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BGP 125
[Switch C-Vlan-interface101]isis enable
[Switch C]interface vlan-interface 100
[Switch C-Vlan-interface100]isis enable
4 Configure
Switch D
[Switch D]isis
[Switch D-isis]network-entity 86.0001.0000.0000.0008.00
[Switch D]interface vlan-interface 102
[Switch D-Vlan-interface102]isis enable
[Switch D]interface vlan-interface 100
[Switch D-Vlan-interface100]isis enable
BGP Border gateway protocol (BGP) is an inter-autonomous system (inter-AS) dynamic
route discovery protocol.
Three early versions of BGP are BGP-1 (RFC1105), BGP-2 (RFC1163) and BGP-3
(RFC1267). The current version is BGP-4 (RFC1771) that is applied to distributed
structures and supports classless inter-domain routing (CIDR). BGP-4 is becoming
the external routing protocol standard of the Internet, which is frequently used
between ISPs.
The characteristics of BGP are as follows:
■ BGP is an external gateway protocol (EGP) that focuses on route propagation
control and selection of best routes, rather than the discovery and calculation
of routes.
■ It eliminates routing loops by adding AS path information to BGP routes.
■ It enhances its own reliability by using TCP as the transport layer protocol.
■ When routes are updated, BGP only transmits updated routes, which greatly
reduces bandwidth occupation by route propagation and can be applied to
propagation of a great amount of routing information on the Internet.
■ BGP-4 supports CIDR, which is an important improvement over BGP-3.
■ In consideration of management and security, users can perform control over
outgoing and incoming routing information of each AS. BGP-4 provides
abundant route policies to implement flexible filtering and selecting of routes.
■ BGP-4 can be scaled easily to support new developments of the network.
CIDR does not distinguish networks of Class A, Class B and Class C. For example,
an invalid Class C network address 192.213.0.0 (255.255.0.0) can be expressed as
192.213.0.0/16 in CIDR mode, which is a valid super network. Here /16 means
that the subnet mask is composed of the first 16 bits from the left.
The introduction of CIDR simplifies route aggregation, which is the process of
aggregating several different routes, and converts the advertisement processes of
several routes to the advertisement of single route to simplify the routing table.