one question in bgp aggregate

[Robbie]

I have router1 and router2 connect to internet ,and router3 connect to r1/r2 ,router4 connects to r1/r2

        | INTERNET    |        
        |                         |        
        |                         |        
        |                         |        
      router1-----router2      
       |     |             |        |     
      |           |   |            |      
     |          |        |            |       
    |       |                 |       |        
  router3              router4

now I wanna do bgp aggregate in r1/r2,is it good to do so?

I am thinking about there are 2 routers facing Internet,how can I do aggregate on it,

for example,if  r1 lost connection to r2 and r4,only connect to r3,but it will still advertise the aggregate route to internet

can you share your experience on this?

Thanks in advance

[RichardF]

Hi Robert,

This is what i do. Write a policy looking for routes from specific peers, if they are being announced to r1/r2 have them announce an aggregate, if not then they don't.

In my instance I look for rootserver networks (something I should always get from an upstream) and if I see them, create a default route to inject into OSPF.

In your case look for loopback route/s from your neighbours.

routing-options {

    generate {
        route 0.0.0.0/0 {
            policy conditional-default;
            discard;
        }
    }

}

    policy-statement conditional-default {
        term 1 {
            from {
                neighbor 10.10.10.10;
                prefix-list root-servers.org;
            }
            then accept;
        }
        term 2 {
            then reject;
        }
    }

    prefix-list root-servers.org {
        192.5.5.0/24;
        192.36.148.0/24;
        192.58.128.0/24;
    }

Cheers,

javelin

[Robbie]

hello,Richard

thanks for your reply

actually my point here is : is it a good idea to aggregate internal route to INTERNET when there are 2 router with 2 links to INTERNET

I have this concern because if part of routers is is lost ,router1 will still advertise aggregate to internet,and it will mislead the Internet

[pad]

It is good practice to aggregate. When the intention behind aggregation is to hide all contributing internal routes, the question of misleading the internet wouldn't arise.

[Robbie]

there are 2 routers facing INTERNET

it will mislead the internet,if r1 only can connect to r3,and with a low metric(we set r1 as active incoming)

so from outside,it will never reach r4 which lost connection to r1

[jasonlai]

Quick question.

1. Is R1-R4 in internal IGP ?

2. It's a practice to annouce the aggregate routes to Internet as those routes are originated to from your AS.

3. Typically users aggregate internal routes to Internet to summarize the direct/IGP IP address which are < 24.

4. Even the link from R1-R3 low metric, you should still able to reach R4 based on iGP.

[mhariry · Accepted solution]

Hi,

from my point of view you have two options here to solve your problem.

A- no need for aggregate and advertise every subnet individual

B- there is another solution but it may take non-optimal-path so if it doesn't make problem for you it will be fine

1- Configure IBGP on your network full mesh or RR where R1,R2 are RR

2- advertise aggregate route to your ISP normally.

lets talk about traffic flow from R1 --> R4 subnet now

3- in case of R1-->R2 & R1-->R4 links went down the BGP session between R1 & R4 still up as there is connectivity between them so R1 still receive update (more specific route) from R4

So traffic will go from R1 ---> R3 ---> R2 ---> R4 long path but will not cause a lot of problems for you just two more hops plus this case is might happened in outage and would be fixed quickly

Regards,

Mohamed Elhariry

JNCIE-M/T # 1059, CCNP & CCIP

----------------------------------------------------------------------------------------------------------------------------------------

If this post was helpful, please mark this post as an "Accepted Solution".Kudos are always appreciated!

[Robbie]

hi,Mohamed

thanks for your reply

with respect to RR,in my case,how to set RR?

set r1 and r2 as RR ,?

how to set client?

pls

[mhariry]

Hi,

for client nothing to do normal BGP peer with only RR (both RR).

for RR just add comman cluster <cluster-id> where cluster-id is any unique ID like 1.1.1.1 on both R1, R2 and be sure that the cluster-id is same on both of them. 

Regards,

Mohamed

[Robbie]

hello

glad to see your reply ,Mohamed

I ;d like to have another 2 questions.

1:r1 need to peer with r2(client relationship)?

r1 is RR ,r2 is RR,what is the relation between r1 and r2?

2:after reflected,r2 reflect route of r4 to r3,

does r3 advertise route to r1 ? r3 is not a RR, and he need to follow horizontal split.

3:RR reslove the horizontal split,but without  horizontal split.,is there any possiblity to cause route loop in IBGP?