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*** A random TCP port over 1023 is not open.
*** BGP configuration error.
*** [[Network congestion]].
*** Flapping network interface.
* '''OpenSent State''':
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Address family identifier (AFI): IPv4 (1)
Subtype: Normal route refresh request [RFC2918] with/without ORF [RFC5291] (0)
Subsequent address family identifier (SAFI): [[Unicast]] (1)
== Internal scalability ==
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'''Route reflectors''' (RRs) reduce the number of connections required in an AS. A single router (or two for redundancy) can be made an RR: other routers in the AS need only be configured as peers to them. An RR offers an alternative to the logical full-mesh requirement of iBGP. The purpose of the RR is concentration. Multiple BGP routers can peer with a central point, the RR{{snd}} acting as an RR server{{snd}} rather than peer with every other router in a full mesh. All the other iBGP routers become RR clients.<ref>{{cite IETF |title=BGP Route Reflection: An Alternative to Full Mesh Internal BGP (iBGP) |RFC=4456 |author=T. Bates |display-authors=etal |date=April 2006}}</ref>
This approach, similar to [[OSPF]]'s DR/BDR feature<ref>{{cite book |last=Chidester |first=Ashlan |title=Advanced OSPF & BGP |publisher=MC Inc |year=2024 |isbn=9798224952625 |url=https://www.worldcat.org/en/title/1476987548 |access-date=2025-08-29}}</ref>, provides large networks with added iBGP scalability. In a fully meshed iBGP network of 10 routers, 90 individual CLI statements (spread throughout all routers in the topology) are needed just to define the remote-AS of each peer: this quickly becomes a headache to manage. An RR topology can cut these 90 statements down to 18, offering a viable solution for the larger networks administered by ISPs.
An RR is a [[single point of failure]], therefore at least a second RR may be configured in order to provide redundancy. As it is an additional peer for the other 10 routers, it approximately doubles the number of CLI statements, requiring an additional {{nowrap|1=11 × 2 − 2 = 20}} statements in this case. In a BGP multipath environment the additional RR also can benefit the network by adding local routing throughput if the RRs are acting as traditional routers instead of just a dedicated RR server role.
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