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CCIE Network Design FAQ: Other Large Scale Cores

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CCIE Network Design FAQ: Large Scale Cores

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Figure: Full Mesh Neighbors

Q1. Is NHRP a routing protocol, or is it a protocol that helps routing protocols do their job?

Answer: A routing protocol.

Q2. How many paths exist through a network with 30 nodes? 40?

Answer: 30 nodes has 870 paths; 40 nodes has 1560 paths.

Q3. What task does a route server in NHRP perform?

Answer: Collects and stores routing information from the routers on the NHRP network.

Q4. When a router on an NHRP network wants to find the SVC to use for a given destination, what does it do?

Answer: It queries the route server.

Q5. What three steps are normally involved in routing a packet?

Answer:

  1. Look up the destination in the routing table.
  2. Perform a longest prefix match to find the correct destination.
  3. Rewrite the MAC header on the packet.

Q6. What type of switching paradigm do ATM and Frame Relay use?

Answer: Label swapping.

Q7. What type of switching paradigm does MPLS use?

Answer: Label swapping.

Q8. What is a push? A pop?

Answer: A push is when a label is pushed onto the top of the label stack; a pop is when a label is removed from the top of the label stack.

Q9. What is a FEC?

Answer: A forwarding equivalence class; a stream or flow of packets between a given set of sources and a given destination.

Q10. Why do you merge FECs?

Answer: FECs, or streams, are merged for scalability. Once several FECs have been merged, downstream LSRs need only to deal with a single label and a single path for multiple source/destination pairs.

Q11. Explain each type of label assignment:

Answer:

  • Host pair— A label is assigned for each source/destination address.
  • Port quadruple— A label is assigned for each source address and port/destination address and port.
  • Port quadruple with ToS— A label is assigned for each source address and port/destination address and port with a given ToS, or class of service.
  • Network pair— A label is assigned for each source/destination network.
  • Destination network— A label is assigned for each destination network.
  • Egress router— A label is assigned for each egress router.
  • Destination AS— A label is assigned for each destination BGP AS.

Q12. Which device assigns labels in an MPLS network?

Answer: The control component.

Q13. Do downstream devices or upstream devices assign labels?

Answer: Downstream devices.

Q14. What are the two ways of driving label assignment?

Answer: · Data driven, where a label is assigned when the first data packet arrives in the network.
· Control driven, where a label is assigned when the routing information changes.

Q15. How is tunneling performed in an MPLS network?

Answer: By stacking labels. An extra label is pushed onto the stack by the LSR at the tunnel entrance and is popped at the egress of the tunnel. (Actually, it could be popped one hop before the tunnel exit.)

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