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myCCNP Switch 300-115 > MultiLayerSwitch-CEF > Flashcards

MultiLayerSwitch-CEF Flashcards

1.1 Configure and verify switch administration address table

1
Q

What is process-based switching?

A

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

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2
Q

What is fast switching?

A

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

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3
Q

What is CEF?

A

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

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4
Q

What are the components CEF is composed of?

A

CEF is composed of the FIB and Adjacency Table.

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5
Q

What does CEF stand for?

A

Cisco Express Forwarding.

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6
Q

What does FIB stand for?

A

Forwarding Information Base.

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7
Q

What is the FIB?

A

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

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8
Q

What is the Adjacency Table?

A

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

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9
Q

What is the Glean Adjacency Type?

A

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.

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10
Q

What is the Null Adjacency Type?

A

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

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11
Q

What is the Drop Adjacency Type?

A

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

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12
Q

What is the Discard Adjacency Type?

A

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

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13
Q

What is the Punt Adjacency Type?

A

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

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14
Q

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

A

Switch# show ip cef
Switch# show adjacency

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15
Q

What command produced this?

A

switch# show ip cef

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16
Q

What command produced this?

A

switch# show adjacency

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17
Q

What command produced this?

A

switch# show adjacency detail

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18
Q

What command produced this?

A

switch# show adjecency interface t1/0/3

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19
Q

What command produced this?

A

switch# show adjacency 10.147.99.52

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20
Q

What command produced this

A

switch# show adjacency vlan 300

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21
Q

What command produced this?

A

switch# show ip cef detail

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22
Q

What command produced this?

A

switch# show ip cef 10.147.97.28 detail

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23
Q

What command produced this?

A

switch# show ip cef 10.147.97.28

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24
Q

What command produced this?

A

switch# show ip cef vlan 311

25
Q

How does CEF work?

A

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

26
Q

What’s the FIB?

A

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

27
Q

What’s the AT?

A

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

28
Q

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

A

It’s processed by the routing engine.

29
Q

Does CEF or MLS need to be configured?

A

No, but IP routing must be enabled.

30
Q

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?

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

A

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

32
Q

CEF is a complete new routing switch technology . Which two table types are CEF components?(Choose two)

A – adjacency tables
B – caching tables
C – neighbor tables
D – forwarding information base

A

Answer: A D

33
Q

How does the CEF L3 engine operate?

A

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

34
Q

What contains the next-hop entries?

A

FIB

35
Q

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

A

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

36
Q

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

A

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

37
Q

what is a CEF punt?

A

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

38
Q

what is accelerated CEF (aCEF)?

A

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

39
Q

what I s distributed CEF (dCEF)?

A

CEF is distributed completely among multiple L3 forwarding engines.

40
Q

what is the adjacency table?

A

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

41
Q

what is a CEF glean?

A

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

42
Q

what is arp throttling?

A

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

43
Q

what is a null adjacency?

A

used to switch packets destined for the null interface

44
Q

what is a drop adjacency?

A

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

45
Q

what is a discard adjacency?

A

packets are discarded due to an ACL or policy action

46
Q

what is a punt adjacency?

A

packets must be sent to the L3 engine for further processing

47
Q

what does the rewrite engine do?

A

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

48
Q

How do you enable CEF?

A

it is enabled by default on all CEF capable switches

49
Q

What is the FIB in multilayer switching?

A

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.

50
Q

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

A

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

51
Q

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

A

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

52
Q

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

A

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

53
Q

What is the command to view the FIB?

A

(exec)show ip cef

54
Q

What is a ‘CEF Glean’?

A

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.

55
Q

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

A

The adjacency table.

56
Q

What is another name for Topology-based Multilayer switching?

A

Cisco Express Forwarding (CEF).

57
Q

When a packet is ‘punted’ what does this mean?

A

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

58
Q

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

A

-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.

59
Q

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

A

-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).

myCCNP Switch 300-115 (73 decks)

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