Exit Zen
68
../../protocols/bgp.rst:283
English
the attributes including the AS-Path
0/360 · 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`.
0/4780 · 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`.
0/6470 · 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.
0/4200 · 420
72
../../protocols/bgp.rst:291
English
Origin of the routes advertised by a BGP router
0/470 · 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 :
0/1830 · 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
0/3040 · 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
0/3560 · 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.
0/2580 · 258
77
../../protocols/bgp.rst:304
English
The BGP decision process
0/240 · 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.
0/3630 · 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.
0/2410 · 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.
0/2840 · 284
81
../../protocols/bgp.rst:315
English
The `local-pref` attribute is often used to prefer some routes over others.
0/750 · 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.
0/2720 · 272
83
../../protocols/bgp.rst:325
English
How to create a backup link with BGP ?
0/380 · 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`
0/1530 · 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.
0/3790 · 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.
0/3590 · 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`.
0/2280 · 228