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CCNP Core > Chap 1 - Forwarding > Flashcards

Chap 1 - Forwarding Flashcards

1
Q

What is Unknown Unicast Flooding?

A

A destination MAC address is not in the MAC table so the switch floods the packet out all ports.

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2
Q

What separates broadcast domains?

A

A router.

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3
Q

What do VLANs do?

A

They create multiple broadcast domains on a single switch.

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4
Q

What 4 fields did IEEE 802.1q add to the VLAN packet structure?

A
  • Tag Protocol Identifier (TID)
  • Priority Code Point (PCP)
  • Drop Eligible Indicator (DEI)
  • VLAN Identifier (VLAN ID)
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5
Q

How many bits in the TID field, what is its hex value, and what does it identify?

A
  • 16 bits
  • set to 0x8100 identifying the packet as an 802.1q packet
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6
Q

What does the Priority Code Point field do?

A
  • 3 bits
  • indicates Class of Service for QOS between switches
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7
Q

How many bits in the Drop Eligible Indicator and what does it do?

A
  • 1 bit
  • indicates whether the packet can be dropped due to congestion
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8
Q

How many bits in the VLAN ID field and what does it do?

A
  • 12 bits
  • specifies the VLAN associated with the packet
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9
Q

What is VLAN 0 reserved for?

A

802.1P traffic

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10
Q

What is different about VLAN 1?

A
  • default VLAN
  • It can’t be deleted.
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11
Q

What is the range of normal VLANs?

A

2 - 1001

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12
Q

What VLANs above 1000 are reserved?

A

1002 - 1005

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13
Q

What is the range of extended VLANs?

A

1006 - 4094

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14
Q

After being in VLAN configuration mode and creating a VLAN at what point does the VLAN actually get created?

A
  • when you leave VLAN config mode
  • when you move to a different VLAN configuration
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15
Q

What VLAN are ports put in by default?

A

VLAN 1

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16
Q

What is the default Native VLAN?

A

VLAN 1

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17
Q

What is the CAM?

A
  • Content addressable memory
  • MAC address table resides here
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18
Q

What is the fallback for CEF?

A

Process switching

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19
Q

What is process switching?

A

Switching in software

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20
Q

What are the 3 types of packets that need to be process switched?

A
  • Packets sourced by or destined for the router itself
  • Packets too complex for hardware (packets with IP options)
  • Packets that need extra information that isn’t already known (like ARP)
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21
Q

What 5 things happen when a packet is punted by CEF to be process switched?

A
  • ip_input process checks the routing table and ARP for next hop routers IP address, outgoing interface and MAC
  • Overwrites source MAC with its own MAC, and destination MAC with next hop router’s MAC
  • Decrements TTL
  • Recomputes checksum of IP header
  • Delivers packet to next hop router
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22
Q

What is another name for the routing table?

A

Routing Information Base (RIB)

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23
Q

What 3 sources build the RIB?

A
  • info from dynamic routing protocols
  • static routes
  • connected routes
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24
Q

How is the ARP table built?

A

From info from the ARP protocol

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25
Q

Where is CEF used?

A
  • Default switching mechanism used on Cisco routers that do packet switching on general purpose CPU
  • Default switching machinism on platforms with ASICs and hardware-based routers
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26
Q

Why do most switches contain multiple TCAM partitions?

A

So that inbound/outbound security, QOS, L2/L3 forwarding decisions occur all at once

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27
Q

Concerning ACLs what does TCAM make possible?

A

ACLs can be processed at the same speed whether the ACL has 10 lines or 500 lines

28
Q

What is Centralized Forwarding Architecture?

A

When a Route Processor is equipped with a Forwarding Engine so it can make all forwarding decisions.

29
Q

What is Distributed Forwarding Architecture?

A

When Line Cards are equipped with Forwarding Engines and can forward without intervention of the Route Processor (RP()

30
Q

What 3 steps occur in Centralized Forwarding?

A
  • Packet received on ingress Line Card, transmitted to Forwarding Engine on RP
  • Forwarding Engine determines where packet will exit
  • Forwards packet to Egress Line Card
31
Q

What are 4 steps in Distributed Forwarding?

A
  • Packet received on Ingress Line Card,
  • Transmitted to Local Forwarding Engine
  • If destination is local Forwarding Engine sends packet out Egress Port
  • If not local, Forwarding Engine sends packet across Switch Fabric directly to Egress Line Card
32
Q

What is another name for Software CEF?

A

Software Forwarding Information Base

33
Q

What does Software CEF consist of?

A
  • Forwarding Information Base (FIB)
  • Adjacency Table (Adjacency Information Base)
34
Q

How is the Forwarding Information Base (FIB) built and what does it contain?

