Let’s give some extra attention to those well-known and mandatory BGP path attributes. This is for good reason of course, they are critical to the operation of BGP.
The first of the three we will examine in great detail is the ORIGIN attribute. It is probably the least familiar to students when compared to AS_PATH and NEXT_HOP.
The ORIGIN attribute really does mean what it sounds like. It is expressing the origin of the BGP prefix – how did the prefix make its way into your BGP table on your Cisco router. But wait, why do we even care? I mean, I really don’t care how the prefix got there, the important thing is IT IS THERE.
Well, it turns out we should all be interested in the ORIGIN path attribute of our prefixes, and this is because it is a component of the BGP Best Path Selection aglgorithm, and the is is a decicion componet that comes fairly easrly in the decision process. So this origin code could really have a priority impact on path selection.
So what aare the possible values for the BGP path attribute of ORIGIN? There are only two you are going to see:
- IGP – this is the highest priority value you can have for the ORIGIN attribute. This means that the prefix was in the routing table of a router and was introduced to BGP using the network command. Why is it called the IGP value? Well, the idea is that you most likely had an Interior Gateway Protocol (IGP) place the prefix in your routing table.
- INCOMPLETE – this value indicates that the prefix was redistributed into BGP. This value is displayed as a ? when you examine the output of your BGP routing table with show ip bgp. Why did they choose the term Incomplete? Well, it is actually pretty accurate since we are not sure exactly what method was specifically used to bring the prefix in. For example, was it an IGP that was redistributed in, or was it a static route, or even a connected interface. Remember, this attribute setting loses out to the IGP option above.
There is a third legacy option for the ORIGIN attribute that you are never going to see in production (or even in labs) today. This is the EGP attribute value. EGP was the predecessor to BGP. It did not last long. BGP is now the exterior gateway protocol of the Internet and large enterprises – period.
Where does the ORIGIN attribute fall in that best path selection algorithm that we discussed? Here is a recap of the first few decision components…
Step 1. The heighest WEIGHT value.
Step 2. Highest LOCAL_PREF.
Step 3. Prefer locally oriningated paths.
Step 4: Shortest AS_PATH.
Step 5. The lowest ORIGIN type. IGP is lower than INCOMPLETE.
Step 6. Lowest MED.