Term
| What is the size of the Route Distinguisher? |
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Definition
The RD is 8 bytes:
2 bytes formats, then one of the following:
2-byte:4byte
4-byte:2byte
4-byte dotted decimal:2 byte |
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Term
| What is the purpose of the RD? |
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Definition
The RD is sent with the prefix in the NLRI. It allows BGP to advertise and distinguish between overlapping prefixes.
"RDs help BGP" |
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Term
| What is the format and purpose of the RT? How is it sent? |
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Definition
The Route Target identifies the VRF into which a route should be placed.
Same format as RD (8 bytes).
Send in Extended COmmunity path attribute.
"RTs help TRansition between BGP and VRF" |
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Term
| What does "show mpls ldp bindings" show? |
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Definition
| "show mpls ldp binding" shows the LIB |
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Term
| What does "show mpls forwarding-table" show? |
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Definition
| "show mpls forwarding-table" shows the LFIB. |
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Term
| What do routers use to forward labeled packets? |
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Definition
| Labeled packets are always forwarded using the LFIB. |
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Term
| How big is the MPLS label header and what fields does it contain? |
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Definition
4 bytes total
20 bits label
3 bits Exp (CoS)
1 bit S (bottom of stack)
8 bits TTL |
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Term
| What is Label 0 and when is it used? |
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Definition
0 = IPv4 explicit NULL label.
When advertised to upstream LSR, the upstream LSR knows to swap the label for Label 0. Label 0 preserves EXP (Cos) bits for the egress LSR. |
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Term
| Does Label 3 ever appear in a label header? |
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Definition
| No. Label 3 is the implicit Null label, advertised to the penultimate LSR. If the PHP sees Label 3 in the LFIB, it pops the label and forwards the packet. This saves the egress LSR from doing an unnecessary lookup. |
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Term
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Definition
| Label 2 is the IPv6 Explicit Null Label -- the IPv6 version of Label 0 |
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Term
What does the FIB do?
When does a router consult the FIB?
How can you see the FIB? |
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Definition
The FIB takes routing info from the RIB, adds next-hop router and outgoing interface for each prefix.
A router uses the FIB to route unlabeled packets.
show ip cef gives the contents of the FIB |
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Term
For what does the router use the LFIB?
How can you see the LFIB? |
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Definition
The router uses the LFIB to forward incoming labeled packets.
"show mpls forwarding-table" shows the LFIB |
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Term
| What command would you use to see LDP timers and version? |
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Definition
Router# show mpls ldp parameters
Protocol version: 1
Downstream label pool: min label 16; max label 100000
Session hold time: 180 sec; keep alive interval: 60 sec
Discovery hello: holdtime: 15 sec; interval: 5 sec
Discovery targeted hello: holdtime: 180 sec; interval: 5 sec LDP for targeted sessions;
peer acl:
1 LDP initial/maximum backoff: 30/240 sec |
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Term
| How does the ingress LSR set the MPLS TTL field? |
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Definition
By default, the IP TTL is copied into MPLS TTL.
"no mpls ip ttl-propagation" will turn this off |
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Term
| What command allow you to hide the structure of the MPLS network from a traceroute command |
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Definition
| "no mpls ip ttl-propagation" |
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Term
| What command on the ingress LSR (PE1) allows the structure of the MPLS network to be hidden from customers but not from the provider? |
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Definition
no mpls ip ttl-propagation forwarded
Only locally sourced packets get IP TTL copied to MPLS TTL and decremented. |
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Term
| Where are all known labels and known prefixes stored (not just the best ones)? |
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Definition
In the LIB.
LFIB and FIB known only labels for best segment. |
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Term
| What does "show mpls ldp bindings" show you? |
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Definition
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Term
| What does the VPNV4 NLRI contain? |
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Definition
| The RD and the IP Prefix. |
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Term
| Give 2 different valid formats for an RD. |
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Definition
123:40 (AS 123)
10.1.1.1:40 (IP based) |
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Term
| Does PHP run in cell mode? |
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Definition
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Term
| What does the LIB contain when this LSR is a PHP for a prefix? |
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Definition
| LIB contains Label 3 (Implicit Null) when it is this PHP. |
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Term
| What are the two labels in a MPLS VPN label stack for? |
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Definition
top label points to the egress router
the second label identifies the outgoing interface or a routing table |
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Term
| On what devices are VRFs used? |
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Definition
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Term
| What are the components of an Edge LSR control plane? |
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Definition
Routing Protocol
IP Routing Table
Label Distribution Protocol |
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Term
| What are the components of an edge LSR data plane? |
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Definition
IP Forwarding Table (FIB)
Label Forwarding Table (LFIB) |
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Term
| What kind of links are used to implement MPLS VPNs? |
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Definition
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Term
| What multicast address is used for LDP Hello packets? |
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Definition
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Term
| What technology was introduced in order to identify VPN memberships when there is overlap? |
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Definition
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Term
| What is the value of the MPLS TTL when "no mpls ip ttl-propagation" is configured? |
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Definition
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Term
| What does "debug mpls ldp peer state-machine" show? |
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Definition
| higher-level deals with setting up and shutting down label advertisements |
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Term
| What are the requirements when using a loopback address for the LDP ID? |
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Definition
| When using a loopback address for the LDP ID it must feature a 32 bit mask and be advertised |
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Term
What does the "imp-null" indicate below?
Router# show mpls ldp bindings
tib entry: 10.0.0.0/8, rev 4
local binding: tag: imp-null
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Definition
| "imp-null" shows up in the LIB when this is the PHP (isntead of Label 3) |
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