一直很困惑SEOS中Circuit和Adjancency之间的关联，询问了好多业务人士，人家要么避而不谈，要么压根儿不清楚。于是我昨天把这个问题帖到了Linkedin.com的redback discussion group中，真有一位热心人士回复了。
原文如下：
The circuit concept is needed to make the virtual router concept work. A virtual router has interfaces. These interfaces are virtual. A circuit is what packets are send and received on. A circuit is "bound" to an interface on a virtual router. Examples of circuits are ethernet port, VLAN, PPP session, pseudo wire, L2TP session. You first create a circuit, then bind it to an interface.
An adjacency is like a completely resolved nexthop. It is a data structure that describes how to forward a packet. When a packet arrives, it does an IP lookup in the FIB or a label lookup in the LFIB. The result is a nexthop. This may be further resolved with an ARP lookup or several other ways. The final result is the adjacency? If the nexthop does not require further resolution, then the nexthop is the adjacency. It describes the header that needs to be put onto the packet and the circuit to send it on to forward it. |
| By Jakob Heitz | |
个人读后理解如下：
1. Circuit就是SE800底层的可收发数据的对象。如以太口、vlan、ppp会话、虛线、l2tp会话等，电路与L2/L3接口绑定到一起的；
2. Adjancency就是IP包被解析出的下一跳，它是一个数据结构，用来指示如何转发包，其中第一个字节代表了包的目标槽位（从0开始算起）。
当IPPA收到包时，设备在FIB表中查找IP的下一跳或在LFIB中查找标签的下一跳；在找到下一跳后，还有可能继续执行ARP的解析（来获得下一跳IP对应的mac地址）；或执行递归查找到真实的下一跳。这里的下一跳就是由目标槽位+邻接索引表示的；
数据包在通过PMA继续发向目标板卡的EPPA时，会将adjancency值中的目标槽位信息删去且只保留邻接索引；EPPA收到这个包后，就能根据邻接索引找到对应的下一跳。
所以，在IPPA上称为下一跳，在EPPA则称为adjancency。
以上为个人理解，抛砖引玉，同时向国外的SE800同行致谢！
