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BGP Basics in cisco

Like EGP, BGP forms a unique, unicast-based connection to each of its BGP-speaking peers. To
increase the reliability of the peer connection, BGP uses TCP (port 179) as its underlying delivery
mechanism. The update mechanisms of BGP are also somewhat simplified by allowing the TCP layer
to handle such duties as acknowledgment, retransmission, and sequencing. Because BGP rides on
TCP, a separate point-to-point connection to each peer must be established.
BGP is a distance vector protocol in that each BGP node relies on downstream neighbors to pass
along routes from their routing table; the node makes its route calculations based on those
advertised routes and passes the results to upstream neighbors. However, other distance vector
protocols quantify the distance with a single number, representing hop count or, in the case of IGRP
and EIGRP, a sum of total interface delays and lowest bandwidth. In contrast, BGP uses a list of AS
numbers through which a packet must pass to reach the destination . Because this
list fully describes the path a packet must take, BGP is called a path vector routing protocol to
contrast it with traditional distance vector protocols. The list of AS numbers associated with a BGP
route is called the AS_PATH and is one of several path attributes associated with each route. Path
attributes are described fully in a subsequent section.
Figure 2-18. BGP Determines the Shortest Loop-Free Inter-AS Path from a
List of AS Numbers Known as the AS_PATH Attribute
Recall from Chapter 1 that EGP is not a true routing protocol because it does not have a fully
developed algorithm for calculating the shortest path and it cannot detect route loops. In contrast,
the AS_PATH attribute qualifies BGP as a routing protocol on both counts. First, the shortest inter-AS
path is very simply determined by the least number of AS numbers., AS7 is receiving

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SHREEJI MAHARAJ

MONU BIRTHDAY

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Who Needs BGP? IN CISCO NETWORKING

Not as many internetworks need BGP as you might think. A common misconception is that whenever an internetwork must be
broken into multiple routing domains, BGP should be run between the domains. BGP is certainly an option, but why complicate
matters by unnecessarily adding another routing protocol to the mix?
Take, for example, a multinational corporate network consisting of 3000 routers and perhaps 150,000 users. Figure 2-9 shows
how such a huge internetwork might be constructed. The entire network is routed with OSPF and is divided into eight
geographic OSPF routing domains for easier manageability. Although the illustration shows only the backbone areas for each
OSPF domain, each of the domains is divided into multiple OSPF areas that also correspond to geographic subreg Even a Very Large Internetwork Can Be Built Using Only Multiple IGP Domains
BGP can be used to provide connectivity between the multiple OSPF domains, but it is unnecessary. Instead, each of the eight
OSPF backbone areas redistributes into a single global backbone. The global backbone is another OSPF domain, consisting of a
single OSPF area. Although this core consists of high-end routers to handle the packet-switching load, the load on these routers
from routing tables and OSPF processing is actually very small. Because of the way the entire internetwork is addressed, each
of the eight OSPF domains advertises only a single aggregate route to the global backbone. In fact, aggregation is fundamental
to making this design work. There are, presumably, such a large number of subnets in such an internetwork that without
aggregation OSPF would "choke" trying to process them all. The result would be very poor performance and possible router
failures.