cnp3-ebook/protocols/bgp — English @ Weblate

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81 ../../protocols/bgp.rst:315

	English The `local-pref` attribute is often used to prefer some routes over others. 75/10000 · 75
82 ../../protocols/bgp.rst:319

	English A common utilization of `local-pref` is to support backup links. Consider the situation depicted in the figure below. `AS1` would always like to use the high bandwidth link to send and receive packets via `AS2` and only use the backup link upon failure of the primary one. 272/10000 · 272
83 ../../protocols/bgp.rst:325

	English How to create a backup link with BGP ? 38/10000 · 38
84 ../../protocols/bgp.rst:327

	English As BGP routers always prefer the routes with the highest `local-pref` attribute, this policy can be implemented using the following import filter on `R1` 153/10000 · 153
85 ../../protocols/bgp.rst:335

	English With this import filter, all the BGP routes learned from `RB` over the high bandwidth links are preferred over the routes learned over the backup link. If the primary link fails, the corresponding routes are removed from `R1`'s RIB and `R1` uses the route learned from `RA`. `R1` reuses the routes via `RB` as soon as they are advertised by `RB` once the `R1-RB` link comes back. 379/10000 · 379
86 ../../protocols/bgp.rst:337

	English The import filter above modifies the selection of the BGP routes inside `AS1`. Thus, it influences the route followed by the packets forwarded by `AS1`. In addition to using the primary link to send packets, `AS1` would like to receive its packets via the high bandwidth link. For this, `AS2` also needs to set the `local-pref` attribute in its import filter. 359/10000 · 359
87 ../../protocols/bgp.rst:346

	English Sometimes, the `local-pref` attribute is used to prefer a `cheap` link compared to a more expensive one. For example, in the network below, `AS1` could wish to send and receive packets mainly via its interdomain link with `AS4`. 228/10000 · 228
88 ../../protocols/bgp.rst:352

	English How to prefer a cheap link over an more expensive one ? 55/10000 · 55
89 ../../protocols/bgp.rst:354

	English `AS1` can install the following import filter on `R1` to ensure that it always sends packets via `R2` when it has learned a route via `AS2` and another via `AS4`. 162/10000 · 162
90 ../../protocols/bgp.rst:363

	English However, this import filter does not influence how `AS3` , for example, prefers some routes over others. If the link between `AS3` and `AS2` is less expensive than the link between `AS3` and `AS4`, `AS3` could send all its packets via `AS2` and `AS1` would receive packets over its expensive link. An important point to remember about `local-pref` is that it can be used to prefer some routes over others to send packets, but it has no influence on the routes followed by received packets. 489/10000 · 489
91 ../../protocols/bgp.rst:365

	English Another important utilization of the `local-pref` attribute is to support the `customer->provider` and `shared-cost` peering relationships. From an economic point of view, there is an important difference between these three types of peering relationships. A domain usually earns money when it sends packets over a `provider->customer` relationship. On the other hand, it must pay its provider when it sends packets over a `customer->provider` relationship. Using a `shared-cost` peering to send packets is usually neutral from an economic perspective. To take into account these economic issues, domains usually configure the import filters on their routers as follows : 671/10000 · 671
92 ../../protocols/bgp.rst:367

	English insert a high `local-pref` attribute in the routes learned from a customer 74/10000 · 74
93 ../../protocols/bgp.rst:368

	English insert a medium `local-pref` attribute in the routes learned over a shared-cost peering 87/10000 · 87
94 ../../protocols/bgp.rst:369

	English insert a low `local-pref` attribute in the routes learned from a provider 73/10000 · 73
95 ../../protocols/bgp.rst:371

	English With such an import filter, the routers of a domain always prefer to reach destinations via their customers whenever such a route exists. Otherwise, they prefer to use `shared-cost` peering relationships and they only send packets via their providers when they do not know any alternate route. A consequence of setting the `local-pref` attribute like this is that Internet paths are often asymmetrical. Consider for example the internetwork shown in the figure below. 467/10000 · 467
96 ../../protocols/bgp.rst:377

	English Asymmetry of Internet paths 27/10000 · 27
97 ../../protocols/bgp.rst:379

	English Consider in this internetwork the routes available inside `AS1` to reach `AS5`. `AS1` learns the `AS4:AS6:AS7:AS5` path from `AS4`, the `AS3:AS8:AS5` path from `AS3` and the `AS2:AS5` path from `AS2`. The first path is chosen since it was learned from a customer. `AS5` on the other hand receives three paths towards `AS1` via its providers. It may select any of these paths to reach `AS1` , depending on how it prefers one provider over the others. 449/10000 · 449
98 ../../protocols/bgp.rst:384

	English BGP convergence 15/10000 · 15
99 ../../protocols/bgp.rst:387

	English In the previous sections, we have explained the operation of BGP routers. Compared to intradomain routing protocols, a key feature of BGP is its ability to support interdomain routing policies that are defined by each domain as its import and export filters and ranking process. A domain can define its own routing policies and router vendors have implemented many configuration tweaks to support complex routing policies. However, the routing policy chosen by a domain may interfere with the routing policy chosen by another domain. To understand this issue, let us first consider the simple internetwork shown below. 618/10000 · 618
100 ../../protocols/bgp.rst:394

	English The disagree internetwork 25/10000 · 25

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