cnp3-ebook/protocols/bgp — English @ Weblate

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32 ../../protocols/bgp.rst:106

	English Simple exchange of BGP routes 29/10000 · 29
33 ../../protocols/bgp.rst:110

	English BGP routers exchange routes over BGP sessions. A BGP session is established between two routers belonging to two different domains that are directly connected. As explained earlier, the physical connection between the two routers can be implemented as a private peering link or over an Internet eXchange Point. A BGP session between two adjacent routers runs above a TCP connection (the default BGP port is 179). In contrast with intradomain routing protocols that exchange IP packets or UDP segments, BGP runs above TCP because TCP ensures a reliable delivery of the BGP messages sent by each router without forcing the routers to implement acknowledgments, checksums, etc. Furthermore, the two routers consider the peering link to be up as long as the BGP session and the underlying TCP connection remain up [#flifetimebgp]_. The two endpoints of a BGP session are called `BGP peers`. 886/10000 · 886
34 ../../protocols/bgp.rst:116

	English A BGP peering session between two directly connected routers 60/10000 · 60
35 ../../protocols/bgp.rst:118

	English In practice, to establish a BGP session between routers `R1` and `R2` in the figure above, the network administrator of `AS3` must first configure on `R1` the IP address of `R2` on the `R1-R2` link and the AS number of `R2`. Router `R1` then regularly tries to establish the BGP session with `R2`. `R2` only agrees to establish the BGP session with `R1` once it has been configured with the IP address of `R1` and its AS number. For security reasons, a router never establishes a BGP session that has not been manually configured on the router. 544/10000 · 544
36 ../../protocols/bgp.rst:122

	English The BGP protocol :rfc:`4271` defines several types of messages that can be exchanged over a BGP session : 105/10000 · 105
37 ../../protocols/bgp.rst:124

	English `OPEN` : this message is sent as soon as the TCP connection between the two routers has been established. It initializes the BGP session and allows the negotiation of some options. Details about this message may be found in :rfc:`4271`. 236/10000 · 236
38 ../../protocols/bgp.rst:125

	English `NOTIFICATION` : this message is used to terminate a BGP session, usually because an error has been detected by the BGP peer. A router that sends or receives a `NOTIFICATION` message immediately shutdowns the corresponding BGP session. 235/10000 · 235
39 ../../protocols/bgp.rst:126

	English `UPDATE`: this message is used to advertise new or modified routes or to withdraw previously advertised routes. 111/10000 · 111
40 ../../protocols/bgp.rst:127

	English `KEEPALIVE` : this message is used to ensure a regular exchange of messages on the BGP session, even when no route changes. When a BGP router has not sent an `UPDATE` message during the last 30 seconds, it shall send a `KEEPALIVE` message to confirm to the other peer that it is still up. If a peer does not receive any BGP message during a period of 90 seconds [#fdefaultkeepalive]_, the BGP session is considered to be down and all the routes learned over this session are withdrawn. 485/10000 · 485
41 ../../protocols/bgp.rst:129

	English As explained earlier, BGP relies on incremental updates. This implies that when a BGP session starts, each router first sends BGP `UPDATE` messages to advertise to the other peer all the exportable routes that it knows. Once all these routes have been advertised, the BGP router only sends BGP `UPDATE` messages about a prefix if the route is new, one of its attributes has changed or the route became unreachable and must be withdrawn. The BGP `UPDATE` message allows BGP routers to efficiently exchange such information while minimizing the number of bytes exchanged. Each `UPDATE` message contains : 602/10000 · 602
42 ../../protocols/bgp.rst:131

	English a list of IP prefixes that are withdrawn 40/10000 · 40
43 ../../protocols/bgp.rst:132

	English a list of IP prefixes that are (re-)advertised 46/10000 · 46
44 ../../protocols/bgp.rst:133

	English the set of attributes (e.g. AS-Path) associated to the advertised prefixes 74/10000 · 74
45 ../../protocols/bgp.rst:135

	English In the remainder of this chapter, and although all routing information is exchanged using BGP `UPDATE` messages, we assume for simplicity that a BGP message contains only information about one prefix and we use the words : 222/10000 · 222
46 ../../protocols/bgp.rst:137

	English `Withdraw message` to indicate a BGP `UPDATE` message containing one route that is withdrawn 92/10000 · 92
47 ../../protocols/bgp.rst:138

	English `Update message` to indicate a BGP `UPDATE` containing a new or updated route towards one destination prefix with its attributes 128/10000 · 128
48 ../../protocols/bgp.rst:143

	English From a conceptual point of view, a BGP router connected to `N` BGP peers, can be described as being composed of four parts as shown in the figure below. 152/10000 · 152
49 ../../protocols/bgp.rst:151

	English Organization of a BGP router 28/10000 · 28
50 ../../protocols/bgp.rst:159

	English In this figure, the router receives BGP messages on the left part of the figure, processes these messages and possibly sends BGP messages on the right part of the figure. A BGP router contains three important data structures : 226/10000 · 226
51 ../../protocols/bgp.rst:161

	English the `Adj-RIB-In` contains the BGP routes that have been received from each BGP peer. The routes in the `Adj-RIB-In` are filtered by the `import filter` before being placed in the `BGP-Loc-RIB`. There is one `import filter` per BGP peer. 236/10000 · 236

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