有些名词在电工书中没有具体解释，这些理解是否正确还请指正！
1. Bearer Capability (ISDN中) (Q184)
B channel 的承载容量 64K.
但对于 Bearer Capability i = 0x8890 indicates a 64k Digital request，不明白是怎么算出来的。
2. ISL (Interior Switching Link) 干道封装协议之一
用于多台SW的 Cisco 专有封装协议，该协议在Switch和Router上被支持。
用于实现交换机间的VLAN中继。它是一个信息包标记协议，在支持ISL接口上发送的帧由一个标准以太网帧及相关的VLAN信息组成。在支持ISL的接口上可以传送来自不同VLAN的数据。
3. 802.xx
IEEE 802.1Q 以太网 干道封装协议之一
IEEE 802.10 FDDI 干道封装协议之一
4. back-door bridge (Q47) IPX
Novell IPX: Client Cannot Connect to Server on Remote LAN
Back-door bridge between segments exists.
1). Use the show ipx traffic exec command on intermediate routers. Determine whether the bad hop count field is incrementing.
2). If the bad hop count counter is incrementing, use a network analyzer to look for packet loops on suspect segments. Look for RIP3 and SAP updates as well. If a back-door bridge exists, you are likely to see hop counts that increment to 16, at which time the route disappears and reappears unpredictably.
3). Look for packets from known remote network numbers that appear on the local network. Look for packets whose source address is the MAC4 address of the remote node instead of the MAC address of the router.
4). Examine packets on each segment. A back door is present on the segment if packets appear whose source address is the MAC address of a remote node instead of that of the router.
5). Remove the back-door bridge to close the loop.
bad hop count
Number of packets discarded because their hop count exceeded 16.
checksum errors
Number of packets containing a checksum error. This number should always be 0, because IPX rarely uses a checksum.
5. format errors (228) IPX
Number of bad packets discarded (for example, packets with a corrupted header). Includes IPX packets received in an encapsulation that this interface is not configured for.
