一、ISDN术语：
* LAPD - provides the data link protocol that allows delivery of messages across that
D-channel to the local switch.
* LAPB - Protocol and is designed primarily to satisfy the signaling requirements of
ISDN basic access. It is defined by ITU-T Recommendations Q.920 and Q.921.
* TE1 - ISDN -capable four-wire cable. Understands signaling and 2B=D. Uses an S
reference point.
* ITU.T.430 - Defines connectors, encoding, framing, and reference points.
* TE2 - Equipment that does not understand ISDN protocols and specifications (no ISDN
awareness). Uses an R reference point, typically an RS-232 or V.35 cable, to connect to a
TA.
* NT1 - CPE equipment in North America. Connects with a U reference point (two-wire)
to the telco.

二、default route:
All cisco routers are classful routers, meaning that they expect a
default subnet mask on each interface of the router. When a router receives a packet
for a destination subnet that's not in the routing table, it will drop packets by default.
If you are using the default routing, you should use the ip classless command
because it is possible that no remote subnet will be in routing table.
In recent version of IOS ip classlsess is enabled by default.
三、show version:
show version command displays the information regarding hardware
of router, IOS name, version as well as registery settings of router.
show flash:
Different Cisco IOS versions and feature sets will ultimately dictate the
size of the IOS file and the amount of Flash and DRAM memory required to run the
IOS. If you are planning to upgrade to a new IOS, you must make sure that you have
enough memory (the more, the better) in your device. To see the amount of Flash you
have and the current IOS file stored in Flash memory, utilize the show flash
四、switch:
When frame receives the frame, it checks the source address on MAC
table if MAC address found in MAC table it tries to forward if not in MAC table
adds the Address on MAC table. After checking the source address, it checks the
destination address on MAC table, if MAC address found on MAC table it forwards
to proper ports otherwise floods on all ports except the source port.
五、loopback address:
It provides stability for the OSPF process.
It specifies that the router ID.
六、clock rate determined：
It is supplied by the CSU/DSU.
七、ethernet frame
Two frame formats can be used on Ethernet:
1. The standard issued in 1978 by Xerox Corporation, Intel Corporation and Digital
Equipment Corporation, usually called Ethernet (or DIX Ethernet).
2. The international IEEE 802.3 standard, a more recently defined standard.
The difference between the two standards is in the use of one of the header fields, which
contains a protocol-type number for Ethernet and the length of the data in the frame for
IEEE 802.3.