cnp3-ebook/protocols/bgp — English @ Weblate

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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
101 ../../protocols/bgp.rst:396

	English In this internetwork, we focus on the route towards `2001:db8::1234/48` which is advertised by `AS1`. Let us also assume that `AS3` (resp. `AS4`) prefers, e.g. for economic reasons, a route learned from `AS4` (`AS3`) over a route learned from `AS1`. When `AS1` sends `U(2001:db8::1234/48,AS1)` to `AS3` and `AS4`, three sequences of exchanges of BGP messages are possible : 373/10000 · 373
102 ../../protocols/bgp.rst:398

	English `AS3` sends first `U(2001:db8:1234/48,AS3:AS1)` to `AS4`. `AS4` has learned two routes towards `2001:db8:1234/48`. It runs its BGP decision process and selects the route via `AS3` and does not advertise a route to `AS3` 219/10000 · 219
103 ../../protocols/bgp.rst:399

	English `AS4` first sends `U(2001:db8:1234/48,AS4:AS1)` to `AS3`. `AS3` has learned two routes towards `2001:db8:1234/48`. It runs its BGP decision process and selects the route via `AS4` and does not advertise a route to `AS4` 219/10000 · 219
104 ../../protocols/bgp.rst:400

	English `AS3` sends `U(2001:db8:1234/48,AS3:AS1)` to `AS4` and, at the same time, `AS4` sends `U(2001:db8:1234/48,AS4:AS1)`. `AS3` prefers the route via `AS4` and thus sends `W(2001:db8:1234/48)` to `AS4`. In the mean time, `AS4` prefers the route via `AS3` and thus sends `W(2001:db8:1234/48)` to `AS3`. Upon reception of the `BGP Withdraws`, `AS3` and `AS4` only know the direct route towards `2001:db8:1234/48`. `AS3` (resp. `AS4`) sends `U(2001:db8:1234/48,AS3:AS1)` (resp. `U(2001:db8:1234/48,AS4:AS1)`) to `AS4` (resp. `AS3`). `AS3` and `AS4` could in theory continue to exchange BGP messages for ever. In practice, one of them sends one message faster than the other and BGP converges. 685/10000 · 685
105 ../../protocols/bgp.rst:402

	English The example above has shown that the routes selected by BGP routers may sometimes depend on the ordering of the BGP messages that are exchanged. Other similar scenarios may be found in :rfc:`4264`. 197/10000 · 197
106 ../../protocols/bgp.rst:404

	English From an operational perspective, the above configuration is annoying since the network operators cannot easily predict which paths are chosen. Unfortunately, there are even more annoying BGP configurations. For example, let us consider the configuration below which is often named `Bad Gadget` [GW1999]_ 303/10000 · 303
107 ../../protocols/bgp.rst:410

	English The bad gadget internetwork 27/10000 · 27
108 ../../protocols/bgp.rst:413

	English In this internetwork, there are four ASes. `AS0` advertises one route towards one prefix and we only analyze the routes towards this prefix. The routing preferences of `AS1`, `AS3` and `AS4` are the following : 210/10000 · 210
109 ../../protocols/bgp.rst:415

	English `AS1` prefers the path `AS3:AS0` over all other paths 53/10000 · 53
110 ../../protocols/bgp.rst:416

	English `AS3` prefers the path `AS4:AS0` over all other paths 53/10000 · 53
111 ../../protocols/bgp.rst:417

	English `AS4` prefers the path `AS1:AS0` over all other paths 53/10000 · 53
112 ../../protocols/bgp.rst:419

	English `AS0` sends `U(p,AS0)` to `AS1`, `AS3` and `AS4`. As this is the only route known by `AS1`, `AS3` and `AS4` towards `p`, they all select the direct path. Let us now consider one possible exchange of BGP messages : 213/10000 · 213
113 ../../protocols/bgp.rst:421

	English `AS1` sends `U(p, AS1:AS0)` to `AS3` and `AS4`. `AS4` selects the path via `AS1` since this is its preferred path. `AS3` still uses the direct path. 148/10000 · 148
114 ../../protocols/bgp.rst:422

	English `AS4` advertises `U(p,AS4:AS1:AS0)` to `AS3`. 45/10000 · 45
115 ../../protocols/bgp.rst:423

	English `AS3` sends `U(p, AS3:AS0)` to `AS1` and `AS4`. `AS1` selects the path via `AS3` since this is its preferred path. `AS4` still uses the path via `AS1`. 151/10000 · 151
116 ../../protocols/bgp.rst:424

	English As `AS1` has changed its path, it sends `U(p,AS1:AS3:AS0)` to `AS4` and `W(p)` to `AS3` since its new path is via `AS3`. `AS4` switches back to the direct path. 160/10000 · 160

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