IS-IS NBMA Solutions in Switched WAN Networks


Integrated IS-IS recognizes three different types of WAN connections: point-to-point leased lines, dialup links, and switched WANs.

Point-to-Point WANs

This first category of WAN connection is usually a leased line solution between two ISs that exist on both sides of a HDLC or PPP circuit. In other words, IS-IS considers this an NBMA network with just two attached devices. A solitary IIH PDU is used to designate a Level 1, Level 2, or L1L2 adjacency on a point-to-point link. After adjacency is defined, the peers transmit CSNP (complete) packets detailing the contents of their link-state databases. PSNPs are then used to request missing LSPs and for acknowledgement as displayed earlier in Figure 6.4. This partial information exchange is another example of the superiority of link-state behavior versus distance-vector full table exchange.

Dialup Links

Dial-on-demand (DDR) solutions can be set up as either point-to-point or point-to-multipoint WANs. It is highly recommended by Cisco that you avoid an IS-IS over dialup scenario with the exception of backup purposes. Legacy DDR, dialer profiles, and dialer virtual profiles are not covered by the BSCI exam and are a remote access solution.

Switched WANs

If your network has been configured in a full-mesh topology, you can consider IS-IS over a multipoint network solution. The inherent possibilities of a full-mesh becoming a partial mesh because of link failures or poor configuration make this scenario problematic and one to be avoided. Cisco highly recommends that you opt for point-to-point subinterface solutions and steer clear of NBMA multipoint altogether when running IS-IS. The subinterface is normally on its own subnet with a 30-bit (/30) subnet mask, as shown in previous chapters for WAN links.

If you decide to deploy an Integrated IS-IS NBMA WAN solution, you will first invoke the IS-IS process and assign NETs. This is covered in detail in the next section, titled "Configuring Cisco Routers for Appropriate Integrated IS-IS Operation." Next , you must design a full (or partial) mesh among the neighbors and configure point-to-point interfaces for every NBMA virtual circuit, including IP addressing. You then need to map the Level 3 addresses/protocols to the virtual circuit, for instance using the frame-relay map commands. Finally, you can start the IS-IS process on each subinterface (not on the main interface!) with the ip router isis command explained in the following section.

Cisco IOS release 12.0 and higher enables you to use the ip unnumbered interface configuration command on point-to-point interfaces because Integrated IS-IS utilizes CLNS for route propagation.




Cisco BSCI Exam Cram 2 (Exam Cram 642-801)
CCNP BSCI Exam Cram 2 (Exam Cram 642-801)
ISBN: 0789730170
EAN: 2147483647
Year: 2003
Pages: 170

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