The leaf router uses default routing to find the backbone and all other valid routes. The leaf router will transmit IGRP information about its routes to the backbone. The backbone router does not transmit any IGRP information to the leaf. The following examples illustrate configurations for leaf and backbone routers:
! Leaf router configuration
router igrp 109
network 131.108.0.0
ip route 0.0.0.0 Serial0
ip route 131.108.0.0 Serial0
! Backbone router configuration
router igrp 109
network 131.108.0.0
passive-interface Serial0
Figure 4-30 illustrates what happens when the preceding leaf and backbone router configurations are used. This configuration does not send routing information to the lower-speed leaf routers, while the backbone retains all valid routes in the network.
Designing SRB Internetworks 4-37
SRB Network Design
Figure 4-30
Effects of using the passive-interface router configuration command.
Before
Rest of topology
Token
Ring
Leaf
Backbone
Local network info
After
Default route
Token
Ring
Leaf
Backbone
Local network info
Note When designing large branch networks based on OSPF routing, OSPF has a natural limit.
When link instability raises broadcast traffic and route recomputation to an unacceptable level, the network is at its limit. Always contact your router technical support representative when designing large OSPF-based internetworks.
SRB Network Design
The key to building predictable and scalable SRB networks is to follow the network design
guidelines in this chapter. Ultimately, there is a limit to the maximum diameter of a single meshed virtual ring, so before you begin designing a network, consider four critical questions. Answering these questions helps you assess the available options.
• How many routers are required? This question assesses the capability to build a simple SRB
network. If you are implementing a large internetwork, contact your technical support
representative for specific information about virtual ring limitations.
• Are there any T1/T3, E1/E3, fractional T1/T3, or fractional E1/E3 links? This question assesses SRB WAN traffic that may reduce a meshed topology to a smaller radius. If you are using T1/T3
or E1/E3 technology, you can take advantage of their increased bandwidth capabilities by
increasing traffic loads to and from the rings, which allows you to reduce the number of routers.
• Is the design for an SNA network, a NetBIOS network, or both? This question helps you
determine whether a partially meshed topology can be used when an FEP-connected ring is a peer
of each Token Ring in the network. The remote token rings are allowed to be peers only of the
FEP rings, not of one another. This topology is called a partially meshed network because certain points can connect only to certain points. Partially meshed SRB networks are much more scalable
than fully meshed networks, in which all rings can reach all rings. Fully meshed topologies are
often required in NetBIOS environments.
4-38
Cisco CCIE Fundamentals: Network Design
SRB Network Design
• Is the network a multiprotocol environment? This question implicitly raises the topic of prioritization. When dealing with a multiprotocol internetwork, you must consider your options for prioritizing traffic to ensure acceptable response time for interactive traffic, while maintaining adequate internetworking resources to handle other types of traffic, such as file transfers.
In general, it is best to design a router network in a hierarchical fashion; there are typically three logical service layers: the backbone (or core) service layer, the distribution service layer, and the access service layer. Figure 4-31 illustrates these basic service layers.
When designing a router network for SRB, consideration should be given to the design of virtual
rings. Two key issues affect the design of virtual rings:
• The type of SRB connectivity required (hierarchical, distributed, or flat)
• The corporate organizational structure
Figure 4-31
Backbone, distribution, and access service layers in an SRB environment.
WAN backbone
Backbone services
Router
Router
Virtual ring 0
Token
Token
Ring
Ring
Central site
Token
Distribution services
Router
Router
Regional hubs
Router
Ring
3745
Virtual ring 1
Virtual ring 2
Campus backbone
Branch offices
Router
Router
Router
Router
Access
Token
Token
services
Ring
Ring
The remainder of this section focuses on network design approaches that help create scalable
networks. The following topics are discussed:
• Hierarchical Design for SNA Environments
• Hierarchical Design for NetBIOS Environments
• Queuing and Prioritization Schemes
Designing SRB Internetworks 4-39
SRB Network Design
Hierarchical Design for SNA Environments
In SNA-only networks, all processing is hierarchical, where a single FEP or a few FEPs (one primary and one secondary) are the target of all remote rings. The SRB topology is focused from all remote rings to a single or a few redundant rings. A topology featuring a single FEP is illustrated in Figure 4-32.
Figure 4-32
Hierarchical topology featuring a single FEP.
Token
Ring
FEP
Token
Token
Token
Token
Ring
Ring
Ring
Ring
A topology featuring duplicate FEPs on duplicate rings is illustrated in Figure 4-33.
Figure 4-33
Duplicate FEPs on duplicate rings.
Token
Token
Ring
Ring
FEP
FEP
Token
Token
Token
Token
Ring
Ring
Ring
Ring