Understanding the network.pdf - Back to Home

dynamic routing protocols are used to manage different parts of a large
internetwork. This condition is often found in large corporate networks where
different sites (and sometimes departments) are managed individually, but need to
interact with the larger corporate whole. Here, two-way route redistribution is a
tremendous win.
Internet or private global network connections are another context where distinct
dynamic routing protocols are in use to enforce the separate routing polices of the
two networks. With single-point connections, static routes can be used to send
packets destined to external networks to the Internet gateway. In situations where
multiple Internet and private network links exist (as an alternative to using BGP), it
is possible to use filtered redistribution to limit the inbound and outbound network
announcements between the different routing policies using distribution lists and
route-maps.
Static route redistribution is often used in dial-access and DDR networks where
dynamic protocols are not always effective. Instead of adding static routes on every
router on the network, a single router can redistribute a collection of static routes.
Static route redistribution can also be used to limit network announcements inside
internal networks. By using multiple dynamic routing processes to manage different
sections of the network, and then statically redistributing on a selective basis, you
will only announce the networks that need to be announced between the separate
routing processes.
The "worse" is the potential for routing loops and the difficulty surrounding the
translation of different routing protocol metrics and distances. Here are some
guidelines to help you avoid the "worse" side of using redistribution:
• Ask yourself, "What is the advantage to using redistribution (in a given
situation) over using a static route?" In most cases, a static route or default
route can be used to provide a network announcement. If you have only a
few networks to redistribute, use static routes. Ideally, you only want to be
running a single routing protocol. In situations where legacy equipment
might reduce your choices in terms of which protocols you can use, it might
be more advantageous to use a single protocol or static routing (to address
legacy hardware issues) than to add the additional processing and network
traffic associated with multiple protocol redistribution.
• If possible, avoid redistributing between classless and classful protocols if
you are using VLSM. Classful protocols do not include network address
masks in their messages. Instead, they apply the mask of the interface that
receives the route announcements. Consequently, only VLSM networks that
have compatible masks will be advertised, and the others will not be
redistributed.
• When using redistribution with multigateway networks (or HSRP) it is
essential that the gateway routers use metrics and distances that favor one