLS Node: End Node (EN)
End Nodes are nodes at the customer site, i.e the CE device
62
Which router requires you to define the label-switched-path (lsp) when using RSVP?
The only router that requires configuration is the ingress routers. The others need to have protocols MPLS & RSVP configured, but do not need configuration specific to the lsp.
63
Does the Junos OS Support traffic engineering on LDP LSPs?
No. However you can use LDP tunneling over RSVP Traffic Engineered LSPs to apply traffic engineering to LDP Traffic
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