Local-AS

Answered Question
Feb 27th, 2007

what is local AS attribute in BGP?

when it is used and how we can utilise it?

and which will be prefered first: AS-Path or MED?

and what is the proper chatecristic of the IGP in the Service Provider Network?

How should IGP interact with IBGP and vice versa?

I have this problem too.
0 votes
Correct Answer by devang_etcom about 9 years 7 months ago

hi dave,

to route the traffic from the local as to some other AS... IGP should have knowledge to reach that router or route in the external AS... now you can have multiple way to achieve the same function... one is to use the redistribution of BGP routes in to the IGP routing table (at that time you need the synchronization) and other way is to go for the full mesh of IBGP session (no need for the synchronization)... so in short synchronization you can define as the full reachability information knowledge between IGP and BGP...

try to search for the NEXT_HOP or SYNCHRONIZATION in BGP in google or in cisco and read some documents on it so you will have proper idea of it...

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4.5 (2 ratings)
Loading.
Jon Marshall Tue, 02/27/2007 - 10:59

Hi Dave

For Cisco BGP implementation there is no attribute called local AS. The attributes in BGP on Cisco routers

origin

as_Path

next_hop

local_pref

atomic_aggregate

aggregator

community

multi-exit-discriminator (MED)

originator_id

cluster_id

AS_path is preferred over MED.

Not sure what you mean by characteristic of IGP in SP network.

IGP and IGBP. You can either have synchronisation on or off under BGP.

If synchronisation is on then before a route that is learned from an IBGP neighbour can be put into the IGP routing or before it can be advertised to another BGP peer the router must know about the route from an IGP.

If synchronisation is off then the above rule does not apply. But then to ensure full connectivity all IGBP routers need to be fully meshed with every other IGBP router.

This is where route reflectors come in.

HTH

Jon

daveporter123 Tue, 02/27/2007 - 11:20

hi jon,

when to enable synchronizationa and when to desable it?

and will you please explain this

"If synchronisation is on then before a route that is learned from an IBGP neighbour can be put into the IGP routing or before it can be advertised to another BGP peer the router must know about the route from an IGP." i m not clear with this.

it something like LOCAL_AS in migration of AS, so what is that exactly?

Jon Marshall Tue, 02/27/2007 - 12:17

Dave

I'll try and explain with an example. You may need to draw this out :-)

R1 is in AS 150.

R2 is in AS 150.

R3 & R4 are also in AS 150

R1 & R2 run IBGP between each other.

R1 connects to R3. R3 connects to R4. R4 connects to R2. There is NO direct connection between R1 & R2.

All 4 routers are running EIGRP.

R1 is peering with REXT1 which is in AS 200

R2 is peering with REXT2 which is in AS 300.

Both these peerings are using EBGP.

So, with synchronisation on

1) R1 learns a route from REXT1 for 192.168.5.0/24

2) R1 advertises this route to R2 via IBGP. R1 advertises the next hop for this route as itself.

3) R2 now advertise this out to REXT2.

NOTE - 3 won't happen but i'm using this as an example. See below for what actually happens.

Now REXT2 wants to send traffic to the 192.168.5.0 subnet and sends it to R2.

R2 looks up the route for 192.168.5.0 and sees it is R1.

R2 then looks up how to get to R1 and sees from it's routing table that it needs to go to R4. This route was entered into the routing table by EIGRP.

R4 now receives the packet. It does a lookup for 192.168.5.0 but it has no entry in it's routing table for this. Remember the route for 192.168.5.0 was advertised from R1 to R2 via IBGP.

So R4 drops the packets as it has no route to that network.

So in the real world step 3 would not happen with synchronisation turned on. When R2 receives the 192.168.5.0 route from R1 it checks it's IGP routing table. If it isn't in there it cannot advertise that route out to REXT2.

The way to solve this is to redistribute BGP into EIGRP on R1. Then when R2 receives the advertisement from R1 for 192.168.5.0 it looks into it's routing table and finds an entry. So it can now advertise out that route to REXT2.

If you turn off synchronisation you must make sure you have a full mesh of IBGP connections between all your interior BGP speaking routers.

Generally speaking you don't want to be redistributing all your BGP learned routes into your IGP so you would want to turn synchronisation off.

Hope this makes sense.

Jon

mikedavi1 Tue, 02/27/2007 - 11:37

While AS-Path is affected, Local-AS is actually not an attribute, so there's not much you can do with regard to policy. It's a knob that allows an eBGP router establish peering, using a an AS# that is not the same as the BGP AS.

YMMV, but essentially, if my AS is 123, and my eBGP neighbor (AS 555) thinks I'm AS 456, (I've recently acquired 456) then I can configure something like the following to establish a peering.

router bgp 123

neighbor x.x.x.x remote-as 555

neighbor x.x.x.x local-as 456

The remote peer will think I'm AS 456 and establish peering.

Routes originating from me to that peer will have an as path that will include 456 123 as if they passed through 123 then 456 before arriving at 555.

See:

http://www.cisco.com/en/US/partner/tech/tk365/technologies_configuration_example09186a00800949cd.shtml

for a more detailed discussion.

HTH clarify,

Mike

daveporter123 Tue, 02/27/2007 - 12:03

you mean to say we can advertise some dummy number or we can establish the session with external peer with the different AS number than the actaul AS in which router is in.

then what is the "dual_as" key word in it?

will you please tell me about the synchronization: when should i enable it or disable it?

royalblues Tue, 02/27/2007 - 21:21

Friend,

Synchronization requires your IGP to synchronize with the BGP.

Say for eg you have 3 routers R1,R2 & R3 connected in a triangle.

If you plan to have a full mesh BGP between all the routers, then in this case you can disable synchronization.

The Idea is to have the all the routers having routing information so that packets are not blackholed

HTH, rate if it does

Narayan

daveporter123 Tue, 02/27/2007 - 22:05

and what are thos community, and what is mean by notation XX:YY in community configuration? i am not able to understand that configuration work.

Correct Answer
devang_etcom Tue, 02/27/2007 - 22:24

hi dave,

to route the traffic from the local as to some other AS... IGP should have knowledge to reach that router or route in the external AS... now you can have multiple way to achieve the same function... one is to use the redistribution of BGP routes in to the IGP routing table (at that time you need the synchronization) and other way is to go for the full mesh of IBGP session (no need for the synchronization)... so in short synchronization you can define as the full reachability information knowledge between IGP and BGP...

try to search for the NEXT_HOP or SYNCHRONIZATION in BGP in google or in cisco and read some documents on it so you will have proper idea of it...

Actions

This Discussion