Tag Archives: 2.1a

2.1.a Implement and troubleshoot switch administration

i/g u/l

i/g means individual/group, u/l means universal/local. examine the mac diagram below:

ig_ul_odom_dia

msb is to the far left (1st byte), while lsb is far right (6th byte). the first three bytes comprise the organizationally unique identifier and the last three bytes comprise the vendor assigned unique value to round out the complete address. this is mandated by the ieee.

we know the mac is 48 bits or 6 bytes.

the 1st bit of the 1st byte (left to right) is considered the most significant bit. the last bit of the 1st byte likewise is the least significant bit. when the frame is transmitted the expected order is 1st byte 1st and so on, however the bits of the individual byte are transmitted in reverse order. this is often referred to as canonical (authorized; recognized; accepted as standard).

if the i/g bit is set to 0 it is a unicast address; if it is set to 1 it is either a multicast or broadcast.

if the u/l bit is zero the mac has been assigned by the vendor; if it is 1 it has been locally assigned (administered), which overrides the original assignment by the vendor.

now consider this address: (from pearson it certification test, ccie v5)

0300.0012.3456

An Ethernet MAC address is always written out in the big endian order, most significant byte first, and is also transmitted on the wire in this order of bytes. However, individual bits of each byte are transmitted in the reverse order, starting with the least significant bit (as stated above). The first byte of the MAC address to be put on wire is 0x03, or 00000011 in binary. In this byte, the least significant bit, or the rightmost bit, is the Individual/Group (I/G) bit, currently set to 1, thereby indicating this is a group MAC address (multicast). The second least significant bit is the Universal/Local (U/L) bit, also set to 1, indicating that this MAC address is locally administered and has not been allocated by IEEE. The bits of the first byte will be transmitted as 1-1-0-0-0-0-0-0 (note reversal). In this sequence, the first transmitted bit is clearly set to 1. (it would be unicast if zero)

 

2.1.a Implement and troubleshoot switch administration

the csma/cd process

carrier sense multiple access with collision detection

1. A device with a frame to send listens until the Ethernet is not busy (in other words,
the device cannot sense a carrier signal on the Ethernet segment).
2. When the Ethernet is not busy, the sender begins sending the frame.
3. The sender listens to make sure that no collision occurred.
4. If there was a collision, all stations that sent a frame send a jamming signal to ensure
that all stations recognize the collision.
5. After the jamming is complete, each sender of one of the original collided frames
randomizes a timer and waits that long before resending. (Other stations that did not
create the collision do not have to wait to send.)
6. After all timers expire, the original senders can begin again with Step 1.

2.1.a Implement and troubleshoot switch administration

2.1.a [iii] L2 MTU

There are 3 types of MTU that can be configured on a switch:
● Layer-2 MTU that affects 10 and 100 Mbps interfaces of a switch. Configured by system MTU {bytes} command in global config mode

● Layer-2 MTU that affects 1000 Mbps and higher speed interfaces of a switch. Configured by system MTU jumbo {bytes} command in global configuration mode

● Layer-3 MTU that affects SVIs and routed interfaces of a switch with IP addresses on them and originating or transit IP traffic that uses these interfaces as GW for routing between networks. Configured by system mtu routing {bytes} command in global config mode

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1513-1521).  . Kindle Edition.

2.1.a Implement and troubleshoot switch administration

2.1.a [ii] errdisable recovery

If the configuration shows a port as enabled, but software on the switch detects an error situation on the port, the software shuts down that port . In other words, the port is automatically disabled by the switch operating system software because of an error condition that is encountered on the port.

When a port is error disabled, it is effectively shut down and no traffic is sent or received on that port. The port LED is set to the amber and if you issue the show interfaces command, the port status shows err-disabled. Here is an example of what an error-disabled port looks like from the command-line interface (CLI) of the switch:

Switch# show interfaces gigabitethernet 5/ 1 status

Port   Name    Status    Vlan   Duplex   Speed   Type

Gi4/ 1 err-disabled 100 full 1000 1000BaseSX

Or, if the interface has been disabled because of an error condition, you can see messages that are similar to these in both the console and the syslog:

%SPANTREE-SP-2-BLOCK_BPDUGUARD:

Received BPDU on port GigabitEthernet4/ 1 with BPDU Guard enabled. Disabling port.

%PM-SP-4-ERR_DISABLE: bpduguard error detected on Gi4/ 1, putting Gi4/ 1 in err-disable state

In order to recover a port from the errdisable state, first identify and correct the underlying cause, and then re-enable the port. If you re-enable the port before you fix the actual problem, the ports could just become error disabled again. After you fix the root problem, the ports are still disabled if you have not configured errdisable recovery on the switch. In this case, you must re-enable the ports manually.

Issue the shutdown command and then the no shutdown interface mode command on the associated interface in order to manually re-enable the ports.

Major reasons for errdisable are:

● EthernetChannel misconfiguration

● Duplex mismatch

● BPDU port guard

● UDLD

● Link-flap error

● Loopback error

● Port security violation

● L2tp guard

● Incorrect SFP cable

● 802.1X security violation

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1492-1509).  . Kindle Edition.
http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/69980-errdisable-recovery.html