Distribute-list and Redistribute in Routing

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In the realm of networking and routing, understanding the nuances of commands like distribute-list and redistribute is very important. These two control how routes are filtered and shared between different routing protocols or routing tables. Here's a breakdown of both:

1. Distribute-List

A distribute-list plays a crucial role in controlling the routes that are permitted to be advertised or received within a routing protocol. Acting as a filter for route updates, it utilizes access control lists (ACLs), prefix-lists, or route maps to achieve this function.

The primary purpose of a distribute-list is to filter the routes being advertised or received by the router. This method can be applied to both incoming (inbound) and outgoing (outbound) routes for specific routing protocols like EIGRP, OSPF, BGP, or RIP.

Understanding the significance and application of distribute-lists is essential for maintaining a secure and efficient routing environment within network configurations.

Example:

• Inbound Filtering: With inbound filtering, you can prevent specific routes from entering the routing table,
• Outbound Filtering: Outbound filtering allows you to restrict which routes are shared with other routers.

router eigrp 80
distribute-list  1 out Serial0/0/0
        

The above command applies an ACL (number 1) to filter outbound routes advertised by EIGRP on interface Serial0/0/0.

2. Redistribute

The redistribute command is used when you want to share routes between different routing protocols or between different routing processes within the same protocol. This function allows routes learned by one protocol to be seamlessly injected into another, promoting efficient communication and connectivity within networks.

• Purpose: Redistribute routes from one protocol to another.
• Example Scenarios: Redistributing between static routes and dynamic routing protocols, or between two dynamic routing protocols (e.g., EIGRP into OSPF, BGP into OSPF, etc.).

Example:

• Redistribute static and connected routes into EIGRP:

router eigrp 90
redistribute static
redistribute connected        

• Redistribute OSPF routes into EIGRP:

router eigrp 100 
redistribute ospf 1 metric 10000 100 255 1 1500        

Here, OSPF routes are being redistributed into EIGRP, and the metric parameters define how the routes will be advertised.

Key Differences:

• Distribute-List: Focuses on filtering specific routes without transferring them between protocols, controlling which routes are shared.
• Redistribute: Redistribute transfers routes from one routing domain or protocol to another, enabling them to be included in the routing tables of other protocols.

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