cnp3-ebook/protocols/bgp — English @ Weblate

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63 ../../protocols/bgp.rst:271

	English Utilization of the BGP nexthop attribute 40/10000 · 40
64 ../../protocols/bgp.rst:274

	English This network contains three routers : `R1`, `R2` and `R3`. Each router is attached to a local IPv6 subnet that it advertises using BGP. There are two BGP sessions, one between `R1` and `R2` and the second between `R2` and `R3`. A `/127` subnet is used on each interdomain link (`2001:db8::4/127` on `R1-R2` and `2001:db8::0/127` on `R2-R3`) in conformance with the latest recommendation :rfc:`6164`. The BGP sessions run above TCP connections established between the neighboring routers (e.g. `2001:db8::5 - 2001:db8::6` for the `R1-R2` session). 546/10000 · 546
65 ../../protocols/bgp.rst:279

	English Let us assume that the `R1-R2` BGP session is the first to be established. A `BGP Update` message sent on such a session contains three fields : 144/10000 · 144
66 ../../protocols/bgp.rst:281

	English the advertised prefix 21/10000 · 21
67 ../../protocols/bgp.rst:282

	English the `BGP nexthop` 17/10000 · 17
68 ../../protocols/bgp.rst:283

	English the attributes including the AS-Path 36/10000 · 36
69 ../../protocols/bgp.rst:285

	English We use the notation `U(prefix, nexthop, attributes)` to represent such a `BGP Update` message in this section. Similarly, `W(prefix)` represents a `BGP withdraw` for the specified prefix. Once the `R1-R2` session has been established, `R1` sends `U(2001:db8:1234::/48,2001:db8::5,AS10)` to `R2` and `R2` sends `U(2001:db8:5678:/48,2001:db8::6,AS20)`. At this point, `R1` can reach `2001:db8:5678::/48` via `2001:db8::6` and `R2` can reach `2001:db8:1234::/48` via `2001:db8::5`. 478/10000 · 478
70 ../../protocols/bgp.rst:287

	English Once the `R2-R3` has been established, `R3` sends `U(2001:db8:acbd::/48,2001:db8::2,AS30)`. `R2` announces on the `R2-R3` session all the routes inside its RIB. It thus sends to `R3` : `U(2001:db8:1234::/48,2001:db8::1,AS20:AS10)` and `U(2001:db8:5678::/48,2001:db8::1,AS20)`. Note that when `R2` advertises the route that it learned from `R1`, it updates the BGP nexthop and adds its AS number to the AS-Path. `R2` also sends `U(2001:db8:abcd::48,2001:db8::6,AS20:AS30)` to `R1` on the `R1-R3` session. At this point, all BGP routes have been exchanged and all routers can reach `2001:db8::1234/48`, `2001:db8:5678::/48` and `2001:db8:abcd::/48`. 647/10000 · 647
71 ../../protocols/bgp.rst:289

	English If the link between `R2` and `R3` fails, `R3` detects the failure as it did not receive `KEEPALIVE` messages recently from `R2`. At this time, `R3` removes from its RIB all the routes learned over the `R2-R3` BGP session. `R2` also removes from its RIB the routes learned from `R3`. `R2` also sends `W(2001:db8:acbd::/48)` to `R1` over the `R1-R3` BGP session since it does not have a route anymore towards this prefix. 420/10000 · 420
72 ../../protocols/bgp.rst:291

	English Origin of the routes advertised by a BGP router 47/10000 · 47
73 ../../protocols/bgp.rst:293

	English A frequent practical question about the operation of BGP is how a BGP router decides to originate or advertise a route for the first time. In practice, this occurs in two situations : 183/10000 · 183
74 ../../protocols/bgp.rst:295

	English the router has been manually configured by the network operator to always advertise one or several routes on a BGP session. For example, on the BGP session between UCLouvain and its provider, belnet_ , UCLouvain's router always advertises the `2001:6a8:3080/48` IPv6 prefix assigned to the campus network 304/10000 · 304
75 ../../protocols/bgp.rst:296

	English the router has been configured by the network operator to advertise over its BGP session some of the routes that it learns with its intradomain routing protocol. For example, an enterprise router may advertise over a BGP session with its provider the routes to remote sites when these routes are reachable and advertised by the intradomain routing protocol 356/10000 · 356
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

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