IBGP Question

EdTheLad

IBGP can be run in an AS without an IGP once you have a full mesh configuration. If you dont want the hastle of the full mesh config you could
deply route reflectors.Now if you have an IGP running i dont see the point in having internal routers running IBGP.All you need to configure is the AS border routers with EBGP to the other remote AS router and IBGP to the internal border router.
Can anyone give me an advantage in having IBGP running on multiple
routers internal to an AS which have no links to an external AS?

darkuser

all bgp routers within an as MUST have ibgp configured between them.
The exception is route reflectors.
also you must have an igp running.

http://www.nanog.org/mtg-0206/ppt/philip/sld012.htm

EdTheLad

darkuser wrote:

all bgp routers within an as MUST have ibgp configured between them.
The exception is route reflectors.
also you must have an igp running.

That statement isnt correct, if you play with your lab you will see all routers in an AS dont need IBGP running.The EBGP router will learn external routes which can then be distributed into an IGP,so if only your gateway routers are running BGP externally and internally you will have
knowledge of all routes.The only requirement is to have the edge router
comunicating to the other edge routers via IBGP.

I've noticed with alot of books they tell you one way to configure something maybe the correct way 99% of the time but in order to really learn this stuff you need to experiment,so maybe you should start giving your own opinions on configs instead of pasteing someone elses

I'll put my question another way to make it more clear what im asking.
You have a transit AS i.e. connected to 2 other AS via 2 edge routers.
Your AS has multiple internal routers.Your 2 edge routers will have both EBGP and IBGP running.In this case i dont see a point to have IBGP running on the rest of the internal routers in your autonomous system.
For me i dont see the point in running BGP on any router in the AS which are not edge routers.Once an IGP is configured it will popogate the routes of all routers throughout the AS and these routes can then be distributed into BGP at the edge routers with the network command.
So what is done in practice on live networks and what are the advantages of having an internal router running IBGP which has no direct link to an external AS.The problem i see is that running IBGP will use alot of resources on internal routers with no apparent benifits!

EdTheLad

OK,for anyone who's interested i found the answer i was looking for.
With very few exceptions,Interior BGP-BGP between peers in the same AS is
only used with multihomed scenarios.
So if your AS is single homed the only routers you should configure with BGP are the edge routers.

darkuser

I don't believe your understanding of bgp is as clear as you think it is .....

or i'm not getting with your example

if your single homed you don't need another bgp router in your as
if your multi homed you have to run ibgp between the two routers representing your as to the internet.

this might be a question of best practices vs commonly taught practices vs
what you can actually do if you want to do it.

personally i wouldnt break best practice in a production network.

EdTheLad

My discussion is regarding internal routers running BGP, a stub AS will need one EBGP router, a transit a minimum of 2.In the transit the 2 EBGP routers will need IBGP running between them.These are the manatory requirements.
My curiousity was about the other internal routers in the AS which are running an IGP i was wondering why or why not to enable IBGP on these routers.
Anyway i'm happy now as i can see the different possible setups and why to use one over the other.Best practice however depending on network topology would be to create a full mesh of IBGP routers and disable synchronization.
As i studying to take the CCIE lab i am covering all possiblities good or bad.

darkuser

bgp requires an igp to function period
rip ... eigrp .... ospf... whatever
in some cases you may need a static route to get to your next hop if its not learned via the igp.
under the rule of synchronization you must always have an underlying igp route for ibgp to work correctly. but .... synchronization is now mostly disabled.
ebgp is for peering inter as ....
ibgp is for peering intra as

so .... if you have more than one router running bgp in an as (666 for example)
they have to full mesh peer ibgp. the only exception is route-reflectors.
confederations are for creating sub -as's

darkuser

the answer is you don't have to .... the only thing the "other" router(s) need to do is find the next hop to the bgp router(s)

But

if you run bgp they have to ibgp peer full mesh or run a route reflector.