Frequently Asked Questions (FAQ)

1:

If a router is running multiple routing protocols, how does it know which one to use?

A1:

The administrative distance is used to determine the preferred route. The following table lists the administrative distance values for each routing protocol. These are automatically calculated values and can be manually changed in the router configurations.

Table 11-1. Routing Protocol Administrative Distance Values

Protocols	Distance Value
Connected Interface	0
Static routes	1
EIGRP Summary routes	5
EBGP	20
Internal EIGRP	90
IGRP	100
OSPF	110
IS-IS	115
RIP	120
EGP	140
External EIGRP	170
Internal BGP	200
Unknown	255

2:

What is route redistribution?

A2:

Redistribution is the process by which routes learned through one routing protocol are distributed into another routing protocol.

Figure 11-3 illustrates two networks with the following routing protocols:

• A,B,C EIGRP

• A,E,D OSPF

Figure 11-3. Routing Protocol Redistribution

Router A is configured to redistribute EIGRP routes into the OSPF cloud so that Routers D and E have visibility into the EIGRP cloud and can connect with Routers B and C. Router A also is configured to redistribute OSPF routes into the EIGRP cloud so that Routers B and C can connect with Routers D and E.

3:

Can a router run multiple routing protocols at the same time?

A3:

Yes. A router operating as a network-to-network interface (NNI), connecting two (or more) networks, can run a separate routing protocol for each network.

Another scenario in which a router can run multiple routing protocols simultaneously would be in support of a migration from one routing protocol to another. For example, suppose that a network initially implemented a distance vector protocol (such as RIP), and as the network has grown, the decision was made that a link-state protocol (such as OSPF) would better suit the internetwork environment. In order to minimize user disruption during this routing protocol migration, the network administrator can enable both routing protocols on each router in the internetwork until the migration is complete. When the migration is complete, the "old" protocol (no longer being used) can be removed from the router, freeing up router resources for the "new" protocol.