[c-nsp] OSPF and BGP relationship

[Clint Wade]

Everything I'm stating below here is under the assumption you're receiving
a full route table from the ISP's and not just a default route. If all
you're getting is a default, you're looking at something like policy based
routing or possibly PFR to fix this as far as I know.

Weight and Local Pref to affect outbound --> You'll want it higher on the
one you want to be the exit point and as long as you have an iBGP
connection between your two BGP edge routers you'll be ok. If no iBGP link
between your two edge routers exists then affecting outbound is impossible
as you're limited by OSPF and the best you can do is force one to be the
outbound for all prefixes. Another way I've seen done what you're doing is
to originate 1 default in OSPF as Type 1 and the other as Type 2, obviously
the exit path to the Type 1 route is preferred, but once it makes it to
that edge router you'll have to rely on BGP path selection to affect which
edge router to egress for specific prefixes, which is why the iBGP link is
required.

AS Path and MED to affect inbound --> Usually done by sending communities
to your providers to affect their routes; Each provider has a list of
communities they accept to perform functions such as 'Add 4x AS# to
existing AS_Path' or 'Set local pref' on the provider side. You'll need to
use a looking glass server to verify these changes, and you'll want to
check them from a couple different providers looking glass to see what
effects it has on routing outside of the provider you're trying to traffic
engineer. Keep in mind you have to be careful as some providers transit to
other provider connections can get saturated which can lead to some
unexpected side affects, so you'll have to keep a close eye on performance
(latency, etc.)

On Mon, Mar 2, 2015 at 4:12 PM, Aaron <aaron1 at gvtc.com> wrote:

> I also have 2 (working on 3) Internet connections and only learn default
> route from upstream provider....
>
> I don’t know if this is best/common practice but if I ever prefer a /32 to
> exit out one of my particular internet connections, I'll point a static /32
> out that internet connection and redistribute it into my igp....my igp
> happens to be mb-ibgp for my l3vpn's to rcv it across my mpls network.
>
> Aaron
>
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
> CWO Network Operations
> Sent: Monday, March 02, 2015 3:40 PM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] OSPF and BGP relationship
>
> I have a question about the common practice of using OSPF and (i)bgp.
>
> Here is my setup:
>
>
> I have 4 Cisco routers (A, B, C & D). All routers are connected to each
> other through metro ethernet connections. The 4 routers have other “stuff”
> behind them speaking only OSPF and require a injected default route.
> Router A and B are connected to different internet backbone providers
> using BGP.
> Internally I use iBGP and OSPF. I do not redistribute OSPF routes into
> BGP, nor do I do the opposite.
> Router A injects a default route into the network using OSPF’s
> default-information originate metric 100.
> Router B also injects a default route into the network using OSPF’s
> default-information originate metric 110.
>
> So, right now all my outbound traffic goes out through router A (because
> of the metric 100). Inbound traffic comes through both internet
> connections, based on the preferred BGP route.
> Since the IGP (ospf) has the lower IGP metric (in comparison to ibgp) the
> ospf default routes (0.0.0.0/0) routes determine the flow of outbound
> traffic. Because of that, I can’t seem to “direct” outbound traffic using a
> local route map (local-preference). Ideally I would like to be able to
> direct outbound traffic as specific as I like.
>
> What is the common setup, in terms of BGP and OSPF, on networks that
> resemble ours?
>
> Thanks
> JB
>
>
>
>
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