CCNP SP MPLS FAQ: Cisco Express Forwarding

CCNP SP MPLS FAQ: Cisco Express Forwarding

Q1. Name the two components of CEF.

Answer: The two components of CEF are the adjacency table and the FIB or CEF table.

Q2. Name the three most common packet switching methods in Cisco IOS.

Answer: Process switching, fast switching, and CEF switching are the three most commonpacket switching methods in Cisco IOS.

Q3. Why does MPLS use CEF?

Answer: MPLS uses CEF because it is the only switching method in Cisco IOS that implements labeling of incoming IP packets.

Q4. What is the adjacency table used for?

Answer: The adjacency table takes care of the Layer 2 rewrite of the frames that are switched by the router.

Q5. What fields of the IP header does CEF use to load-balance IP packets?

Answer: CEF uses the destination IP address and the source IP address to load-balance IP packets.

Q6. How does CEF perform equal and unequal cost load balancing?

Answer: CEF uses a hashing algorithm that looks at the source and destination IP address of the packet. The hash result points to one or more of the 16 hash buckets, which in turn indicates which adjacency to use to forward the packet.

Q7. How does a prefix in the CEF table get the imposed label stack?

Answer: Each label in the imposed label stack can be assigned directly via LDP, BGP, or RSVP, or it can be inherited from recursion.

Q8. How is load balancing of labeled packets performed in Cisco IOS?

Answer: If the MPLS payload is an IPv4 or IPv6 packet, CEF hashing is performed on the source and destination IP address. If the MPLS payload is anything else, the load balancing is determined by the value of the bottom label in the label stack.

Q9. Name two reasons not to use CEF per-packet load balancing.

Answer: Two reasons for not using CEF per-packet load balancing are these:

  • Receipt of out-of-sequence packets, resulting in reordering of the packets or dropping of the packets
  • Jitter (variable delay)

Q10. Name two huge differences between the fast switching and the CEF switching methods.

Answer: Fast switching and CEF switching are different in the following two ways:

  • Fast switching uses an on-demand route cache, whereas the CEF switching table is prebuilt.
  • The CEF table holds the MPLS label information to label IP packets before switching them out of the router; fast switching does not.

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