Hi Ondrej,

Thanks for getting back. Yes, this would only affect routes that are propagated to eBGP peers. So when a route is propagated from a true eBGP peer to an internal eBGP peer, then the MED value would be preserved. This is useful for networks where you have eBGP peers and would still like to preserve the MED attribute beyond AS boundaries. 

Although we are not aware of Cisco/Juniper having this option, this RFC on MED considerations mentions such behavior: https://tools.ietf.org/html/rfc4451#section-3.3. Thanks for the suggestion on using export filters to set this attribute. For our use case, it feels simpler to enable a knob for this, but we will test using export filters for this. 

Thanks,
Trisha
--

Trisha Biswas | Sr. Software Engineer, Network Systems


On Wed, Dec 9, 2020 at 8:03 PM Ondrej Zajicek <santiago@crfreenet.org> wrote:
On Tue, Dec 08, 2020 at 09:02:15AM -0800, Trisha Biswas wrote:
> Hi all,
>
> The attached patch helps preserve MED values to/from eBGP peers. When
> propagating a route from a true eBGP peer to an internal peer, this avoids
> resetting the MED attribute to allow propagation of MED to/from external
> peers. This is enabled via a configuration line: "preserve med", which goes
> under BGP configs. Without the configuration line this would be a no-op.

Hi

I am bit hesitant with this patch. Seems to me that it is something that
could be easily done with 'bgp_med = bgp_med;' export filter, but that
may be tricky for users that are not thoroughly familiar with bgp_med
behavior.

Also, your description seems confusing. When a route is propagated from
EBGP to an internal peer, then MED is preserved regardless of this.
It is just propagation to EBGP peer when original MED is stripped.

What is a use case for this option? Does other BGP implementations
(Juniper, Cisco) have some similar option?

--
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago@crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."