Forwarding Architecture (CEF, CAM, SSO, etc)

Question 1

What is a CAM?

Answer 1

Content Addressable Memory - high speed memory where the MAC Address table is stored

Question 2

Describe Process Switching

Answer 2

• uses the CPU
• aka Slow Path or Software Switching
• backup for CEF

Question 3

Describe CEF

Answer 3

Cisco Express Forwarding
- default switching mechanism on all Cisco devices

Question 4

What command can you use to verify CEF on a cisco device?

Answer 4

show ip cef {prefix | ip-address}

Question 5

What are the two key hardware components used by CEF?

Answer 5

• Hardware CEF
  
  • Application Specific Integrated Circuits (ASICs)
  • Network Processing Units (NPUs)

Question 6

What is TCAM?

Answer 6

Ternary Content Addressable Memory - an enhancement of CAM that allows for Layer 2 and 3 source and destination, protocol, QoS marking and other such identification

Question 7

What are the two main components of Software CEF?

Answer 7

• FIB (Forwarding Information Base)
• Adjacency Table

Question 8

What is the FIB?

Answer 8

Forwarding Information Base
- built directly from the Routing Table (RIB)
- contains next-hop address for each network destination
- used by CEF for destination prefix forwarding

Question 9

What is the Adjacency Table?

Answer 9

• aka Adjacency Information Base (AIB)
• contains directly connected next-hop IP address and corresponding MAC Address

Question 10

What is SSO (in forwarding terms)?

Answer 10

Stateful SwitchOver - a redundancy feature where two switches mirror each other’s configurations and the standby switch takes over if the primary switch fails

Question 11

What is a SDM Template?

Answer 11

Switching Database Manager Template - used by switch stacks to store and modify TCAM tables

Question 12

What is the command to configure an SDM template?

Answer 12

SW1(config)# sdm prefer {vlan | advanced}

Question 13

What is PBR?

Answer 13

Policy-Based Routing - a routing feature that uses route-maps to route packets.

Question 14

What are the basic steps to enable policy-based routing?

Answer 14

• create ACLs for the traffic you want to match
• create a route-map to match the ACLs and set next-hop
• apply the policy to an interface using the command “ip policy route-map ROUTE-MAP-NAME”

Question 15

What makes policy-based routing so flexible?

Answer 15

The ability to match on traffic based on factors other than shortest path, particularly to be able to match on protocol.

Question 16

What traffic will not be caught by policy-based routing (by default)?

Answer 16

Traffic generated by the device itself. This is because ACLs will not catch this traffic by nature of ACL behavior.

Question 17

What commands are used to configure a route-map for a policy-based route?

Answer 17

R1(config)# route-map RMAP_NAME permit [seq-num]
R1(config-route-map)# match ip address ACL_NAME
R1(config-route-map)# set ip next-hop {ip-address}