查看完整版本: JunOS中BGP路由的优选原则(译自AJRE)

JunOS中BGP路由的优选原则(译自AJRE)

原文:

1. The router first must verify that it has a current route in the inet.0
routing table to the IP address in the BGP next-hop attribute field.
2. The router then compares routes for the highest local preference (the
only choice based on a higher, rather than lower, value).
3. The router evaluates the AS-path attribute next, where a shorter path is
preferred. This attribute is often a common tiebreaker for routes.
4. The router evaluates the origin code. The lowest origin code is preferred.
5. If any of the remaining routes are advertised from the same neighboring
AS, the router checks the multiple exit discriminator (MED) attributes for
a lowest value. The absence of a MED value is interpreted as a MED of 0.
6. If multiple routes remain, the router prefers any routes learned via an
EBGP peer over routes learned via an IBGP peer. If all remaining routes
were learned through EBGP, the router skips to Step 9.
7. If the remaining routes were learned through IBGP, the router uses the
path with the lowest IGP cost to the IBGP peer.
8. The router then examines the cluster-list attribute for the shortest length.
The cluster list is similar in function to an AS path.
9. The router prefers the route advertised from the peer with the lowest
router ID. However, for EBGP-received routes only, the router prefers the
current active route when comparing routes received from different
neighboring ASs.
10. The router prefers routes from the router with the lowest peer ID.
Note that the local-preference value (which configures each AS) is more important
than the length of the AS path used to get to the destination prefix. This example is an
illustration of the way BGP is designed to give great flexibility in enforcing a routing
policy, rather than to choose the shortest path.


译文:

1.router首先确定在inet.0路由表里已经有到BGP下一跳地址的路由
2.router比较路由里谁的Local Preference属性值更大
3.然后router评估AS-Path属性,优选更短的路径.这个属性是必遵的.
4.router评价origin code,origin code值更小的将被优选.
5.如果经过之前的比较剩下的路由是从同一个邻居AS通告而来,则router将对比MED属性来选取一个更小的值.MED值缺省为0(即不设置则该值为0)
6.如果还剩下多条路由,则router将优选从EBGP邻居学习到的路由.如果所有的待比较路由都是从EBGP邻居学习到的,则router将直接跳到第9步进行比较
7.如果剩下的待比较路由是从IBGP邻居学到的,则router选择到IBGP邻居的IGP cost值较小的路由
8.router将选取cluster-list属性最短的.cluster-list属性的功能类似AS-Path
9.router将优选router ID 小的(这只对EBGP邻居有效,且在存在多条从不通邻居AS收到的路由的时候,router将选取当前active的路由
10.router优选邻居peer ID值更小的.
注意Local-preference值比ASpath的长度更优先被比较,这说明BGP的设计初衷是为路由策略提供更高的灵活性,而不是仅仅去选择最短的路径.

不错，都翻译过来了，学习起来就更简单的了哦。谢谢