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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.
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: BGP Weight Attribute,
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.
Origin Attribute
The origin attribute indicates how BGP learned about a particular route. The origin attribute can have one of three possible values:- IGP - The 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.
- EGP - The route is learned via the Exterior Border Gateway Protocol (EBGP).
- Incomplete - The origin of the route is unknown or learned in some other way. An origin of incomplete occurs when a route is redistributed into BGP.
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.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.
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