higher local preference value. The local preference value is simply a numerical

value that a network operator in the local AS can assign to a particular

route. This attribute is purely local, it

does not get transmitted between autonomous systems,

so it is dropped in EBGP route

advertisements. But it allows a local network operator

the ability to explicitly state that one

route should be preferred over the other. Among

routes with equally high local preference values,

BGP prefers routes with shorter AS path length.

The idea is that a path might be better if it traverses a fewer number of

autonomous systems. The third step involves comparison

of multiple routes advertised from the same

autonomous system. The multi-exit discriminator value allows

one AS to specify that one exit

point in the network is more preferred than another. So lower MED values are

preferred, but this step only applies to

compare routes that are advertised from the same

autonomous system. Because the neighboring AS sets the MED value on routes that

it advertises to a neighbor, MED

values are not inherently comparable across routes

advertised from different ASs. Therefore this

step only applies to routes advertised from

the same AS. Fourth, BGP speaking routers

inside an autonomous system will prefer a

BGP route with a shorter IGP path cost to the IGP next up. The idea here is

that if a router inside an autonomous system

learns two routes via IBGP then it wants to

prefer the one that results in the shortest

path the to the exit of the network. This

behavior results in what is called hot potato routing,

where an autonomous system sends traffic to the neigjboring

autonomous system via a path that traverses as

little of its own network as possible. Finally, if

there are multiple routes with the highest possible local

preference, the shortest AS path and the shortest IGP

path, the router uses a tiebreak to pick a

single breaking route. This tiebreaking step is arbitrary. It

might be the most stable, or the route that's

been advertised the longest. But often, to induce determinism,

operators typically prefer that this tie breaking step

is performed based on the route advertisement from the

router with the lowest router ID, which is

typically the neighboring router's IP address. Let's now take

a closer look into local preference, AS path

length, muli-exit discriminator and hot potato routing. Now as

I mentioned the first step in the router

selection process is for routers to prefer routes with

higher local preference values. Now an operator can

actually set the local preference value on incoming

BGP route advertisements to affect which route a

router ultimately selects. Let's see how this works.