2.1.b Flash Cards

CCIE R&S 5.1 2.1.b Flash Cards


Here are my flash cards for 2.1.a Implement and troubleshoot layer 2 protocols. I am giving these flash cards in TXT and in an ANKI package.

If you don’t have or know what Anki is, it is my favorite , free flash card program:

Here is the Anki package you may import and start using the flash cards right away:

What happens to the autoneg side of a link if one side is set for 100 Mbps full duplex static?
The autoneg side cannot detect duplex if both sides are not set for auto, however it can detect speeds of 10/100.
(1000Mbps requires autoneg not static)

How can CDP detect duplex issues?
If both sides are configured with CDP (cisco devices) then they send their duplex as part of their msg.

What is the main difference between Ethernet II/DIX and 802.3 Ethernet (and its variants)? Which is mostly used?
The difference between Ethernet II and 802.3 Ethernet is that while ethernet II has a ethtype field, the others have DSAP and SSAP. Basically they have more fields that are used for seq and setting up the connection at layer 2. This method of ethernet never took off much because it had unnecessary overhead. We still see this method of ethernet today in IEEE protocols or Cisco protocols like CDP, DTP, STP, LLDP.
However most traffic will be Ethernet II.

What does CDP stand for?
Cisco discovery protocol

Is CDP encapsulated in Ethernet II or 802.3 Ethernet?
CDP is encapsulated in 802.3 Ethernet

What is the default transmit and hold time of CDP?
60 sec transmit
180 sec hold time

How does CDP traverse a link or L2 network?
CDP cannot be routed, and it cannot traverse more than the directly connected neighbor.

What show command will allow us to verify CDP timers and version?
show cdp

Is CDP enabled by default on Cisco devices?

How do we turn CDP off per interface?
no cdp enable

How do we turn CDP off globally? How do we turn it back on?
Turn it off:no cdp run
Turn it back on:cdp run

How can we change the CDP timer to 5 and hold time to 30 ?
conf t
cdp timer 5
cdp holdtime 30

How do we view CDP neighbors?
show cdp neighbors

How do we view a more detailed cdp neighbor table?
show cdp neighbors detail

How do we view the cdp neighbor info for just g0/1 in a detailed way?
In IOS/IOS XE:show cdp neighbors g0/1 detail
In NXOS :show cdp neighbors int g0/1 detail

What 9 fields are in a CDP message?
device type
MGMT address
directly connected interface
duplex of directly connected interface
Native VLAN

What does LLDP stand for?
Link layer discovery protocol

What is the IEEE standard for LLDP?

What is the ethertype for LLDP?

Is LLDP enabled by default?
No LLDP is NOT enabled by default

How do we turn on LLDP globally?
conf tlldp run

How do we prevent an interface from sending LLDP messages, but allow it to receive?
1. confirm LLDP is on globally

conf t
int g0/1
no lldp transmit
lldp receive

What 3 LLDP timers do we have available to tune?
The timers available to tune are:lldp holdttime – how long to wait before an lldp message expires
lldp timer – how long to wait before retransmitting an lldp message
lldp reinit – how long  to wait after an interface comes up to transmit the first message

How do we reconfigure the lldp transmit timer to 5, initialization timer to 2, and holdtime to 30?
conf t
lldp timer 5
lldp reinit 2
lldp holdtime 5

What does UDLD stand for?
unidirectional link detection

Is UDLD available on other vendor gear?
No UDLD is cisco proprietary

What two things needs to happend for UDLD to initialize?
Both sides must be configured, and both must receive each other’s hellos to begin.

What 4 things does UDLD protect us from?
UDLD protects us from:
1. Connecting cables to ourselves by accident
2. A link connected to a HUB or dumb switch (multiple UDLD peers on one link is not good)
3. Cut transmit or receive (in aggressive mode)
4. Misconnected fiber strands (connected to proper device but wrong ports)

What does UDLD do before it declares a neighbor dead?
UDLD will send 8 quick hellos to the neighbor to verify they are indeed down.

What is the default interval for UDLD hellos?
15 sec

Is UDLD enabled by default? No

What two ways can we enable udld? What modes are available? Type it.
1. globally viaudld
2. per port via
int g0/1
udld port

Is enabling UDLD globally required for the per interface config to work?

What is true about enabling UDLD globally regarding copper and fiber links?
Enabling UDLD globally causes it to only be enabled for fiber links

What two ways can we recover a UDLD errdisable port?
1.”shut” and “no shut” the link
2. The global config command “UDLD reset”

What show command can we use to quickly verify our udld neighbors?
show udld neighbors

What show command will display all details regarding UDLD, its modes, timers, etc…
show udld

Which LLDP MED-TLV sends the voice vlan info to vendor neutral phones? Is it enabled by default?
Yes all MED-TLV’s are sent by default

What is LLDP-MED?
LLDP-MED is LLDP Media endpoint discovery extension.
It was an extension to LLDP to give it more functionality so that it can transmit things like CoS, DSCP, VLAN info, PoE settings.

Which 3 LLDP TLVs are mandatory?
Chassis ID, Port ID, and time to live