AS-path prepending and Internet bgp table

[mathieuploton]

I am trying to loadshare my inbound traffic using AS-prepending between two ISPs. As it does not seem to work, I decided to have a look on my internet bgp routes using the very useful http://lg.level3.net.

Switching of one ISP, then the other, I was able to realize that the metric and localpref of my routes were different when coming through one or the other provider.

Realizing this, I have no idea why is this and I would really appreciate if someone can light me up.

This is the anonymised output from level3 :

ISP1

Show Level 3 (Washington, DC) BGP routes for 32.45.23.23.1

BGP routing table entry for 32.45.23.23.0/21

Paths: (2 available, best #1)

  1273 4251 9231 6754 6754 6754 6754 >> I think I cannot make the path longer J

  AS-path translation: { CW-ECRC APNIC-AS-BLOCK APNIC-AS-3-BLOCK MYISP MYISP MYISP MYISP }

    edge1.Washington4 (metric 40)

      Origin IGP, metric 0, localpref 100, valid, internal, best

      Community: 1273:12250 North_America  Lclprf_100 Level3_Customer United_States Washington_DC Suppress_to_AS1239 Prepend_3_to_Peers

      Originator: edge1.Washington4

  1273 4651 9931 6754 6754 6754 6754

  AS-path translation: { CW-ECRC APNIC-AS-BLOCK APNIC-AS-3-BLOCK MYISP MYISP MYISP MYISP } (prepending seems to work)

    edge1.Washington4 (metric 40)

      Origin IGP, metric 0, localpref 100, valid, internal

      Community: 1273:12250 North_America  Lclprf_100 Level3_Customer United_States Washington_DC Suppress_to_AS1239 Prepend_3_to_Peers

      Originator: edge1.Washington4

ISP2

BGP routing table entry for 32.45.23.23.0/21

Paths: (2 available, best #1)

  3320 7232 55218 6754

  AS-path translation: { RIPE-ASNBLOCK4 APNIC-AS-2-BLOCK INTERNET1 MYISP }

    car4.Washington1 (metric 40)

      Origin IGP, metric 100000, localpref 86, valid, internal, best

      Community: North_America  Lclprf_86 United_States Level3_Peer Washington_DC

      Originator: car4.Washington1

  3320 7552 55508 6754

  AS-path translation: { RIPE-ASNBLOCK4 APNIC-AS-2-BLOCK INTERNET1 MYISP }

    car4.Washington1 (metric 40)

      Origin IGP, metric 100000, localpref 86, valid, internal

      Community: North_America  Lclprf_86 United_States Level3_Peer Washington_DC

      Originator: car4.Washington1

AS-prepending is working but localpref and metric make ISP1 always preferred in the bgp path selection.

[Mahesh Gohil]

Hi,

you can achieve what you want is by dividing your pool:32.45.23.23.0/21 into two parts of /22 and advt. each pool to each ISP.

The reason is every ISP has their own policy of local preference for customers.

Regards

Mahesh

[mathieuploton]

This is the output for level3 bgp table using a different upstream isp, this is not the bgp table from my upstream isp.

It seems that is not related to my isp policy but to my isp connectivy to Tier1 :

- Localpref for non-customer peers is generally set
to 86.

"http://onesc.net/communities/as3356/"

[Mahesh Gohil]

Yes,

That is the reason i recommended to do changes of dividing pool.

Upstream-----ISP1-----

                               Customer

Upstream---- ISP2-----

Here you can ask ISP1 or ISP2 for any policy changes but not upstream as you are not their direct customer and they will be having their own policy of

local preference.

Regards

Mahesh

[mathieuploton]

Yes but if I do that, I won't have any resiliancy ? If one link fails, half of my network will lose internet connectivity...

[Atif Awan]

mathieuploton wrote:
Yes but if I do that, I won't have any resiliancy ? If one link fails, half of my network will lose internet connectivity...

You can have resiliency by advertising the supernet to both upstream providers. For example, let's say you have a /23 that you split into two /24s. You will advertise one /24 to ISP-A and the other /24 to ISP-B. You would also advertise the /23 to both ISP-A and ISP-B. This should give you the desired behavior as well as provide redundancy in case one of your upstream connectivities fail.

Atif

[Mahesh Gohil]

Hi,

Or

Divide your /21 pool into two of /22..say A and B

Advt. A normally on ISP-1

Advt. B with as-path prepend on ISP-1

Advt. A with as-path prepend on ISP-2

Advt. B normallu on ISP-2

This way you will have backup as well as load sharing.

Re