MultiLayerSwitch-CEF

Question 1

What is process-based switching?

Answer 1

Tasks are switched using the CPU and this is known as the “IP Input” process.

Question 2

What is fast switching?

Answer 2

First packet is sent to the CPU for processing while Fast Switching cache is updated with the information.

Question 3

What is CEF?

Answer 3

CEF is prepopulated the L2 and L3 caches while CPU is rarely bothered. THis is done by tracking the ARP and Routing tables.

Question 4

What are the components CEF is composed of?

Answer 4

CEF is composed of the FIB and Adjacency Table.

Question 5

What does CEF stand for?

Answer 5

Cisco Express Forwarding.

Question 6

What does FIB stand for?

Answer 6

Forwarding Information Base.

Question 7

What is the FIB?

Answer 7

The FIB is a shadow copy of the IP Routing Table and tracks the table for automatic updates.

Question 8

What is the Adjacency Table?

Answer 8

The Adjacency Table is populated by L2 tables such as ARP and Frame-Relay Mapping.

Question 9

What is the Glean Adjacency Type?

Answer 9

Glean occurs when no specific information about an address in a subnet exists in ARP. This results in the CPU being tasked to trigger an ARP reply.

Question 10

What is the Null Adjacency Type?

Answer 10

Null occurs when there are valid packets that must be dropped as they are destined for the Null0 interface.

Question 11

What is the Drop Adjacency Type?

Answer 11

Drop occurs when there are invalid packets that must be dropped. This can occur when there is no route or there’s bad encapsulation.

Question 12

What is the Discard Adjacency Type?

Answer 12

Discard occurs when there are valid packets that must be dropped because there is a security policy in place such as an ACL.

Question 13

What is the Punt Adjacency Type?

Answer 13

Punt occurs when there are packets that are destined for the CPU itself such as a routing update from a neighbor.

Question 14

What are some useful show commands that can be used to look up information in CEF?

Answer 14

Switch# show ip cef
Switch# show adjacency

Question 15

What command produced this?

Answer 15

switch# show ip cef

Question 16

What command produced this?

Answer 16

switch# show adjacency

Question 17

What command produced this?

Answer 17

switch# show adjacency detail

Question 18

What command produced this?

Answer 18

switch# show adjecency interface t1/0/3

Question 19

What command produced this?

Answer 19

switch# show adjacency 10.147.99.52

Question 20

What command produced this

Answer 20

switch# show adjacency vlan 300

Question 21

What command produced this?

Answer 21

switch# show ip cef detail

Question 22

What command produced this?

Answer 22

switch# show ip cef 10.147.97.28 detail

Question 23

What command produced this?

Answer 23

switch# show ip cef 10.147.97.28

Question 24

What command produced this?

Answer 24

switch# show ip cef vlan 311

Question 25

How does CEF work?

Answer 25

By having the hardware route all packets, saving CPU resources. This is accomplished with FIB and AT.

Question 26

What's the FIB?

Answer 26

Forwarding information database. Contains all of the information that the IP routing table does, but in a format usable by ASICs.

Question 27

What's the AT?

Answer 27

Adjacency table. Contains next-hop layer 2 information learned through ARP.

Question 28

In CEF L3 switching, how is excessive information handled after the AT and TCAM tables are filled?

Answer 28

It's processed by the routing engine.

Question 29

Does CEF or MLS need to be configured?

Answer 29

No, but IP routing must be enabled.

Question 30

Assume that a host sends a packet to a destination IP address and that the CEF-based switch does not yet have a valid MAC address for the destination. How is the ARP entry (MAC address) of the next-hop destination in the FIB get? ## Footnote A – The sending host must send an ARP request for it B – All packets to the destination are dropped C – The Layer 3 forwarding engine (CEF hardware) must send an ARP request for it D – CEF must wait until the Layer 3 engine sends an ARP request for it

Answer 30

Answer: D Explanation: If a valid MAC address for the destination is not found, the Layer 3 forwarding engine can’t forward the packet in hardware due to the missing Layer 2 next-hop address. Therefore the packet is sent to the Layer 3 Engine so that it can generate an ARP request (this is called the “CEF glean” state) CEF\_ARP

Question 32

CEF is a complete new routing switch technology . Which two table types are CEF components?(Choose two) ## Footnote A – adjacency tables B – caching tables C – neighbor tables D – forwarding information base

Answer 32

Answer: A D

Question 33

How does the CEF L3 engine operate?

