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Get ready for multihoming!

Posted in Cisco by vcappuccio on 15/07/2014

Internet businesses, especially those which enable VOIP, e-commerce or cloud services, require IP redundancy. For them, network performance is crucial as it is directly connected to their quality of service. Any routing anomaly causing downtime or outages results in financial loss and might severely affect provider’s reputation. Deploying redundant IP connectivity is one of the most frequent solutions to minimize downtime, and this post will screen the most important steps in setting up redundancy for an IP network.

A redundant network is one connected to multiple internet providers. Such networks are commonly called multihomed. The Border Gateway Protocol (BGP) is used to connect to transit providers via eBGP sessions. The protocol is able to asses all the available routes, and find the shortest path to an end-user. Eventually, traffic is routed through the shortest available paths to achieve maximum performance.

Prepare your BGP Network:

BGP is quite similar to the Routing Information Protocol (RIP); however, instead of choosing the shortest path based on router hops, it relies on the shortest path among Autonomous Systems (AS). Autonomous System Numbers are associated with the BGP routing domains and are identified by an AS Number (ASN), provided by a Regional Internet Registry (RIR).

As you get to understand the BGP basics, configuring a multihomed network becomes simple. As soon as your network’s internet connections are up and running, you can follow these common steps to achieve BGP multihoming:

1. Get your own ASN. You can acquire one from your Regional Internet Registry, and identify your network on the internet, as a separate authority, running its own policies.
2. Purchase some IP address space from your RIR.
3. When using a static route to link with your provider, the network is single-homed (using one internet connection) and the internet provider is not sending any BGP routes to your network. In order to multihome, you must ask the internet provider to announce BGP routes towards your AS. Keep in mind, your ASN and the remote router’s neighbor address will be required by your internet provider. The static route can be removed as soon as you get the internet provider’s BGP routes in your routing table. As soon as you have all these in place, you can start advertising your network via BGP.
4. Once you are multihomed on a single route, add a link to an alternative internet provider, and ask it to advertise BGP routes towards your AS. The second internet provider will also require your ASN and the remote router’s neighbor address, so have them ready.

As soon as you have followed these steps, routes from each of your internet providers will appear within your edge router’s BGP table. According to BGP’s algorithm, routes having the shortest AS path towards a destination will be used to send the traffic through.

If one of your Internet providers goes down, the BGP session that enables connectivity with that provider will be reset and all of the advertised routes, originating from the offline provider shall be withdrawn from your routing table. Eventually, better alternative routes shall be selected from routes announced by the alternative internet provider.

Given to the BGP’s algorithm, all of your traffic might be sent out towards a particular provider, since it is the best one to route through. If the amount of traffic exceeds the internet provider’s link capacity, you might need to perform some tuning, to balance the traffic among your internet provider’s links. This task might be quite hard to accomplish, since BGP alone does not imply load balancing. As an alternative, you could use specific hardware or some route optimization​ solutions such as Noction’s Intelligent Routing Platform (IRP), to optimize BGP decision-making

BGP Usage and considerations:

When using BGP, there are several things to keep in mind:
e- Since BGP advertises network fluctuations to routers outside your AS, you must maintain your network to be as stable as possible.
– Advertise only a specific set of prefixes you own. Other networks might suffer service loss if you are advertising prefixes other than yours.
– Plan your architecture before engaging in BGP routing. Your network needs to be configured according to several BGP aspects in meeting multihoming requirements.
– Choose your edge routers. The Internet’s BGP tables involve huge amounts of data, especially with multihoming in place. Therefore, your edge routers must have enough memory to store and process all those routing tables.

While BGP alone can empower your network to deliver fair performance, it is still not enough when delivering performance sensitive applications, such as VOIP or e-commerce. Under some circumstances, the shortest path BGP selects, could be congested or affected by other network anomalies. However, traffic gets re-routed from from the shortest path only when it is the destination is completely unreachable. As a result, an end user might experience service delivery issues, since traffic is routed through a reachable, yet underperforming internet path.

To avoid such scenarios, BGP tuning must be performed at a network’s edge, which involves manipulating various BGP attributes to spot the issues and re-route specific prefixes, from those underperforming paths to alternative routes with better performance metrics. Best practices, recommend deployment of intelligent routing systems like Noction IRP, which can address most of your BGP challenges in a multihomed environment.

As soon as you have a redundant BGP network which is empowered by automation, you are ready to meet your customer’s demand for 100% uptime and outstanding network performance.

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