Cisco: Engineer’s trick to avoid suboptimal path

I was explaining in the previous post what is the difference between optimal and suboptimal path and how to avoid the use of not such a good path in your routed environment. Also there I presented this so call “dirty trick” you can use to force the routing protocol to choose the path that you want, based on the Administrative distance modification.

As I said there is another way (for sure more than one) to do it, using a more elegant approach and from the networking point of view more safe considering the complex routing environment. I will use the same topology like in the previous post to offer you the possibility to compare these 2 methods presented and to choose the one that you understand and fit better to your needs. Also there are some other ways to do it and please feel free to discussed them in the comments section and maybe to present them here in a future post.

We will achive the desired results by setting one community on R1 for the advertised network and dropping the prefixes, marked with the same community, on R2. Please be aware that for this method to work you have to allowed BGP peers to send communities list with the command “neighbor xx.xx.xx.xx send-community …” under “router bgp xxx” process.

Please see the example by clicking the image below:

Optimal path engineer trick

Cisco: Dirty trick to force optimal path in routed environment

Everywhere in the world people try to find the optimal path to achieve something.If we speak about roads, trips and in our case networking, choosing the best path to an end point can have only advantages.

I took the term optimal / suboptimal path from the routing issues that can appear in the OSPF network environment and which are called by the experts suboptimal routing. What I want to explain here, maybe you already seen it, is that in some network environment the best path to a destination is not always preferred by the routing protocol due to some unhappy situations. To understand better what I’m talking here, please have a look at the topology that I will use for this tutorial and for the next one regarding optimal path.

In the example below I will show you a simple and dirty trick how you can escape from this situation. I recommend to use this dirty trick only in urgent cases and only temporary as this can lead to more problems if you have a complex network environment. In the next days I will show you a more elegant method to escape from suboptimal path problem.

As you can see in the topology we have a network environment formed by 4 routers. On R1 we have configured a BGP session with the peer R3 and OSPF with R2. Since the peers are in different autonomous systems the BGP session will be external. For OSPF this does not matter.The Loopback100 interface in R1 is advertised into BGP and OSPF, and it is learned by R2. On R2 the interface arrives on BGP table and OSPF table, but since the Administrative Distance of the eBGP (20) is better that the one of OSPF (110) on the routing table will appear the route through R4. This is bad because without considering the obvious longer path through R4, we can see that the links between R2-R4 and R1-R3 are Serial interfaces and definitely with more limited bandwidth than FastEhernet interface R1-R2 (we assume that we do no have any QoS or other limitation). Last note before we begin, all the routing processes on the devices are completely configured and functional.

Optimal path dirty trick