1.1.a Switch Administration

CCIE Enterprise Infrastructure Exam Topics

1.1.a i Managing MAC address table
1.1.a ii Errdisable recovery
1.1.a iii L2 MTU

Managing MAC address table

Looking at the MAC Addresses on the Switch is simple as typing one command. You get the VLAN it is assigned to, the MAC Address, the way it has been learned (static/dynamic) and on which port this MAC Address has been seen.

SW1# sh mac address-table
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    5000.000c.0006    DYNAMIC     Gi1/2
   1    5000.000c.0007    DYNAMIC     Gi1/3
   1    5000.000d.0002    DYNAMIC     Gi0/2
   1    5000.000d.0003    DYNAMIC     Gi0/3
[output omitted]
   1    5000.000e.0007    DYNAMIC     Gi0/2
Total Mac Addresses for this criterion: 12

The address table can store a number of unicast and multicast address entries without flooding any frames.
The switch uses an aging mechanism, defined by a configurable aging timer, so if an address remains inactive
for a specified number of seconds, it is removed from the address table. Depending on the platform the default ageing time may vary. The following example (IOSv) has an aging time of 300 seconds. Other platforms have 1800 seconds as their default (Cisco Nexus).

SW1# sh mac address-table aging-time
Global Aging Time:  300
...

Changing the timer:

SW1#conf t
SW1(config)#mac address-table aging-time ?
  <0-0>         Enter 0 to disable aging
  <10-1000000>  Aging time in seconds

SW1(config)#mac address-table aging-time 600

Verifying the setting

SW1# sh mac address-table aging-time
Global Aging Time:  600

Errdisable recovery

Errdisable is a feature that automatically disables a port on a Cisco switch. When a port is error disabled, it is effectively shut down and no traffic is sent or received on that port. The show interfaces command, the port status shows as errdisabled.

The reasons a switch can go into Errdisable mode and shutdown a port are many and include:

SW1#sh errdisable recovery
ErrDisable Reason            Timer Status
-----------------            --------------
arp-inspection               Disabled
bpduguard                    Disabled
channel-misconfig (STP)      Disabled
dhcp-rate-limit              Disabled
dtp-flap                     Disabled
gbic-invalid                 Disabled
inline-power                 Disabled
l2ptguard                    Disabled
link-flap                    Disabled
mac-limit                    Disabled
link-monitor-failure         Disabled
loopback                     Disabled
oam-remote-failure           Disabled
pagp-flap                    Disabled
port-mode-failure            Disabled
pppoe-ia-rate-limit          Disabled
psecure-violation            Disabled
security-violation           Disabled
sfp-config-mismatch          Disabled
storm-control                Disabled
udld                         Disabled
unicast-flood                Disabled
vmps                         Disabled
psp                          Disabled
dual-active-recovery         Disabled
evc-lite input mapping fa    Disabled
Recovery command: "clear     Disabled

Timer interval: 300 seconds

Manual recovery: To recover a port that is in an err-disable state the administrator must access the switch and configure the specific port with ‘shutdown’ followed by the ‘no shutdown’ command. This command sequence will enable the port again, however, if the problem persists expect to find the port in err-disable state again soon.

Automatic recovery: It is also possible to enable auto recovery for this feature, so that the switch will try to recover from the issue after the timer interval has expired (300 seconds in the above output). To enable the recovery for a specific feature, configure it as documented below:

SW1#conf t
SW1(config)#errdisable recovery cause ?
  all                   Enable timer to recover from all error causes
  arp-inspection        Enable timer to recover from arp inspection error
                        disable state
  bpduguard             Enable timer to recover from BPDU Guard error
  channel-misconfig     Enable timer to recover from channel misconfig error
....

To change the default timer-interval:

SW1#conf t
SW1(config)#errdisable recovery interval ?
  <30-86400>  timer-interval(sec)

L2 MTU

The Ethernet frame is on Layer 2 of the Open Systems Interconnection (OSI). The standard size MTU for Ethernet is 1,500 bytes (Payload). This does not include the Ethernet header of 18 or 20 bytes, and is the theoretical maximum amount of data that can be transmitted by the physical link. The MTU of any higher-level protocols must fit within this MTU.

In computer networking, jumbo frames are Ethernet frames with more than 1500 bytes of payload, the limit set by the IEEE 802.3 standard. Commonly, jumbo frames can carry up to 9000 bytes of payload, but smaller and larger variations exist and some care must be taken using the term.