bgp to the isp for dual internet routers to 2 isps

[carl_townshend]

Hi all

I have a quick qeustion about bgp to the isp

1.What do most customers do to influence the routing inbound to the company from the isp? AS Path prepend or MED ?

cheers

Carl

[Giuseppe Larosa]

Hello Carl,

to influence how inbound traffic enters on the internet links AS path prepending is the more used tool.

MED can be used if the internet links connect to the same ISP AS in that case it is effective

Hope to help

Giuseppe

[carl_townshend]

hi

so if you connect to different isp's cant you use the MED value ?

[cadet alain]

Hi, normaly no but there is the command bgp always-compare-med that permits to do so.

You could also use communities to influence inbound path by discussing with your ISP.

Regards.

Alain

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[Giuseppe Larosa · Accepted solution]

Hello Carl,

the MED cannot be passed to a third AS, unless the two ISPs have an agreement between themselves the MED would be not effective in your network scenario for inbound traffic

Hope to help

Giuseppe

[carl_townshend]

so

for same ISP use MED

for different ISP use AS path

can you give me a example config ?

[Giuseppe Larosa]

Hello Carl,

you will need to apply route-maps outbound on eBGP sessions

router bgp

neighbor ISP1 route-map SETVALUES out

route-map SETVALUES permit 10

! match statement as needed

set as-path prepend

or for MED setting :

route-map SETVALUES permit 10

! match statement as needed

set metric 

in case of AS path prepending you will prepend the ISP that you want to have a lower probability to be the return path

for MED you have to set on both neighbors with lower value that will be the preferred link for inbound traffic

see

http://www.cisco.com/en/US/docs/ios/11_3/np1/configuration/guide/1cbgp.html#wp14844

A good filter for routes to be advertised in multihoming is the following

ip as-path access-list 1 permit ^$

this matches an empty AS path attribute and allows to advertise only locally generated routes

in the route-map you would put in the match section

match as-path 1

this avoids to pass routes from ISP1 to ISP2 or from ISP2 to ISP1 to avoid to become a transit AS

Hope to help

Giuseppe