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IP Routing: BGP Configuration Guide, Cisco IOS Release 15M&T -Cisco BGP Overview. This document contains five Border Gateway Protocol (BGP) case studies. Example: Configuring a BGP Process and Customizing Peers Examples: Example: Resetting BGP Peers Using 4-Byte Autonomous System Numbers


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EGP (Exterior Gateway Protocol) - BGP = Routing protocol used to exchange .. Cisco routers maintain a separate routing table to hold BGP routes: show ip bgp. BGP is a standardized exterior gateway protocol (EGP), as opposed to RIP,. OSPF, and A Cisco router running BGP can belong to only one AS. The IOS will. Introduction. Prerequisites. Requirements. Components Used. Conventions. BGP Case Studies 1. How Does BGP Work? eBGP and iBGP.

Phase 3: Route Dissemination Overlapping Routes Update-Send Process Controlling Routing Traffic Overhead Frequency of Route Advertisement Frequency of Route Origination Efficient Organization of Routing Information Information Reduction Aggregating Routing Information Route Selection Criteria Originating BGP routes BGP Timers Comparison with RFC Implementation Recommendations Multiple Networks Per Message Reducing Route Flapping Path Attribute Ordering Control Over Version Negotiation This information is sufficient for constructing a graph of AS connectivity for this reachability, from which routing loops may be pruned and, at the AS level, some policy decisions may be enforced.

These mechanisms include support for advertising a set of destinations as an IP prefix and eliminating the concept of network "class" within BGP. Routing information exchanged via BGP supports only the destination- based forwarding paradigm, which assumes that a router forwards a packet based solely on the destination address carried in the IP header of the packet. This, in turn, reflects the set of policy decisions that can and cannot be enforced using BGP.

BGP can support only those policies conforming to the destination-based forwarding paradigm. Definition of Commonly Used Terms This section provides definitions for terms that have a specific meaning to the BGP protocol and that are used throughout the text. Autonomous System AS The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol IGP and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes.

Since this classic definition was developed, it has become common for a single AS to Rekhter, et al.

The use of the term Autonomous System stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASes to have a single coherent interior routing plan, and presents a consistent picture of the destinations that are reachable through it.

External peer Peer that is in a different Autonomous System than the local system. Feasible route An advertised route that is available for use by the recipient.

Internal peer Peer that is in the same Autonomous System as the local system. IGP Interior Gateway Protocol - a routing protocol used to exchange routing information among routers within a single Autonomous System.

BGP (Border Gateway Protocol)

Route A unit of information that pairs a set of destinations with the attributes of a path to those destinations. The set of Rekhter, et al.

Unfeasible route A previously advertised feasible route that is no longer available for use. Honig for their contributions to the earlier version BGP-1 of this document.

We would like to specially acknowledge numerous contributions by Dennis Ferguson to the earlier version of this document. We would like to explicitly thank Bob Braden for the review of the earlier version BGP-2 of this document, and for his constructive and valuable comments. Johns, and Paul Tsuchiya, acted with a strong combination of toughness, professionalism, and courtesy.

We would like to specially acknowledge Andrew Lange for his help in preparing the final version of this document. Finally, we would like to thank all the members of the IDR Working Group for their ideas and the support they have given to this document.

This information is sufficient for constructing a graph of AS connectivity, from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced.

In the context of this document, we assume that a BGP speaker advertises to its peers only those routes that it uses itself in this context, a BGP speaker is said to "use" a BGP route if it is the most preferred BGP route and is used in forwarding. All other cases are outside the scope of this document. Note that some policies cannot be supported by the destination-based forwarding paradigm, and thus require techniques such as source routing aka explicit routing to be enforced.

Such policies cannot be enforced using BGP either. Standards Track [Page 7] RFC BGP-4 January traffic to a neighboring AS for forwarding to some destination reachable through but beyond that neighboring AS, intending that the traffic take a different route to that taken by the traffic originating in the neighboring AS for that same destination. On the other hand, BGP can support any policy conforming to the destination-based forwarding paradigm. These mechanisms include support for advertising a set of destinations as an IP prefix and eliminating the concept of a network "class" within BGP.

The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol IGP and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes.

Since this classic definition was developed, it has become common for a single AS to use several IGPs and, sometimes, several sets of metrics within an AS. The use of the term Autonomous System stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of the destinations that are reachable through it.

