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A Border Gateway Protocol 4 (BGP-4), March Canonical URL: https://www. ; File formats: Plain Text PDF; Status: DRAFT. Connected: An Internet Encyclopedia RFC RFC Network Working Group Request for Comments: A Border Gateway Protocol 4 (BGP-4). RFC A Border Gateway Protocol 4 (BGP-4) (Q). request for comments publication. RFC; A Border Gateway Protocol 4.

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To work around this problem, BGP administrators of bg multihomed network may divide a large contiguous IP address block into smaller blocks and tweak the route announcement to make different blocks look optimal on different paths, so that external networks will choose a different path to reach different blocks of that multi-homed network.

If AS1 later decides to drop the route Increasingly, BGP is used as a generalized signaling protocol to carry information about routes that may not be part of the global Internet, such as VPNs. In the “Active” state, the router resets the ConnectRetry timer to zero and returns to the “Connect” state. Due to the extended attribute range, its usage can be manifold.

Again depending on implementation, separate tables may be kept for each view of a different peer AS. BGP may be used for routing within an autonomous system.

MEDs, defined in the main BGP standard, were originally intended to show to another neighbor AS the advertising AS’s preference as to which of several links are preferred for inbound traffic. We show that the current damping design leads to the intended behavior only under persistent route flapping.

Some of the information carried that is explicitly intended to be used in policy decisions are communities and multi-exit discriminators MED. The additional information tells the BGP process such things as whether individual entries belong in the Adj-RIBs for specific neighbors, whether the peer-neighbor route selection process made received policies eligible for the Loc-RIB, and whether Loc-RIB entries are eligible to be submitted to the local router’s routing table management process.

If so, it replaces them. While it is common to say that BGP allows an administrator to set policies on how prefixes are handled by ISPs, this is generally not possible, strictly speaking. For each peer-to-peer session, a BGP rgc maintains a state variable that tracks rcf of these six states the session is in.


Depending on the implementation of that process, the BGP route is not necessarily selected. At the second occurrence, BGP shuns that prefix for a certain length of time; subsequent occurrences are timed out exponentially. Consider AS1 has been allocated the big address space of By eligible to be submittedBGP will submit the routes that it considers best to the main routing table process.

Such manipulation is outside the scope bbp the standard but is commonly used.

RFC A Border Gateway Protocol 4 (BGP-4) – Wikidata

Other commercial routers may need a specific software executable image that contains BGP, or a license that enables it.

With the introduction of 32 bits AS numbers, some issues were immediately obvious with the community attribute that only defines a ffc bits ASN field, which prevents the matching between this field and the real ASN value.

Views Read Edit View history. Both confederations and 1717 reflectors can be subject to persistent oscillation unless specific design rules, affecting both BGP and the interior routing protocol, are followed.

Due to the extent to which BGP is embedded in the core systems of the Internet, and the number of different networks operated by many different organizations which collectively make up the Internet, correcting this vulnerability such as by introducing the use of cryptographic keys to verify the identity of BGP routers is a technically and economically challenging problem. A single rfcc or two for redundancy can be made a route reflector: In the “OpenSent” state, the router sends an Open message and waits for one in return in order to transition to the “OpenConfirm” state.

Until latethe global routing table was growing exponentiallythreatening an 11771 widespread breakdown of connectivity.

The BGP Extended Community Attribute was added inin order to extend the range of such attributes and to provide a community attribute structuring by means of a type field. Even simple multihoming can have modest routing table size.

With damping, a route’s flapping is exponentially decayed. The k problem was predicted in advance by a number of IT professionals.

The new routes appear to have been reaggregated within 5 minutes, but instability across the Internet apparently continued for a number of hours.

Nevertheless, these are common tools for experienced BGP network architects. Confederations are sets of autonomous systems.


This pattern of repeated withdrawal and re-announcement known as route flapping can cause excessive activity in all the other routers that know about the broken link, as the same route is continually injected and withdrawn from the routing tables. Retrieved from ” https: RFC corrected errors, clarified ambiguities and updated the specification with common industry practices.

BGP actually carried the information with which rules inside BGP-speaking routers could make policy decisions. Confederations are used in very large networks where a large AS hgp be configured to encompass smaller more manageable internal ASs. This requires a reboot on most routers.

Border Gateway Protocol

This allows for automatic and decentralized routing of traffic across the Internet, but it also leaves the Internet potentially vulnerable to accidental or malicious disruption, known as BGP hijacking. The action could be to drop the route, or it could be to modify some attributes of the route before inserting it in the routing table.

Theory and Applications” PDF. These devices, however, may be perfectly reasonable and useful when used for BGP routing of some smaller part of the network, such as a confederation-AS representing one of several smaller enterprises that are linked, by a BGP backbone of backbonesor a small enterprise that announces routes to an ISP but only accepts a default route and perhaps a small number of aggregated routes.

If the router does not have a route to that destination from any non-BGP source, the withdrawn route will be removed from the main routing table.

The BGP defines the messages that each peer should exchange in order to change the session from one state to another. A BGP router used only for a network with a single point of entry to the Internet may have a much smaller routing table size and hence RAM and CPU requirement than a multihomed network.

A feature known as route flap damping RFC is built into many BGP implementations in an attempt to mitigate the effects of route flapping.