• Interim during conversion
• Application-specific protocols
- One size does not always fit all.
• Political boundaries
- Groups that do not work well with others
• Mismatch between devices
- Multivendor interoperability
- Host-based routers
Multiple routing protocols may be necessary in the following situations:
■ When you are migrating from an older interior gateway protocol (IGP) to a new IGP, multiple routing protocols are necessary. Multiple redistribution boundaries may exist until the new protocol has completely displaced the old protocol.
■ When use of another protocol is desired, but the old routing protocol is needed for host systems, multiple routing protocols are necessary, for example, UNIX host-based routers running RIP.
■ Some departments might not want to upgrade their routers to support a new routing protocol.
■ In a mixed-router vendor environment, you can use a routing protocol specific to Cisco such as EIGRP in the Cisco portion of the network and a common standards-based routing protocol, like OSPF, to communicate with devices from other vendors.
When multiple routing protocols are running in different parts of the network, there may be a need for hosts in one part of the network to reach hosts in the other part. One solution is to advertise a default route into each routing protocol, but that is not always the best policy. The network design may not allow default routes.
If there is more than one way to get to a destination network, routers may need information about routes in the other parts of the network to determine the best path to that destination. Additionally, if there are multiple paths, a router must have sufficient information to determine a loop-free path to the remote networks.
5-6 Building Scalable Cisco Internetworks (BSCI) v3.0 © 2006 Cisco Systems, Inc.
Cisco routers allow internetworks using different routing protocols, referred to as routing domains or autonomous systems, to exchange routing information through a feature called route redistribution.
Redistribution is how routers connect different routing domains so that they can exchange and advertise routing information between the different autonomous systems.
Note The term autonomous system (AS), as used here, denotes internetworks using different routing protocols. These routing protocols may be IGPs or exterior gateway protocols (EGPs), which is a different use of the term "AS" than when in Border Gateway Protocol (BGP).
© 2006 Cisco Systems, Inc. Manipulating Routing Updates 5-7
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