Information Technology Reference
In-Depth Information
BGP next hop. Physical topology presents less of an issue in a traditional iBGP environ-
ment than in an RR environment, because all the peers are fully meshed and have all the
routing information in the domain.
In an RR environment, BGP speakers have only a partial view of the network topology—
specifically, the exit paths to the neighboring autonomous systems. Therefore, designing an
architecture with congruent logical and physical topologies becomes much more important.
The following specific examples demonstrate why:
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Following physical topology
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Session between an RR and a nonclient should not traverse a client
•
Session between an RR and its client should not traverse a nonclient
Following the Physical Topology
In an RR environment, it is important to keep physical and logical topologies congruent.
When the two topologies are not congruent, several solutions can be considered:
•
Changing the physical topology
•
Modifying the logical topology
•
Following the physical topology
The first solution to consider is changing the physical topology to fit the logical topology
and thereby provide an optimal design. However, this solution might not always be
acceptable because of circuit cost and geographic limitations.
Another solution is to modify the logical topology to follow the physical topology, perhaps
resulting in a design that is less optimal but more cost-efficient. Because network design is
often a compromise between various constraints and design goals, a set of possible solutions
can be proposed. Which solution is preferable might depend on specific requirements for
each problem.
Figure 7-12 shows an example of the need to maintain congruence between the two topol-
ogies. Both R1 and R2 are RRs, but their iBGP session goes through a client, R4. If there
is any problem in R4 or links from R1 to R2, two RRs and their clients are isolated. One
solution (shown in the center topology of Figure 7-12) is to have a direct physical link
between R1 and R2.
Another solution is to follow the physical topology, as shown in the right topology in Figure 7-12.
In this solution, R4 is an RR, and both R2 and R5 are clients of R4. Note that the session
between R4 and R5 does not have a physical link, but because both R2 and R5 are clients
and there is no physical redundancy, no additional risk is introduced.
