Showing posts from June 17, 2007




ATOMIC_AGGREGATE is a well-known discretionary attribute.

When a BGP speaker aggregates several routes for the purpose of advertisement to a particular peer, the AS_PATH of the aggregated route normally includes an AS_SET formed from the set of ASes from which the aggregate was formed. In many cases, the network administrator can determine if the aggregate can safely be advertised without the AS_SET, and without forming route loops.

If an aggregate excludes at least some of the AS numbers present in the AS_PATH of the routes that are aggregated as a result of dropping the AS_SET, the aggregated route, when advertised to the peer, SHOULD include the ATOMIC_AGGREGATE attribute.

ATOMIC_AGGREGATE這個屬性一旦存在(並無內含任何的數值,僅僅為一個指標意義而已)就代表這筆路由中可能發生了AS-Path Information Lost的情況,比如將AS123, AS321的路由合併之後只留下|123|的AS-Path;因此ATOMIC_AGGREGATE跟BGP command中的AS-SET這個參數互斥,只要設定了AS-SET這個參數,AS123, AS321的路由合併之後,AS-Path會變成|{123, …

BGP Site of Origin

Site of Origin (SoO) is one of the attributes a PE router assigns to a prefix prior to redistributing any VPNv4 prefixes. SoO in BGP is configured at neighbor level and is used to manage MPLS VPN traffic and to prevent transient routing loops from occurring in complex and mixed network topologies. SoO uniquely identifies the site from which the PE router learned the prefix. All prefixes learned from a particular site must be assigned the same site of origin attribute, even if the site is multiply connected to a single PE, or is connected to multiple PEs. The objective of this case study is to demonstrate SoO configuration when BGP PE-CE routing protocol is used In this case study, CE1-A and CE2-A belong to the same site, and this site is connected to multiple PE routers PE1-AS1 and PE2-AS1. Figure 14-55 shows the topology where all Customer A sites belong to BGP AS 65001.

Figure 14-55. SoO in BGP PE-CE Based MPLS VPN Network

Figure 14-56 shows the relevant configuration to configure and…

RFC 4271 - A Border Gateway Protocol 4 (BGP-4)

也許有人會覺得我無聊,沒事把催眠文RFC搬到blog上來…,我從以前一開始學習網路就很不喜歡讀RFC,因為太多的內容在實務上用不到,不過由於工作性質的轉換,學習的方式及方向都有所改變之後,我發覺RFC的文章真的才是寶,有太多的東西在一般的論壇及archive中找不到完整的解釋,此時建議各位轉換一下心態,如果真的有心想了解書本文字背面的深層意義及來由,真的還是必須花時間好好K一下RFC,不是要各位從頭讀一遍,你可以善加利用Search功能,找到你想了解的部份就可以了;比方說,BGP Route Selection中的Prefer lowest origin code(IGP < EGP < Incomplete),為何IGP會小於EGP,EGP又小於Incomplete呢? 請參考這份RFC,你就會釣到大魚了~

Network Working Group Y. Rekhter, Ed.
Request for Comments: 4271 T. Li, Ed.
Obsoletes: 1771 S. Hares, Ed.
Category: Standards Track January 2006

A Border Gateway Protocol 4 (BGP-4)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardi…