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•  
• Introduction
• 
  The Border Gateway Protocol (BGP) is an interautonomous system routing protocol. An autonomous
  system is a network or group of networks under a common administration and with common routing
  policies. BGP is used to exchange routing information for the Internet and is the protocol used between
  Internet service providers (ISP).
•  
• Customer networks, such as universities and corporations, usually employ an Interior Gateway Protocol
• (IGP) such as RIP or OSPF for the exchange of routing information within their networks.
•  
• Customers connect to ISPs, and ISPs use BGP to exchange customer and ISP routes. When BGP is used
• between autonomous systems (AS), the protocol is referred to as External BGP (EBGP). If a service provider
• is using BGP to exchange routes within an AS, then the protocol is referred to as Interior BGP (IBGP).
•  
• Figure illustrates this distinction

• BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing
  protocol employed on the Internet. At the time of this writing, the Internet BGP routing tables number
  more than 90,000 routes. To achieve scalability at this level, BGP uses many route parameters, called
  attributes, to define routing policies and maintain a stable routing environment.
• 
  In addition to BGP attributes, classless interdomain routing (CIDR) is used by BGP to reduce the size
  of the Internet routing tables. For example, assume that an ISP owns the IP address block 195.10.x.x
  from the traditional Class C address space. This block consists of 256 Class C address blocks, 195.10.0.x
  through 195.10.255.x.
•  
• Assume that the ISP assigns a Class C block to each of its customers. Without CIDR, the ISP would advertise
• 256 Class C address blocks to its BGP peers. With CIDR, BGP can supernet the address space and advertise
• one block, 195.10.x.x. This block is the same size as a traditional Class B address block. The class distinctions
• are rendered obsolete by CIDR, allowing a significant reduction in the BGP routing tables.
• 
  BGP neighbors exchange full routing information when the TCP connection between neighbors is first
  established. When changes to the routing table are detected, the BGP routers send to their neighbors only
  those routes that have changed. BGP routers do not send periodic routing updates, and BGP routing
  updates advertise only the optimal path to a destination network.
•  
• BGP Attributes
• 
  Routes learned via BGP have associated properties that are used to determine the best route to a
  destination when multiple paths exist to a particular destination. These properties are referred to as BGP
  attributes, and an understanding of how BGP attributes influence route selection is required for the
  design of robust networks. This section describes the attributes that BGP uses in the route selection
  process:
• 
  . Weight
  . Local preference
  . Multi-exit discriminator
  . Origin
  . AS_path
  . Next hop
  . Community
• 
  Weight Attribute
• 
  Weight is a Cisco-defined attribute that is local to a router. The weight attribute is not advertised to
  neighboring routers. If the router learns about more than one route to the same destination, the route with
  the highest weight will be preferred.
•  
• In Figure Router A is receiving an advertisement for network 172.16.1.0 from routers B and C. When Router
• A receives the advertisement from Router B, the associated weight is set to 50. When Router A receives the
• advertisement from Router C, the associated weight is set to 100. Both paths for network 172.16.1.0 will be in
• the BGP routing table, with their respective weights. The route with the highest weight will be installed in the
• IP routing table.
• 
  Figure BGP Weight Attribute

• Local Preference Attribute
• 
  The local preference attribute is used to prefer an exit point from the local autonomous system (AS).
  Unlike the weight attribute, the local preference attribute is propagated throughout the local AS. If there
  are multiple exit points from the AS, the local preference attribute is used to select the exit point for a
  specific route.
•  
• In Figure  AS 100 is receiving two advertisements for network 172.16.1.0 from AS 200. When Router A
• receives the advertisement for network 172.16.1.0, the corresponding local preference is set to 50.
• When Router B receives the advertisement for network 172.16.1.0, the corresponding local preference
• is set to 100.
•  
• These local preference values will be exchanged between routers A and B. Because Router B has a higher
• local preference than Router A, Router B will be used as the exit point from AS 100 to reach network
• 172.16.1.0 in AS 200.
•  
• BGP Local Preference Attribute

