Re: IXP BGP routing

[Brian Candler <B.Candler at pobox.com>]

On Tue, May 21, 2002 at 09:34:48AM +0000, Antonio Godinho wrote:
> Hi,
> 
> We have just set up an IXP in Mozambique but now I am having a 
> difficult problem with the routing.
> The ISPīs are connected to the IX directly and they peer with each 
> other normally but the problem that has come up is that some of 
> the ISPīs are also connected to the same Telcom for internet 
> access. The problem is that the telcom is running EIGRP with the 
> ISpīs and they are running BGP through the IX. Their routers 
> always choose the EIGRP routes because of the lower admin 
> distance of 90 over the BGP internal distance of 200 (this is 
> because they are running iBGP between the IX router and their 
> router). How can I change their routing so that they choose the IX 
> routes?

[Forwarded to the list with permission]

The short answer: if a Cisco gets two identical routes from two different
routing protocols, as you have discovered it uses the "administrative
distance" to choose between them, with the default values you have seen. You
should be able to change them so that BGP routes are preferred over EIGRP
routes. Someone else on this list should be able to give you the exact
syntax if you can't find it.

The longer answer: your Telcom is leaving itself (and you) open to enormous
risk by using EIGRP to exchange routes with its customers. Any one customer
will be able to take down their entire network by injecting bad routing
information. You should try to persuade them to change this situation
_before_ their network is broken, rather than after a major outage occurs.

Possible solutions are:

(1) Use static routes - usually the preferred option if the customer does
not have their own public AS and is not multi-homed to Telcom.
In other words: at the Telcom router which terminates the leased line, put
a single static route for the customer's netblock pointing down that leased
line. EIGRP will then carry that route within Telcom's network to all their
other routers. The customer just points a static defaultroute at Telcom.

The customer can run EIGRP, OSPF or whatever they like within their own
network. The Telco and each customer are all running separate, isolated
routing domains which cannot interfere with each other.

       EIGRP                             OSPF(say)
   <----------->                         <--->
   R ----- R     static rt                   R
             \   --------->                /
               R ----------------------- R
             /                  <-------   \       BGP
           R                     static      R --------- IXP
                                 default
                                           

  TELCOM NETWORK        LEASED LINE         CUSTOMER NETWORK

Since the only routes carried in OSPF are your own routes plus the static
default, the routes for other ISPs received via the IXP will be more
specific and therefore will always take preference (longest match wins).

(2) Use BGP (and filter the received routes)

This is the preferred solution if the customer has their own public AS,
their own provider-independent address space, and/or is multi-homed (either
to the same provider or to multiple providers). BGP with prefix filtering
and flap dampening ensures that the routing domains are sufficiently
isolated.

Regards,

Brian.



> Cheers,
> 
> 
> 
> Antonio Godinho
> B.Sc., MCP, MCP+Internet, MCSE, CCNA
> Address:Av. Julius Nyerere 947 3rd floor esq 
> Maputo - Mozambique
> Phone  : 258-82-300392
> e-mail : ANTONIO at nambu.uem.mz
> 
> An expert is one who knows more and more 
> about less and less until he knows absolutely 
> everything about nothing.
>                      
> 

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