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Operation of EGP

《TCP IP路由技术(第2卷)(第2版)英文版》本书是有关Cisco外部路由协议和高级IP路由主题的权威指南,是Cisco路由与交换领域实属罕见的经典著作。本书在上一版的基础上进行了全面更新,其可读性、广度和深度相较于上一版有了相当大的改进。本节为大家介绍Operation of EGP。

作者:【美】Jeff Doyle(杰夫 多伊尔)来源:人民邮电出版社|2017-11-05 18:11

开发者盛宴来袭!7月28日51CTO首届开发者大赛决赛带来技术创新分享

Operation of EGP

Version 1 of EGP was proposed in RFC 827. Version 2, slightly modified from version 1, was proposed in RFC 888 , and the formal specification of EGPv2 is given in RFC 904 .

EGP Topology Issues

EGP messages are exchanged between EGP neighbors, or peers.2 If the neighbors are in the same AS, they are interior neighbors. If they are in different autonomous systems,they are exterior neighbors. EGP has no function that automatically discovers its neighbors; the addresses of the neighbors are manually configured, and the messages they exchange are unicast to the configured addresses.

RFC 888 suggests that the Time-To-Live (TTL) of EGP messages be set to a low number because an EGP message should never travel farther than to a single neighbor. However, nothing in the EGP functionality requires EGP neighbors to share a common data link. For example, Figure 1-1 shows two EGP neighbors separated by a router that speaks only RIP. Because EGP messages are unicast to neighbors rather than broadcast or multicast, they can cross router boundaries. These concepts of interior and exterior peers and of unicast messages are essential to understand because they are also used by BGP.

EGP gateways are either core gateways or stub gateways . Both gateway types can accept information about networks in other autonomous systems, but a stub gateway is restricted to sending information about networks in its own AS. Only core gateways can send information they have learned about networks in autonomous systems other than their own.

To understand why EGP defines core and stub gateways, you need to understand the architectural limitations of EGP. As previously mentioned, EGP is not a routing protocol. Its updates list only reachable networks, without including enough information to determine shortest paths or to prevent routing loops. Therefore, the EGP topology must be built with no loops.Figure 1-2 shows an EGP topology. There is a single core AS to which all other autonomous systems (stub autonomous systems) must attach. This two-level tree topology is similar to the two-level topology requirements of OSPF, and its purpose is the same. Recall from Routing TCP/IP, Volume I that inter-area OSPF routing is essentially a distance vector and therefore vulnerable to routing loops. Requiring all traffic between nonbackbone OSPF areas to traverse the backbone area reduces the potential for routing loops by forcing a loop-free inter-area topology. Likewise, requiring all EGP reachability information between stub autonomous systems to traverse the core, AS reduces the potential for routing loops in the EGP topology.


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