Implementing BGP Selecting a BGP Path

BGP Path Attributes BGP metrics are called path attributes. Characteristics of path attributes include: –Well-known( 公认 ) versus optional( 可选 ) –Mandatory( 强制 ) versus discretionary( 自由选择 ) –Transitive( 传递 ) versus nontransitive( 非传递 ) –Partial

Well-Known Attributes Well-known attributes – 所有厂商在实现 BGP 时必须支持 – 被传递到其他邻居 Well-known mandatory attributes( 公认强制属性 ) – 必须在所有的更新消息中包含 Well-known discretionary attributes( 公认自由选择属性 ) – 可能在更新消息中包含

Optional Attributes Optional attributes – 它们可能是私有属性 (could be private); 不要求所有厂商在实 现 BGP 时支持. – 基于该可选属性的含义, 可能被传输给其他 BGP 邻居. Optional transitive attributes( 可选传递属性 ) – 对于不支持的可选传递属性, 路由器将属性标记为 partial( 部分 ) 并原封不动的传递给其他邻居. Optional nontransitive attributes( 可选非传递属性 ) – 对于可选非传递属性, 路由器将该属性丢弃, 而不将其传递给其他 邻居

BGP Attributes BGP attributes include the following: AS path * Next-hop * Origin * Local preference (Well-known discretionary attribute) MED (Optional nontransitive attribute) Others * Well-known mandatory attribute

AS Path Attribute A list of autonomous systems that a route has traversed: –For example, on router B, the path to is the AS sequence (65500, 64520). The AS path attribute is well-known, mandatory.

Next-Hop Attribute The IP address of the next AS to reach a given network: Router A advertises network to router B in EBGP, with a next hop of Router B advertises in IBGP to router C, keeping as the next-hop address. The next-hop attribute is well-known, mandatory.

Origin Attribute IGP (i) –network command EGP (e) –Redistributed from EGP Incomplete (?) –Redistributed from IGP or static The origin attribute informs all autonomous systems in the internetwork how the prefixes were introduced into BGP. The origin attribute is well-known, mandatory.

Example: Origin Attribute RouterA# show ip bgp BGP table version is 14, local router ID is Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> / i * i i *> / i *>i / i *> / i * i * i i *> / i * i * i i r> / i r i r i i *> / i

Local Preference Attribute Paths with highest local preference value are preferred: Local preference is used to advertise to IBGP neighbors about how to leave their AS. The local preference is sent to IBGP neighbors only (that is, within the AS only). The local preference attribute is well-known and discretionary. Default value is 100.

The paths with the lowest MED (also called the metric) value are the most desirable: –MED is used to advertise to EBGP neighbors how to exit their AS to reach networks owned by this AS. The MED attribute is optional and nontransitive. MED Attribute

Paths with the highest weight value are preferred Weight not sent to any BGP neighbors; local to this router only Weight Attribute (Cisco Only)

BGP Path Selection BGP 转发表中通常包含到达每一个网络的多条路径. BGP 并不设计用于负载均衡 : – 路径选择基于策略. – 路径选择不是基于带宽. BGP 在路径选择过程中排除多条路径, 只留下一条最佳路径. 最佳路径服从路由表管理器进程, 路由表管理器进程将其同其他路 由选择协议提供的到达同一网络的路径进行比较 ( 使用管理距离 ). 管理距离最小的路由来源被加入到路由表中.

Route Selection Decision Process Consider only (synchronized) routes with no AS loops and a valid next hop, and then: 1. 首选 weight 最大的路由 ( 本地路由器 ). 2. 首选 local preference 最大的路由 (AS 内 ). 3. 首选来源于本地路由器的路由 (next hop = ). 4. 首选 AS path 最短的路由. 5. 首选 origin code 最小的路由 (IGP < EGP < incomplete). 6. 首选 MED 值最小 (AS 间交换 ). 7. 从 EBGP 学习到的路径优于从 IBGP 学习到的路径. 8.Prefer the path through the closest IGP neighbor. 9.Prefer oldest route for EBGP paths. 10.Prefer the path with the lowest neighbor BGP router ID. 11.Prefer the path with the lowest neighbor IP address.

Summary BGP metrics are called path attributes and describe the paths to reach each network. These attributes are categorized as well- known mandatory, well-known discretionary, optional transitive, and optional nontransitive. The AS path attribute is a well-known mandatory attribute that lists the AS numbers that a route has traversed to reach a destination. The BGP next-hop attribute is a well-known mandatory attribute that indicates the next-hop IP address to use to reach a destination. The origin attribute is a well-known mandatory attribute that defines the origin of the path information. The local preference attribute is a well-known discretionary attribute that provides an indication to routers in the AS about which path is preferred to exit the AS.

Summary (Cont.) The MED attribute, also called the metric, is an optional nontransitive attribute that is an indication to EBGP neighbors about the preferred path into an AS. The MED is sent to EBGP peers; those routers propagate the MED within their AS. The routers within the AS use the MED, but do not pass it on to the next AS. The weight attribute is an attribute that Cisco defines for the path selection process. Routes with a higher weight are preferred when multiple routes exist to the same destination. Paths for a network that are determined not to be the best are eliminated from the selection criteria but are still kept in the BGP forwarding table in case the best path becomes inaccessible. BGP follows a multiple-step process when selecting the best route to reach a destination.