[isp-bgp] RE: "equal" routes

[Carmen Mingolelli <carmen.mingolelli@...>]

You could do the following:

conf t
access-list <aaa> permit ip <netblock> <wildcard bits>
router bgp <your ASN>
route-map <provider #1> permit 10
match ip address <aaa>
set local-preference 100
set community <your ASN>:100 <your ASN>:<provider #1 ASN>
route-map <provider #1> permit 20
match ip address <bbb>
set local-preference 90
set community <your ASN>:90 <your ASN>:<provider #1 ASN>
route-map <provider #1> permit 30
set community <your ASN>:<provider #1 ASN>
route-map <provider #2> permit 10
match ip address <aaa>
set local-preference 90
set community <your ASN>:90 <your ASN>:<provider #2 ASN>
route-map <provider #2> permit 20
match ip address <bbb>
set local-preference 100
set community <your ASN>:100 <your ASN>:<provider #2 ASN>
route-map <provider #2> permit 30
set community <your ASN>:<provider #2 ASN>

Finally apply the route maps to their respective BGP session and do a soft
clear:
neighbor <IP address of provider #1> route-map <provider #1> in
neighbor <IP address of provider #2> route-map <provider #2> in
clear bgp <IP address of provider #1> soft in
clear bgp <IP address of provider #2> soft in

An explaination of the above cammands:
access-list <aaa> permit ip <netblock> <wildcard bits> 'define what IP
address(es) you want to be applied to the route map that will prefer
provider #1 over provider #2.
route-map <provider #1> permit 10 'config a route map that will change local
pref for the prefixes that match ACL <aaa>.
match ip address <aaa> 'matches incoming prefix announcements with the
prefixes in ACL <aaa>.
set local-preference 100 ' set the local pref on the prefix to 100.
set community <your ASN>:100 <your ASN>:<provider #1 ASN> 'tags the route
with communities so you can look up routes easily.
route-map <provider #1> permit 20 'config a route map that will change local
pref for the prefixes that match ACL <bbb>.
match ip address <bbb> 'matches incoming prefix announcements with the
prefixes in ACL <bbb>.
set local-preference 90 'set the local pref on the prefix to 90.
set community <your ASN>:90 <your ASN>:<provider #1 ASN> 'tags the route
with communities so you can look up routes easily.
route-map <provider #1> permit 30 'config a route map that will handle all
other prefixes advertised by provider #1.
set community <your ASN>:<provider #1 ASN> 'tags the routes again.

access-list <bbb> will be used in the future that will define what IP
address(es) you want to be applied to the route map that will prefer
provider #2 over provider #1.
route-map <provider #2> permit 10 'config a route map that will change local
pref for the prefixes that match ACL <aaa>.
match ip address <aaa> 'matches incoming prefix announcements with the
prefixes in ACL <aaa>.
set local-preference 90 'set the local pref on the prefix to 90.
set community <your ASN>:90 <your ASN>:<provider #2 ASN> 'tags the route
with communities so you can look up routes easily.
route-map <provider #2> permit 20 'config a route map that will change local
pref for the prefixes that match ACL <bbb>.
match ip address <bbb> 'matches incoming prefix announcements with the
prefixes in ACL <bbb>.
set local-preference 100 'set the local pref on the prefix to 100.
set community <your ASN>:90 <your ASN>:<provider #2 ASN> 'tags the route
with communities so you can look up routes easily.
route-map <provider #2> permit 30 'config a route map that will handle all
other prefixes advertised by provider #1.
set community <your ASN>:<provider #2 ASN> 'tags the routes again.


The command below will show you what prefixes have the local pref set to 100
in your network.  When you run the command you will see the next hop IP
address, Metric, LocalPref, Weight, and AS Path.  The net blocks that come
up in this should have a next hop IP address of the preferred provider you
want for that given net block.

sh ip bgp community <your ASN>:100

You can also perform the command below and again you should see the prefixes
that have a next hop IP address of the provider you want to use only if the
other preferred connection for that given net block goes down.

sh ip bgp community <your ASN>:90

There are many advantages of using communities in your network, they can be
a powerful tool to manipulate how you want traffic to flow out of your
network and they can be scaled very easily.  There is information on Cisco's
site and there are a few RFC's on using BGP communities that you may want to
look up for more explainations.

Carmen



 

-----Original Message-----
From: Bill Pitz [mailto:pitz@...]
Sent: Friday, March 05, 2004 5:46 PM
To: isp-bgp@isp-bgp.com
Subject: [isp-bgp] "equal" routes





I've got a simple network running BGP with two upstreams.  This is on a 
Cisco 3662 running IOS 12.2.

I'm in a position where I want to give incoming advertisements only for a 
certain block of IP addresses (possibly several blocks later) precedence in 
the BGP selection process when they are received from my second upstream.

The situation is this..

Upstream #1 and Upstream #2 are both providing me with advertisements for a 
netblock.  The AS Paths are the same length from both upstreams, so 
upstream #1 is getting chosen because my peer at #1 has a lower IP 
address.  However, Upstream #2 generally has better connectivity to this 
particular netblock so I would like to give it priority.  I don't want to 
add a static route because I still want normal BGP selection to take over 
in the event that my link to Upstream #2 goes down.

What's the easiest way to accomplish this?

Thanks,

-Bill 







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