cnp3-ebook/protocols/bgp — English @ Weblate

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76 ../../protocols/bgp.rst:298

	English The first solution is the most frequent. Advertising routes learned from an intradomain routing protocol is not recommended, this is because if the route flaps [#fflap]_, this would cause a large number of BGP messages being exchanged in the global Internet. 258/10000 · 258
77 ../../protocols/bgp.rst:304

	English The BGP decision process 24/10000 · 24
78 ../../protocols/bgp.rst:306

	English Besides the import and export filters, a key difference between BGP and the intradomain routing protocols is that each domain can define its own ranking algorithm to determine which route is chosen to forward packets when several routes have been learned towards the same prefix. This ranking depends on several BGP attributes that can be attached to a BGP route. 363/10000 · 363
79 ../../protocols/bgp.rst:311

	English The first BGP attribute that is used to rank BGP routes is the `local-preference` (local-pref) attribute. This attribute is an unsigned integer that is attached to each BGP route received over an eBGP session by the associated import filter. 241/10000 · 241
80 ../../protocols/bgp.rst:313

	English When comparing routes towards the same destination prefix, a BGP router always prefers the routes with the highest `local-pref`. If the BGP router knows several routes with the same `local-pref`, it prefers among the routes having this `local-pref` the ones with the shortest AS-Path. 284/10000 · 284
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

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