BGP multihoming single site, two isp

Hi all,

We are soon starting up bgp and multihoming.
Two ISPs and we have one site.
We have already PI number and ASN assign to us.
A Cisco 3825 with 3 ethernet interface with IOS advanced feature pack
is order.
No full routing table.

Any sugestion for a config?

Regards Jan Rockstedt

 

Hi Jan

You could do it two ways:

1. Use one ISP as primary and one as secondary
2. Use both ISPs and recieve default routes from both ISP along with
their "own" IPs

For both ISPs you setup standard BGP settings as described in the
documentation from Cisco (take a look at CCO and do a search on BGP).

With option 1 you tag the inbound routes from both ISPs, setting the
local-preference for 200 on the primary routes and leave the secondary
ISP with default local-preference (that'll be 100). When you advertise
your own routes, do a prepend to the routes you advertise to the
secondary ISP.


With option 2 you simply accept any routes advertised by the ISP and
advertise your own routes with default settings.


If you need further information / help, please respond to this group.

Cheers,
Lars Christensen

 

Thanks, that is interesting to me too.

Is it possible to find out how many AS numbers to
"prepend" to effectively guarantee that one path
will become primary for all ISPs?

I have made a stab at 4 relating to 2 connections within a single
city.
How many would be needed to guarantee this globally?

Is there a limit to the number of ASs in a path and
if so what is a reasonable number to limit our prepends
to?

Thanks.

 

Hi Lars,

Thanks for your respond.

I have found so many doc on the google.
Many are old quit .....

Our primary isp are the link 50 Mbit and our second isp we have a 10
Mbit.
So two default routes, one with static 0.0.0.0/0.0.0.0 with default
100 to the 50 Mbit and one with 0.0.0.0/0.0.0.0 with 200 to the 10
Mbit
Redistribute static to bgp?

router bgp our-ASN
network 1.0.0.0
network 2.0.0.0

neighbor 10.10.10.10 remote-as ISP1-ASN
neighbor 10.10.10.10 route-map localonly out
neighbor 20.20.20.20 remote-as IPS2-ASN
neighbor 20.20.20.20 route-map localonly out

//Jan

 

A few comments...

Don't worry about the age of the docs on the web! Not much has changed with
BGP since version 4 was released last century. If you can track down a copy
of my book, read the chapter on connecting to an ISP, it still applies.

As for your proposal, I do not recommend it if your goal is to minimize
potential for black holes. Rather than using static default routes, have
your ISPs advertise a default route to you (that way, you only get a
default route if your link to the ISP is up). There are even better ways to
protect against problems in your ISP or upstream, but they take more
effort, as does setting up reasonable load balancing.

Good luck and have fun!