Question on same AS on different site

pipsadmin · ‎10-17-2008

lets say you have an AS17000.

network 206.10.10.0/24

You have 2 sites geographicaly seperated and with a different ISP serving the 2 sites (ISP1 for site1 and ISP2 for site2)

is it possible to advertise AS17000 network 206.10.10.0/25 (.0 to .127) on site1 and advertise AS17000 network 206.10.10.128/25 (.128 to .255) on site2?

Is this possible?

Collin Clark · ‎10-17-2008

Yes, take a look at AS-Override-

http://www.cisco.com/en/US/docs/ios/12_3/switch/command/reference/swi_n1.html#wp1034057

Hope that helps

Giuseppe Larosa · ‎10-17-2008

Hello Nelson,

in the global internet a /25 prefix can be seen as too specific by Tier1 /Tier2 ISPs in a effort to limit the BGP table size.

So the visibility of the two prefixes /25 is not guaranteed on the whole internet.

They can use a prefix-list and deny prefixes longer then 24 for example.

the two ISPs you peer to can make agreements to let you exchange traffic between sites using some BGP tricks (AS override on ISP side or allow-as-in 1 on your side) but this is not enough for internet visibility

I would consider to buy an EoMPLS point-to-point service to build a connection between the two sites (even using the same two ISPs) and to advertise the /24 on both and use routing on the EoMPLS to forward traffic if needed

Hope to help

Giuseppe