Answer 33

it's a router that maintains routes (static or dynamic), and populates the FIB

Question 34

What contains the next-hop entries?

Answer 34

FIB

Question 35

what does the version number indicate when looking at CEF entries?

Answer 35

the number of times the entry has been updated since the table was generated

Question 36

what does the epoch number indicate when looking at cef entries?

Answer 36

the number of times the CEF table has bee flushed and regenerated as a whole

Question 37

what is a CEF punt?

Answer 37

a packet can't be switched in hardware with the FIB and must go to the L3 engine

Question 38

what is accelerated CEF (aCEF)?

Answer 38

CEF is distributed across multiple L3 forwarding engines, typically on Catalyst 6500 line cards each having only a subset

Question 39

what I s distributed CEF (dCEF)?

Answer 39

CEF is distributed completely among multiple L3 forwarding engines.

Question 40

what is the adjacency table?

Answer 40

a router keeps a routing table and an ARP table. The FIB combines them for every next-hop entry

Question 41

what is a CEF glean?

Answer 41

the L3 forwarding engine can't forward the packet in hardware because there is no L2 next-hop address, so the packet is sent to the L3 engine to generate the ARP response

Question 42

what is arp throttling?

Answer 42

during the time that the FIB entry is in CEF glean waiting for ARP entries, subsequent packet to that host are dropped to keep input queues from filling

Question 43

what is a null adjacency?

Answer 43

used to switch packets destined for the null interface

Question 44

what is a drop adjacency?

Answer 44

used to switch packets that can't be forwarded normallly due to an encapsulation failure, unresolved address, unsupported protocol, etc.

Question 45

what is a discard adjacency?

Answer 45

packets are discarded due to an ACL or policy action

Question 46

what is a punt adjacency?

Answer 46

packets must be sent to the L3 engine for further processing

Question 47

what does the rewrite engine do?

Answer 47

it updates the L2 headers with the proper src and dst MAC, L3 TTL, L3 checksum, and L2 checksum information

Question 48

How do you enable CEF?

Answer 48

it is enabled by default on all CEF capable switches

Question 49

What is the FIB in multilayer switching?

Answer 49

The FIB is the Forwarding Information Base, and is a table of layer-3 routing information used to hardware switch packets. It contains the next hop address for each entry.

Question 50

Will a FIB contain host routes (/32) entries?

Answer 50

Yes. Even though most routing tables do not (unless manually configured) the FIB does.

Question 51

What are the four types of adjacency's that can be found in the FIB adjacency table?

Answer 51

Null, Drop (actual error), Discard (ACL or policy), and Punt.

Question 52

What are the two techniques of CEF that use specialized hardware?

Answer 52

-aCEF, functions as a FIB cache.-dCEF, has its own FIB table and forwarding engine!

Question 53

What is the command to view the FIB?

Answer 53

(exec)show ip cef

Question 54

What is a 'CEF Glean'?

Answer 54

When a L3 forwarding engine cannot forward a packet because of missing L2 info, the time it takes to have the L3 engine generate an ARP request and figure out the L2 information is the CEF Glean.

Question 55

What table contains the Layer 2 information for every next-hop entry in the FIB?

Answer 55

The adjacency table.

Question 56

What is another name for Topology-based Multilayer switching?

Answer 56

Cisco Express Forwarding (CEF).

Question 57

When a packet is 'punted' what does this mean?

Answer 57

That the packet was unable to be forwarded naturally and is sent to the switch CPU for process switching.

Question 58

What are the 9 reasons that a packet can be 'punted'?

Answer 58

-ARP requests and replies-IP packets needing a response (TTL, MTU, frag-IP broadcasts-Routing protocol updates-CDP packets-IPX routing packets-Packets needing encryption-Packets triggering NAT-Non-IP and non-IPX packets.

Question 59

What three things are contained in the Hex-value when looking at the FIB adjacency table?

Answer 59

-The first 6 octets are the Hex value of the next hop MAC address.-the second 6 octets are the Source MAC for the layer 3 engine interface (like VLAN 99).-the last two octets are the EtherType (0800 for Ethernet).