A
  • Built from Routing Table to have a mirror image
  • Contains next hop IP address for each destination in routing table
35
Q

How does CEF use the FIB?

A

CEF uses the FIB to make switching decisions based on IP destination prefixes

36
Q

How is the Adjacency Table (AIB) built and what does it contain?

A
  • Built from ARP and other L2 protocols
  • Contains directly-connected next hop IP/MAC addresses and egress interfaces MAC
37
Q

What is a Glean Adjacency in CEF and when does it occur?

A
  • CEF punts to CPU where ARP is invoked
  • Glean Adjacency occurs when a packet arrives but no entry in FIB
38
Q

In CEF switching what 6 things happen during packet forwarding process?

A
  • Packets headers are re-written
  • Destination MAC overwritten with info from Adjacency Table
  • Source MAC overwritten with MAC from egress interface
  • TTL is decremented
  • Header checksum is re-computed
  • Packet delivered to next hop router
39
Q

What happens when a router receives a packet with TTL = 0 ?

A

Packet is discarded

40
Q

What is done to prevent starving of CPU cycles by packets with an invalid or imcomplete adjacency?

A

Rate limiter

41
Q

What is an NPU?

A

Network Processing Unit

42
Q

What is the difference between an NPU and an ASIC?

A

The NPU is programmable but the ASIC is not.

43
Q

In a Distributed Forwarding Architecture what mechanism is used for packet switching?

A

It uses Distributed CEF (dCEF)

44
Q

What does dCEF stand for and what 2 things are true about it?

A
  • Distributed CEF
  • CEF data structures are downloaded to ASICs and CPUs of all the Line Cards
  • Increases packet throughput of the router
45
Q

What is Hardware CEF?

A

CEF switching done in ASICs and NPUs

46
Q

In hardware-based platforms what is Software CEF used for?

A

For programming Hardware CEF in ASICs / NPUs

47
Q

What is Stateful Switchover?

A

Redundancy feature that allows 2 RPs to synchronize configuration and control plane info

48
Q

What is the process called for mirroring information between RPs?

A

Checkpointing

49
Q

What does Non-Stop Forwarding (NSF) do?

A

Informs routers to maintain CEF entries and to continue forwarding packets during an RP failure until the control plane recovers

50
Q

What is SDM

A

Switching Database Manager

51
Q

What do SDM templates do?

A

Used to adjust the Allocation Ratios of memory allocated to various TCAM tables

52
Q

What is particular about SDM templates when used in a switch stack?

A

The switches all need to use the same SDM template.

53
Q

What is the ‘show’ command that allows you to see the current SDM template?

A

show sdm prefer’

54
Q

What 2 components make up CEF?

A
  • Forwarding Information Base (FIB)
  • Adjacency Table
55
Q

Display the MAC address table for a switch

A
  • show mac address-table address (mac_address)
  • show mac address-table dynamic
  • show mac address-table vlan (vlan_id)
56
Q

Display the Layer 2 configuration for a specific switchport.

A

show interfaces (interface_id) switchport

57
Q

Display the ARP table.

A
  • show ip arp (mac_address)
  • show ip arp (ip_address)
  • show ip arp vlan (vlan_id)
  • show ip arp (interface_id)
58
Q

Display the SDM template.

A

show sdm prefer

59
Q

How is TCAM different than CAM?

A

TCAM allows for the matching and evaluation of a packet on more than one field.

60
Q

Architecturally, describe what the TCAM is.

A

It is an extension of the CAM table but enhanced so that it can do upper-layer processing.

61
Q

Name 3 things TCAM can identify.

A
  • L2 / L3 source/destination address
  • Protocol
  • QOS markings
62
Q

Name 2 differences between TCAM and CAM.

A
  • TCAM (ternary) provides more flexibility in searching than CAM (binary)
  • TCAM does upper-layer processing, CAM does not
63
Q

What are the states TCAM can match on?

A

1’s, 0’s, and X’s where X is ‘don’t care’

example:

TCAM can search for a value that has 0 1 1 1 X X 1 0 . This will match:

01110010

01110110

01111010

01111110

64
Q

What 3 components make up a TCAM entry?

A
  • Value 172.19.0.0
  • Mask 255.255.0.0
  • Result Deny
65
Q

What are 3 things that the TCAM Result can be?

A
  • Permit or deny
  • Pointer to a next hop in the routing table
  • An index to a QOS Policer
66
Q

What 2 factors allow TCAM to be really fast?

A
  • TCAM operates in hardware
  • Each of the 5 TCAM partitions can be processing simultaneously allowing multiple operations to be done at the same time

CCNP Core (46 decks)

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