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If the import filter accepts the BGP message, the pseudo-code distinguishes two cases. If this is an `Update message` for prefix `p`, this can be a new route for this prefix or a modification of the route's attributes. The router first retrieves from its `RIB` the best route towards prefix `p`. Then, the new route is inserted in the `RIB` and the `BGP decision process` is run to find whether the best route towards destination `p` changes. A BGP message only needs to be sent to the router's peers if the best route has changed. For each peer, the router applies the `export filter` to verify whether the route can be advertised. If yes, the filtered BGP message is sent. Otherwise, a `Withdraw message` is sent. When the router receives a `Withdraw message`, it also verifies whether the removal of the route from its `RIB` caused its best route towards this prefix to change. It should be noted that, depending on the content of the `RIB` and the `export filters`, a BGP router may need to send a `Withdraw message` to a peer after having received an `Update message` from another peer and conversely.
Let us now discuss in more detail the operation of BGP in an IPv6 network. For this, let us consider the simple network composed of three routers located in three different ASes and shown in the figure below.
Utilization of the BGP nexthop attribute
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).
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 :
the advertised prefix
the `BGP nexthop`
the attributes including the AS-Path
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`.
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`.
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.
Origin of the routes advertised by a BGP router
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 :
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
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
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.
The BGP decision process
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.
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.
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.