by Tony Mattke on April 5, 2010
Policy based routing is the process of altering a packets path based on criteria other than the destination address, commonly referred to as ‘policy routing’. PBR (Policy Based Routing, not Pabst Blue Ribbon… ) can be used to affect the flow of traffic based on source address, the size of the packet, the protocol of the payload, or any other information available in a packet header or payload. In effect, it is a method to allow a predefined policy override routing protocol decisions. While some of this may seem similar, it should not be confused with source routing.
Within the Cisco world, policy routing is implemented using route maps, these route maps can apply different policies based on the match statements used. Generally policy routing is used to change the next-hop of the packet, but ToS markings or IP precedence may be applied as well. Building a route map for PBR is the same two step process used with any route map.
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by Tony Mattke on March 27, 2010
Recently a “colleague”, I use that term very loosely here, was reviewing my recommendations for changes on his network. Since they’re rather tight on public IP space, and require it for a few of their PTP links, I had suggested changing them to /31 subnets. His response was less than accepting of the changes as he did everything but call me a liar.
Nonetheless, RFC 3021 first introduced the idea of thirty-one bit subnets as a means to increase the utilization of IP space in point-to-point address space. Since then most manufacturers have made a move to support the standard. Cisco’s IOS has supported the feature since version 12.2(2)T.
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by Tony Mattke on September 14, 2009
One of the most common questions I get concerns path selection within the router. Everyone knows (or at least they should know) that a more specific prefix will be preferred, but until you learn the basics, you’ll never be able to debug why a router is using a sub optimal path. There are a couple layers to processes when building the routing table, first we have to consider the individual routing protocols. Within each protocol there are several considerations for route selection. First, lets discuss selection within an IGP.
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by Tony Mattke on May 9, 2009
Recently one of my clients asked me to help resolve an issue at an aggregation point on their network. They had several connections that converged onto a single unlicensed link on their network, not only was the link saturated, but it had lots of bi-directional traffic going across it. While I would’ve preferred to move them to a licensed setup, the associated costs were astronomical when compared to implementing a little ospf-fu.
The basic idea here is to create two ospf paths we can use, and on each side ‘de-prefer’ one of them. This allows us to one radio for sending data, and one for receiving data. (although OSPF traffic will still be sent and recieved across either link independently, it should not cause you any issues) This setup creates a simulated full-duplex link. Lets take a look at an example that will for work for Cisco or ImageStream (Quagga) routers…
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