Tag: bgp

Cisco: How to improve BGP table stability with route dampening

[UPDATE Feb 13, 2013]
After gaining some more years of experience, playing more with BGP also from SP perspective, I would recommend to be careful with this feature. It can help in your enterprise environment, where you have access to BGP routers and can clean the dampened prefixes. If you have your Service Provider involved in routing your prefixes, I would prefer that the SP does not enable this feature. Imagine that because of some flaps your provider dampen all your prefixes. Or negotiate your SLA in such way that the provider can support if the BGP dampening feature is active and you need support.

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One of the issues that can affect BGP table stability is link flapping. Imagine that if a link to a network is flapping very often, BGP process has to remove the route to that network from the BGP table and implicit from the routing table and then we the link is available again to re-introduce the prefix in these tables. All this means some BGP operations that consume CPU and memory of the machine.

A way to improve the BGP table stability is to use route dampening. This BGP feature monitor the prefixes in the BGP table and when a route to some prefixes flaps more than BGP dampening is set to allow, it will take out the prefixes from the BGP table. In the following tutorial I will show you a way to configure BGP dampening with some explanations.

For this tutorial we will use the same topology like in the post “Cisco: BGP path selection for outgoing traffic” where we have already a working BGP environment. I took out the configuration for BGP path selection, so we have a simple BGP config running. If you do not have the topology, you can download it here and the initial configuration files here.

Please see the tutorial below:

Cisco: BGP path selection for outgoing traffic

Before going further on, please download the topology and the initial configuration files and have a close look into them (mostly into topology drawing). Also, this tutorial is addressed to people that know what is BGP and how to do a basic configuration for peering establishing.

For modifying the BGP automate selection of the best path, we have to modify some of BGP attributes from the table below:
Since today we will deal with outgoing traffic, we are interested in Weight and Local preference attributes, which we have to modify to reach our desired result. For the incoming traffic I will make another tutorial in some days.

So, let assume that we have some networks in the LAN after R1 in the topology and we advertise this into the BGP network. On R3, we will learn this prefixes by the way of R1 as it is the shortest path. But in our case the shortest path does not mean the fastest one. As you can see the connection between R1 and R3 is a Serial connection (let’s assume E1). The connections between R1 – R2 and R2 – R3 are FastEthernet connections. Even if this a longer path, for us would be better to reach the LAN subnets in behind R1 through R2, as this is more faster (bigger bandwidth and lower latency). We will force the outgoing traffic through R2 by modifying the Weight and Local preference attribute.

Please see the tutorial below: