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34 Cards in this Set
- Front
- Back
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mpls fwds packets based on ___
instead of the usual way using ___ |
labels, instead of destination addresses
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what r control plane protocols?
why does mpls need them? |
OSPF or LDP... they are used to learn labels
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do hosts send packets labelled?
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nope
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what underlying switch technology does MPLS use?
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CEF
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what does CEF itself do normally?
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creates a FIB which is organised information for fwding packets, hence reducing time - it ignores the routing table
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do computers that use MPLS know that they are using it?
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nope
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what do PE n CE stand for?
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PE = provider (P in ISP) edge
CE = customer edge |
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what is applying a label called?
and what is removing a label called in mpls lingo? |
pushing and popping respectively
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what is an LSR? (i.e. what does it do?)
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label switch router that pushes pops or just forwards labelled packets
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what is an ELSR? (i.e. what does it do different?)
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edge label switch router. it is on the edge of the mpls network and processes both unlabelled and labelled packet
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What is the difference between FIB and LFIB?
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FIB - used fr incoming unlabelled packets
LFIB - used for incoming labelled packets (keep in mind a router identifying a unlabelled packet through FIB may label it) |
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how big is the mpls header and where is it?
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4 bytes and immediately before the IP header
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what r the components of an MPLS header?
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Label - 20bits
Experimental (EXP) - 3bits Bottom of stack(s) - 1bit TTL - 8bits |
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what two things do an IP header's TTL field support?
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mechanism to identify looping packets
AND method for traceroute to find I of each router in end to end route |
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why does MPLS need a TTL field?
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so LSR's can completely ignore the encapsulated IP header when forwarding IP packets
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what do ingress LSR's do to the MPLS TTL ?
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they copy the packet's IP TTL value on to the MPLS TTL
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what do regular LSR do to the MPLS TTL ?
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it decrements TTL per hop
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what do egress LSR's do to the MPLS TTL?
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they copy the MPLS TTL onto the IP TTL field!
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Can cisco routers be configured to disable MPLS TTLS configuration?
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yes. ingress ELSR's set MPLS TTL to 255. the IP TTL remains unchanged always.
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which control plane routing protocols do MPLS Unicast IP forwarding use?
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an IGP and LDP
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what control protocols do MPLS VPN's use?
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LDP and MP-BGP (multiprotocol)
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what is LDP?
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Label distribution protocol. it sends advertisements with IP prefix and label.
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what does an LSR do upon recieving a route through LDP?
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it creates a local label for the route
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what is a LSP?
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Label switched path. it is a combined set of labels that can be used to forward packets correctly to the destination
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where do LSRs store their label and related info?
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in LIB (not LFIB)
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Which forwarding databases contain labels only for the currently used best LSP segment?
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FIB and LFIB
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which forwarding databases contain all labels known to LSR whether the label is currently used for forwarding or not?
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LIB
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how does the LSR make the decision about the best label to use?
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they use the routing protocol's decision aboutbest hop. they look up information in the LIB and add it in the FIB and LFIB.
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how does LDP discover neighbors and determine what IP address the ensuing TCP connection should be made?
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using a hello protocol
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LDP hello uses what mcast IP?
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224.0.0.2
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how do LDP hellos transmit?
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UDP 646
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does LDP use TCP?
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yeah for updates. UDP is used for hellos
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who initiates TCP connection in an LDP session?
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the highest LDP ID
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MPLS VPNs depend on which underlying MPLS technology?
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MPLS Unicast forwarding
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