• Multi-Exit Discriminator Attribute
• 
  The multi-exit discriminator (MED) or metric attribute is used as a suggestion to an external AS
  regarding the preferred route into the AS that is advertising the metric.
• 
  The term suggestion is used because the external AS that is receiving the MEDs may be using other BGP
  attributes for route selection. We will cover the rules regarding route selection in the next section. In
  Figure Router C is advertising the route 172.16.1.0 with a metric of 10, while Route D is
  advertising 172.16.1.0 with a metric of 5. The lower value of the metric is preferred, so AS 100 will
  select the route to router D for network 172.16.1.0 in AS 200. MEDs are advertised throughout the local
  AS.
• 
  Origin Attribute
• 
  The origin attribute indicates how BGP learned about a particular route. The origin attribute can have
  one of three possible values:
• 
  IGPThe route is interior to the originating AS. This value is set when the network router
  configuration command is used to inject the route into BGP.
• 
  EGPThe route is learned via the Exterior Border Gateway Protocol (EBGP).
• 
  IncompleteThe origin of the route is unknown or learned in some other way. An origin of
  incomplete occurs when a route is redistributed into BGP.
•  
• BGP Multi-Exit Discriminator Attribute

• AS_path Attribute
• 
  When a route advertisement passes through an autonomous system, the AS number is added to an
  ordered list of AS numbers that the route advertisement has traversed. Figure  shows the situation
  in which a route is passing through three autonomous systems.
• 
  AS1 originates the route to 172.16.1.0 and advertises this route to AS 2 and AS 3, with the AS_path
  attribute equal to {1}. AS 3 will advertise back to AS 1 with AS-path attribute {3,1}, and AS 2 will
  advertise back to AS 1 with AS-path attribute {2,1}. AS 1 will reject these routes when its own AS
  number is detected in the route advertisement. This is the mechanism that BGP uses to detect routing
  loops. AS 2 and AS 3 propagate the route to each other with their AS numbers added to the AS_path
  attribute. These routes will not be installed in the IP routing table because AS 2 and AS 3 are learning a
  route to 172.16.1.0 from AS 1 with a shorter AS_path list.

       BGP AS-path Attribute

• 
  Next-Hop Attribute
• 
  The EBGP next-hop attribute is the IP address that is used to reach the advertising router. For EBGP
  peers, the next-hop address is the IP address of the connection between the peers. For IBGP, the EBGP
  next-hop address is carried into the local AS, as illustrated in Figure

        BGP Next-Hop Attribute

•        
•        
• Router C advertises network 172.16.1.0 with a next hop of 10.1.1.1. When Router A propagates this
  route within its own AS, the EBGP next-hop information is preserved. If Router B does not have routing
  information regarding the next hop, the route will be discarded. Therefore, it is important to have an IGP
  running in the AS to propagate next-hop routing information.
•  
• Community Attribute
• 
  The community attribute provides a way of grouping destinations, called communities, to which routing
  decisions (such as acceptance, preference, and redistribution) can be applied. Route maps are used to set
  the community attribute. Predefined community attributes are listed here:
• 
  . no-export Do not advertise this route to EBGP peers.
  . no-advertise Do not advertise this route to any peer.
  . internet Advertise this route to the Internet community; all routers in the network belong to it.
• 
  Figure illustrates the no-export community. AS 1 advertises 172.16.1.0 to AS 2 with the community
  attribute no-export. AS 2 will propagate the route throughout AS 2 but will not send this route to AS 3
  or any other external AS.
• 
  BGP no-export Community Attribute

• 
  AS 1 advertises 172.16.1.0 to AS 2 with the community attribute no-advertise. Router B
  in AS 2 will not advertise this route to any other router.

•        
• BGP Path Selection
• 
  BGP could possibly receive multiple advertisements for the same route from multiple sources. BGP
  selects only one path as the best path. When the path is selected, BGP puts the selected path in the IP
  routing table and propagates the path to its neighbors. BGP uses the following criteria, in the order
  presented, to select a path for a destination:
• 
  . If the path specifies a next hop that is inaccessible, drop the update.
• 
  . Prefer the path with the largest weight.
• 
  . If the weights are the same, prefer the path with the largest local preference.
• 
  . If the local preferences are the same, prefer the path that was originated by BGP running on this
    router.
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  . If no route was originated, prefer the route that has the shortest AS_path.
• 
  . If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is
    lower than EGP, and EGP is lower than incomplete).
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  . If the origin codes are the same, prefer the path with the lowest MED attribute.
• 
  . If the paths have the same MED, prefer the external path over the internal path.
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  . If the paths are still the same, prefer the path through the closest IGP neighbor.
• 
  . Prefer the path with the lowest IP address, as specified by the BGP router ID.

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