This eliminates the need to implement explicit update fragmentation, retransmission, acknowledgement, and sequencing. A TCP connection is formed between two systems. They exchange messages to open and confirm the connection parameters. The initial data flow is the portion of the BGP routing table that is allowed by the export policy, called the Adj-Ribs-Out see 3.

Incremental updates are sent as the routing tables change. BGP does not require a periodic refresh of the routing table. A peer in a different AS is referred to as an external peer, while a peer in the same AS is referred to as an internal peer.

If a particular AS has multiple BGP speakers and is providing transit service for other ASes, then care must be taken to ensure a consistent view of routing within the AS. This document specifies the base behavior of the BGP protocol. This behavior can be, and is, modified by extension specifications. When the protocol is extended, the new behavior is fully documented in the extension specifications.

Routes: Advertisement and Storage For the purpose of this protocol, a route is defined as a unit of information that pairs a set of destinations with the attributes of a path to those destinations. If a BGP speaker chooses to advertise a previously received route, it MAY add to, or modify, the path attributes of the route before advertising it to a peer.

Changing the attribute s of a route is accomplished by advertising a replacement route. The replacement route carries new changed attributes and has the same address prefix as the original route. Their contents represent routes that are available as input to the Decision Process. These are the routes that will be used by the local BGP speaker.

The choice of implementation for example, 3 copies of the information vs 1 copy with pointers is not constrained by the protocol.

Routing information that the BGP speaker uses to forward packets or to construct the forwarding table used for packet forwarding is maintained in the Routing Table.

The Routing Table accumulates routes to directly connected networks, static routes, routes learned from the IGP protocols, and routes learned from BGP.

Whether a specific BGP route should be installed in the Routing Table, and whether a BGP route should override a route to the same destination installed by another source, is a local policy decision, and is not specified in this document. In addition to actual packet forwarding, the Routing Table is used for resolution of the next-hop addresses specified in BGP updates see Section 5. A message is processed only after it is entirely received.

The maximum message size is octets. All implementations are required to support this maximum message size.

The smallest message that may be sent consists of a BGP header without a data portion 19 octets. All multi-octet fields are in network byte order. Message Header Format Each message has a fixed-size header.

There may or may not be a data portion following the header, depending on the message type. Length: This 2-octet unsigned integer indicates the total length of the message, including the header in octets. Thus, it allows one to locate the Marker field of the next message in the TCP stream.

Type: This 1-octet unsigned integer indicates the type code of the message. The current BGP version number is 4. My Autonomous System: This 2-octet unsigned integer indicates the Autonomous System number of the sender. Hold Time: This 2-octet unsigned integer indicates the number of seconds the sender proposes for the value of the Hold Timer.

Optional Parameters Length: This 1-octet unsigned integer indicates the total length of the Optional Parameters field in octets.

If the value of this field is zero, no Optional Parameters are present. Type Parm. Parameter Type is a one octet field that unambiguously identifies individual parameters.

Parameter Length is a one octet field that contains the length of the Parameter Value field in octets. Parameter Value is a variable length field that is interpreted according to the value of the Parameter Type field. The minimum length of the OPEN message is 29 octets including the message header. By applying rules to be discussed, routing Rekhter, et al.

An UPDATE message is used to advertise feasible routes that share common path attributes to a peer, or to withdraw multiple unfeasible routes from service see 3. High quality Cisco Preparation Materials. Immediately shouts heard, stature does not go back and forth, blinking into the army, the sector reappears, began the fishy killings. Once the tragedy reproduction, even more intensified than ever before, Su. He said he was not a lustful one.

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Beginner's Guide to Understanding BGP

Towards a small North An Jiang Nana s pleasant past, eyes cold and indifferent said. Jiang Nana. The four cabinet leaders and the eight elders who came by the news.Unfeasible route A previously advertised feasible route that is no longer available for use.

A TCP connection is formed between two systems. What is BGP?

BGP has no automatic discovery mechanism, which means connections between peers have to be set up manually, with peer addresses programmed in at both ends. Rene, I really appreciate your effort to share knowledge and experience. This eliminates the need to implement explicit update fragmentation, retransmission, acknowledgement, and sequencing.

Border Gateway Protocol

Since this classic definition was developed, it has become common for a single AS to use several IGPs and, sometimes, several sets of metrics within an AS. These are the routes that will be used by the local BGP speaker.

A peer in a different AS is referred to as an external peer, while a peer in the same AS is referred to as an internal peer. When announcing a Rekhter, et al.

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