Now a lot of people would argue that this next item,
this next bullet point, this next selection
criteria that we're about to cover, autonomous system path,
is the most important selection criteria in the BGP algorithm.
The reason why is that if we don't change anything
like the weight, the local preference, or AIGP,
this is the criteria that will most of the time
be used to select the next stop.
Well, our lab environment is relatively simple.
Think about the size of the internet for just a moment.
How many advertisements for one prefix WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
How many advertisements for one prefix
are we going to receive from all of our peers?
We're going to receive that advertisement so many times
from multiple carriers, and all of them
are going to have varying AS path hop counts.
So this is the selection criteria
that most commonly is used to determine the best path.
Let's show up on a whiteboard and see how it looks in action.
Let's pretend for just a moment that each one of these routers
also represents the number of their autonomous system. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
also represents the number of their autonomous system.
So R1 here represents AS1, R2 here represents AS2, and so on.
When we receive prefixes from R8, R3, and R2
regarding a route to R5, this is when the selection algorithm is
going to make a determination.
You see, if we were to follow this path here,
it would hop to autonomous system 7, 4, 8, and then 1.
So our AS path here is going to look like 7, 4, 8. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So our AS path here is going to look like 7, 4, 8.
And then it would end up on 5 followed by an I,
meaning that the network was originated with a network
statement within autonomous system 5.
The path going from the top would
look like 5, 6, 2, and then the I
here, whereas the path here would look like 5 and 3 and I.
So the AS path would determine that this path right here
is the best one because it only has WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
is the best one because it only has
to traverse the two autonomous systems, 3 and then 5.
Up here, it would have to traverse autonomous system
2, 6, and 5, and down here, it would have
to go through 8, 4, 7, and 5.
So as long as we're not setting something
like wait or local preference or maybe using an AIGP somewhere,
this rule, rule number 5, the AS path,
is what's going to fire off and make
a selection that the next hop address of R3 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
a selection that the next hop address of R3
is going to be what's installed in the routing table.
So while that seems pretty basic,
it is worth pointing out that you
can use a route map to add autonomous system
hops to the AS path.
So we can absolutely manipulate an advertisement outbound
to include more hops than would be necessary
as part of our path attributes in an NLRI.
It's just something that you should
know about that, as per usual, you can use a route
map to manipulate just about any part of a path attribute, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
map to manipulate just about any part of a path attribute,
including the AS hops.
So that's been rule number 5, the AS path.
I hope this has been informative for you,
and I'd like to thank you for viewing.