Interaction of Non-BGP Routers with BGP Routers

Non-BGP routers inside the AS can reach the outside world by using the following two methods:

  • Injecting BGP into the IGP

  • Following default routes inside the AS

Injecting BGP into the IGP

Injecting full BGP routes into an IGP is not recommended; doing so will add excessive routing overhead to any IGP. Interior routing protocols were never meant to handle more than the networks inside your AS, perhaps in addition to a small number of exterior routes from other IGPs.

This does not mean that BGP routes should never be injected into IGPs. Depending on the number of BGP routes and how critical it is for them to be in the IGP, injecting partial BGP routes into IGP might be appropriate. If you follow this course, you should exercise caution to control routes leaked into the IGP. Discussing all the potential issues associated with BGP redistribution into IGPs is beyond the scope of this book. However, here are some things worthy of consideration: the amount of available memory, CPU resources available for calculating paths and processing routing updates, link utilization from routing control traffic, impact on convergence, IGP limitations, and network topology. All of these factors, and numerous others, should be considered.

Injecting partial BGP routes into the IGP from specific points of the AS can help direct the corresponding outbound traffic toward specific exit points. Outbound traffic toward other Internet routes will still have to follow defaults toward the BGP routers. Although injecting BGP routes into the IGP seems like the optimal routing solution, it has its drawbacks. For instance, if the IGP is classful (such as RIP-1 or Interior Gateway Routing Protocol [IGRP]), information about classless interdomain routing (CIDR) blocks will be lost. The other major problem is the potential for instability in the injected BGP routes, causing further instability of the IGP. Some major network meltdowns have been caused by IGPs failing due to fluctuations of a large number of external routes.

Following Defaults Inside an AS

The more practical solution for non-BGP routers inside the AS to reach the outside world is to follow defaults inside your AS to the closest exterior gateway router that can get you outside the AS. A default route can be injected into the AS from each autonomous system border router. Each IGP router might receive the default route from one or multiple routers. Each IGP router chooses the best path to an exterior destination based on the internal cost or metric to reach the default. After the traffic reaches the BGP routers, the traffic propagates according to how BGP has determined the best path. Figure 8-1 illustrates non-BGP routers inside an AS following defaults to reach the closest BGP router.

Figure 8-1. Example of Following Defaults


RTC and RTD are BGP border routers that are injecting the default 0/0 inside AS1. RTB is an internal BGP transit router running a full IBGP mesh with RTC and RTD. Internal non-BGP routers, such as RTA, receive the default from different IGP sources; they prefer the default with the smallest IGP metric. In Figure 8-1, RTA is receiving the 0/0 from RTB with a metric of 10, from RTE with a metric of 20 (RTA-to-RTE: 10 + RTE-to-RTB: 10), and from RTF with a metric of 30 (RTA-to-RTF: 10 + RTF-to-RTG: 10 + RTG-to-RTB or RTC: 10). RTA prefers the default via its link to RTB because it has the lowest internal metric (10). After traffic arrives at RTB, the BGP routing table of RTB is used to reach external destinations of the AS.

Tip

See Examples 12-38 through 12-41, “Following Defaults Inside an AS“, beginning on page 397.


Running IBGP inside the AS is an important element both to help direct traffic that must exit the AS and to carry transit traffic in previously mentioned cases such as a partner AS providing backup transit during failures. Also, most of the symmetry techniques discussed in the preceding chapter cannot be implemented if multiple BGP routers are not running IBGP.

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