選擇BGP最佳路徑的標(biāo)準(zhǔn)

henry05

                                                                                                理解路由器上BGP最佳路徑選擇算法的工作過程
BGP creates the routes that allow us to connect to whatever
Internet data we desire within milliseconds. David Davis explains how
the BGP Best Path Selection Algorithm actually works on your router.
——————————————————————————————————
The Border Gateway Protocol (BGP) allows us to surf the Web, receive
our e-mail, and read this article. BGP is a path-vector routing
protocol, which simply means that its job is to exchange routing
information between autonomous systems to find the most efficient path
for your data to the Internet. Each BGP router connects to its neighbor
(or neighbors) with neighbor statements. Once it forms that neighbor
relationship, the BGP routers exchange routes. Last time I checked, the
full Internet routing table was about 125,000 routes, so you need a
strong router to receive the full Internet BGP routing table.
//注意:BGP是path-vector 路由協(xié)議; 在不同AS間交換路由信息,并找到最佳路徑.使用neighbour statement來連接鄰居.作者檢查BGP路由器時,發(fā)現(xiàn)全互聯(lián)網(wǎng)路由有125,000條.
I have authored three different TechRepublic articles about BGP:

• 
  Five Things You Should Know about Cisco IOS BGP Configuration
  
• 
  How to Use BGP to Achieve Internet Redundancy
  
• 
  Build Your Skills: Use IP and BGP to Troubleshoot Internet Connectivity
  

Now, I’d like to focus on a different aspect of BGP — how BGP
selects the best path, through the Internet, out of all the routes that
it might receive.
What is the BGP Best Path Selection Algorithm?
With the full Internet BGP routing table being upward of 200K routes
and with a BGP router having the potential to be receiving multiple
copies of that routing table from multiple providers, it has to have
some way to compare those multiple BGP routing tables and select only
the best route to go into the IP routing table on the router. It uses
the BGP Best Path Selection Algorithm to do this.
You should note that Cisco BGP routers have weight as the first
criteria in the table where other brands of routers do not. Cisco’s
official
BGP Best Path Selection Algorithm documentation
lists these criteria, but below I will list each of them with my own commentary and examples.
Unless there are no options in place to influence the result, the
BGP Best Path selects the best path based on the shortest Autonomous
System (AS), one or more networks under a single administrative
control. However, many times options like weight, local preference, and
MED are put in place by network administrators.
//基于最短AS的選擇原則,而其他情況如 weight, local preference, MED也被考慮.
Let’s look at the selection criteria, in order, that BGP uses to select the best routes to install into the IP Routing table:
//注意其順序性.
#1 Weight — This is a Cisco-defined attribute that
is assigned locally to your router and does not get carried through to
the router updates. If there are multiple paths to a particular IP
address (which is very common), then BGP looks for the path with the
highest weight. There are several ways to set the weight parameter,
such as the neighbor command, the as-path access list, or route maps.
//cisco所屬的本地路由屬性. BGP選擇highest weight的路徑.可通過 neighbor命令 / as-path access表 / route maps去設(shè)定 weight參數(shù).
#2 Local Preference — This is an indicator to the
AS as to which path has local preference, with the highest preference
being preferred. The default is 100. For example:
bgp default local-preference 150
//path中包含local preference,指明選擇AS要做的考慮.
#3 Network or Aggregate — This criterion prefers
the path that was locally originated via a network or aggregate. The
aggregation of specific routes into one route is very efficient and
saves space on your network. For more information on aggregate, please
see the Cisco article “
Understanding Route Aggregation in BGP
.”
//通過網(wǎng)絡(luò)或收斂而產(chǎn)生的本地path作為標(biāo)準(zhǔn). (收斂能節(jié)省空間)
#4 Shortest AS_PATH — BGP uses this one only when
there is a “tie” comparing weight, local preference, and locally
originated vs. aggregate addresses.
//僅當(dāng)與#1 #2 #3比較后才采用.
#5 Lowest origin type — This deals with protocols
such as Interior Gateway Protocol (IGP) being a lower preference than
Exterior Gateway Protocol (EGP).
//低等級的IGP 協(xié)議 的優(yōu)先級高于 EGP
#6 Lowest multi-exit discriminator (MED) — This is also known as the external metric of a route. A lower MED value is preferred over a higher value.
//MED作為一個路由的外部權(quán)值. (MED低等級值對應(yīng)于higher value)
#7 eBGP over iBGP — Similar to #5, BGP AS Path prefers eBGP over iBGP.
//eBGP 高于 iBGP
#8 Lowest IGP metric — This criterion prefers the path with the lowest IGP metric to the BGP next hop.
//低的IGP權(quán)值對應(yīng)的路徑 優(yōu)于 BGP 的下一跳.
#9 Multiple paths — This determines if multiple paths require installation in the routing table. Check out this link for information on
BGP Multipath
.
//多路徑情況匯 參考 BGP Multipath說明
#10 External paths — When both paths are external, it prefers the path that was received first (the oldest one).
//兩外部路徑情況, 首個接收的路徑優(yōu)先.
#11 Lowest router ID — This prefers the route that comes from the BGP router with the lowest router ID.
//來自 低ID 路由器的路由優(yōu)先
#12 Minimum cluster list — If the originator or router ID is the same for multiple paths, it prefers the path with the minimum cluster list length.
//如果 針對多路徑的 originator 或 路由器ID 相同, 那么 帶最小的 cluster list 長度的路徑優(yōu)先.
#13 Lowest neighbor address — This prefers the path that comes from the lowest neighbor address.
//來自最低鄰居地址的路徑優(yōu)先.
There are some commands that you will use over and over with the BGP metrics. They are show ip bgp and show ip bgp summary. In fact, I usually create aliases for these commands to make using them quicker (see my article “
Enter Commands More Efficiently with Cisco Command Aliases
“).
These commands will give you a quick snapshot of BGP and its
relationship to the routers. Here is a truncated example of output from
the show ip bgp command:
RTA# show ip bgp
BGP table version is 14, local router ID is 203.250.15.10
Status codes: s suppressed, d damped, h history, * valid, > best, i -internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network          Next Hop          Metric LocPrf Weight Path
*>i128.213.0.0      128.213.63.2           0    200      0 200 i
> 192.208.10.0     192.208.10.5           0    300      0 300 i
*>i200.200.0.0/16   128.213.63.2                200      0 200 400 i
>i203.250.13.0     203.250.13.41          0    100      0 i
*>i203.250.14.0     203.250.13.41          0    100      0 i
*> 203.250.15.0     0.0.0.0                0         32768 i
The routes with the * beside them were selected as the best path by the BGP Best Path Selection Algorithm.
Conclusion
Whether you are a network administrator who uses BGP or you are just
interested in BGP for CCNP or CCIE certification, understanding how it
works is valuable knowledge. As you have seen, using the metrics and
the show commands can really narrow down the criteria for easy
resolution.
//使用metrics 和 show命令能減少使用多標(biāo)準(zhǔn)來判斷.
作者:David Davis has worked in the IT industry for 15+ years and
holds several certifications, including CCIE, CCNA, CCNP, MCSE, CISSP,
VCP. He has authored hundreds of articles and numerous IT training
videos. Today, David is the Director of Infrastructure at
Train Signal.com
. Train Signal, Inc. is the global leader in video training for IT Professionals and end users. Read his
full bio and profile
.
               
               

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