Discussion

the physical interface. It allows you to correlate the values in the interface MIBOIDs
with actual interfaces. You can also see this information on the router:
aviva@router1> show snmp mib walk ifTable
ifDescr.29 = fe-0/0/0
The next line gives more information about the physical properties of the interface:
Link-level type: Ethernet, MTU: 1514, Speed: 100mbps, Loopback: Disabled,
This shows the Layer 2 (link-level) encapsulation type and Maximum Transmission
Unit (MTU) of the interface. Both are using the default encapsulations for their interface
types. For the IPv4 address family, Ethernet is the only allowed link-level type. If
you look in the logical interface section of the output, you see the encapsulation that
is being used on the logical interface. The physical encapsulation applies to all protocols
running on the interface, and logical interface encapsulation applies only to that
address family. You can use the encapsulation statement in both portions of the configuration
to modify the defaults:
[edit interfaces fe-0/0/0]
aviva@router1# set encapsulation vlan-ccc
[edit interfaces fe-0/0/0 unit 0]
aviva@router1# set encapsulation ppp-over-ether
The MTU is 1,514 bytes. This is the default media (Layer 2) MTU size, which is the
size of the largest packet that the router can transmit through this interface. It applies
to all protocols that use this interface. The protocol MTU size is in the logical interfaces
portion of the output. Both logical interfaces are running IPv4 (Protocol inet),
and the IPv4 MTU size is the standard 1,500 bytes for both interfaces. The media
MTU is the sum of the IP MTU and the encapsulation overhead, which is 14 bytes
for Ethernet interfaces. All devices on the Ethernet LAN must support the same
MTU size. If you notice that a device is dropping traffic, check its MTU.
The speed for the Ethernet shows that this is Fast Ethernet, running at 100 Mbps.
The next line of output shows whether source filter and flow control are configured
on the interface:
Source filtering: Disabled, Flow control: Enabled
Source address filtering blocks traffic from specific Ethernet MAC addresses. This is
off by default. Flow control, which is on by default on Fast Ethernet interfaces, regulates
the amount of traffic sent out on a full-duplex interface. (Flow control, also
called pause frames, is an optional clause on the specification for full-duplex Ethernet,
defined by the IEEE 802.3x Task Force in Annex 31B.) Source filtering and flow
control are explained more in Recipe 7.13.
The device flags list information about the physical device. You see this line for all
router interfaces:
Device flags
: Present Running
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