Fundamentals

Question 1

Where are MAC address Stored?

Answer 1

CAM Table

Question 2

What happens if a CAM table is full?

Answer 2

A switch will flood all frames like a hub

Question 3

What determines where a packet is forwarded?

Answer 3

Destination IPv4 Address. The routing table is used for the routing decision.

Question 4

What happens at every hop that an IP packet hits?

Answer 4

L2 frame header is de-capsulated to remove L2 header. It updates source MAC (outbound interface) and destination MAC address (Next hop) then encapsulates the L2 header for transmission

Question 5

When would a L3 header be changed

Answer 5

When it is used in PAT or NAT.

Question 6

Name the slowest type of switching?

Answer 6

Process Switching. Because the CPU processes every frame.

Question 7

What is Fast Switching?

Answer 7

Fast switching uses CPU to lookup the first frame and put it into a cache. It uses the cache to forward the rest of the frames in the data stream. This is faster than process switching but slower than CEF.

Question 8

What is CEF?

Answer 8

Cisco Express Forwarding is the fastest switching method. Forwarding table created in hardware beforehand. All packets will be switched using hardware. Multilayer switches and routers use CEF.

Question 9

Explain the L2 Switching Process.

Answer 9

1. The switch will verify the checksum on the Ethernet frame to amke sure it’s not corrupted.
2. The switch receives an ethernet frame and adds the source MAC address to the MAC address table.
3. The switch forwards the ethernet frame to the correct interface if it knows the destination MAC address, if not it will flood the frame.

There is NO ALTERATION of the ethernet frame.

Question 10

Explain receiving a IP Packet on a multilayer switch.

Answer 10

1. Verify checksum on Frame
2. Verify checksum on Packet
3. Check routing table
4. Check ARP table for L2 to L3 Mapping to H2.
5. Change destination MAC from FFF to BBB
6. Change source MAC from AAA to GGG
7. Decrease TTL by one, recalculate checksum
8. The ethernet frame carrying the IP packet will be sent out of the interfce toward H2.

Question 11

Where is the ASIC Located?

Answer 11

In The Dataplane. Information like MAC addresses, the routing table or access-lists are stored into these ASICs. The tables are stored in content-addressable memory (CAM) and ternary content addressable memory (TCAM).

Question 12

What is the recursive lookup process (example: 10.10.10.5)

Answer 12

Preform recursive lookup

10. 10.10.5 via 11.11.11.1
11. 11.11.1 via 12.12.12.1
12. 12.12.1 directly connected, FastEthernet 0/0

Question 13

What are the 3 main steps in moving traffic.

Answer 13

1. Routing-Longest matching of destination address
   
   1. Recursive Lookups until you find the outbound interface
2. Switching
   
   1. Moving packets between interfaces
3. Encapsulation
   
   1. Build new layer 2 deader based on outgoing media
   2. two types of interfaces, P2P or Multipoint