Section 7.5. BGP and Area Design


7.5. BGP and Area Design

You probably have heard that IS-ISat least when used in support of IPis seldom found outside carrier and Internet service provider networks. The IGP is the foundation of all but a few very large enterprise networks, and so the many features OSPF offers for multi-area topologies are attractive: The network is likely to have many low-powered, low-memory routers and low-bandwidth links, calling for area designs that can protect these routers and links from being overloaded.

However, the mission of carrier and ISP networks is not routing between points internal to the network but transit routing: receiving packets from outside the network and forwarding them as efficiently as possible to destinations also outside of the network. The fundamental protocol for these networks is BGP, which is designed to manage very large numbers of prefixes and complex routing policies.

The role of the IGP in such networks is much simpler than it is in enterprise networks. The IGP must provide the means for internal BGP to find the endpoints for the TCP sessions it operates over and the next-hop addresses of the BGP routes being advertised through the network (almost always, in both cases, the loopback interfaces of the internal routers). The IGP design should therefore be kept as simple as possible in keeping with the role it plays.

One way to keep the IGP design simple is to put the entire network in a single area. Carrier and ISP networks are, because of their mission, built with high-end routers and highspeed links.[13] The designers of carrier and ISP networks often choose IS-IS over OSPF both because of its relative simplicity and because of the characteristics of the protocol that make it scalable and stable within large areas. So although this chapter examines the details of multi-area IS-IS, the reality is that IS-IS-based IP networks are usually single area.

[13] Core links in carrier networks are seldom below DS-3 or OC-3, and range up to OC-192. As of this writing, the high end of core links is moving toward OC-768.

When designing single-area networks, whether with OSPF or IS-IS, one key piece of advice is offered: Build the single area with L2 adjacencies or as area 0. Then if you ever need to attach another area, adding it as an L1 area or a nonbackbone area is straightforward.




OSPF and IS-IS(c) Choosing an IGP for Large-Scale Networks
OSPF and IS-IS: Choosing an IGP for Large-Scale Networks: Choosing an IGP for Large-Scale Networks
ISBN: 0321168798
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Jeff Doyle

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