Tag: bgp

BGP dampening – punishment for unstable BGP prefixes

BGP prefix flapping can be caused by different issues in network. Basically every unstable network where links are unreliably and are going up and down here and there can cause BGP prefix flapping. Every prefix flap will cause some networks to became unreachable. BGP process will then need to recalculate best-path in order to hopefully find other way to get towards unreachable network.

Impact on the network can be enormous as one network prefix missing can mean that huge number of other networks will change the path on which they are reachable. In the situation of prefix flap we have the prefix going up and down all the time. After every status change all those efforts of finding new best-path are done, and when the prefix comes back, everything is calculated again and becomes like before. We have a way to cut the impact of flapping prefixes.

By implementing BGP prefix dampening. When configured, dampening will punish those prefixes that are changing state from reachable to unreachable few time in determined time period. After every flap BGP will give to that prefix a defined penalty of 1000 by default. The penalty points will immediately start to be reduced exponentially but if the prefix flaps more times in little time period he will surely collect enough point to reach Suppress Limit and BGP will at that point mark the prefix as damped. It means that it will immediately suppress the advertisement of the prefix until the penalty points do not fall below Reuse Limit which is 1000 by default. Suppress limit is 2000 by default and yes, there will be needed for prefix to flap three times in order to be suppressed by default (you need to count that the first penalty will be 1000, next one also 1000, but the first one will fall at least by 1 to 999 before second flap occurs.)

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BGP communities

O men, when you start to write about BGP it is probably the time then you seriously start questioning yourself where did I go with myself. That is probably the moment in which you realize that there is a network geek sitting somewhere inside you. At least that is what happened to me when I finished to write this huge post. Don’t be scared, it’s fun to know about this thing below.

Simple start

Every local network is managed by his own network administrator. If the network become big enough and there are more than few sub-segments inside that network there will probably be some kind of routing protocol running inside. That routing protocol will be IGP or interior gateway protocol more probably OSPF as it’s vendor independent.

When we want to connect our network to other networks across the world, we are trying to connect it to the internet. The Internet is the network connecting most of the networks today and in that way it became the biggest inter-networking system in the world. To be able to get that huge network to function and get our LANs to act jointly there must be a routing protocol that enables it.

BGP – Border Gateway Protocol is that one.

Every individual network has his own policies that are enabling that network to behave as the administrator want. When connection networks to the internet network all those policies need to be tied together with BGP protocol in order to influence outside communication entering the local network and communications initiated from the local network going outside somewhere on the internet. This is done using more that few different BGP attributes. Those attributes are forwarded across specific prefixes. Sometimes those attributes are not only forwarded but also modified on the way, one of which is the community attribute.

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BGP – Border Gateway Protocol

BGP is the Internet routing protocol. He is making the Internet work.

BGP protocol performs actions by maintaining IP networks table for the biggest network in the world – The Internet. The BGP protocol, as a code of behavior, supported core routing decisions over the Internet.  Instead of using traditional IGP (Interior Gateway Protocol) metrics, BGP protocol relies upon available path, network guidelines and rule-sets for routing decision making purposes. And just for this feature, it is sometimes expressed as a reachability protocol. The main idea behind the BGP creation was to replace the EGP (Exterior Gateway Protocol) to permit a complete decentralized routing. NSFNET backbone is the best example of a decentralized system.

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