[c-nsp] Showing alternate BGP paths

[Oliver Boehmer (oboehmer)]

Karol,

your output shows a lab router receiving two paths from two different
neighbors. This router can do multipath (and if it does multipath, it
can perform unequal-cost multipath using dmzlink-bw), but it will still
select *one* best path and will only send this one best-path to its
neighbors.
You usually run into this limitation when you have route reflectors,
i.e. the following BGP topology:

eBGP GW1  --\
             ---- RR --- router1
eBGP GW2  --/

Assuming the RR is not in the forwarding path: Even if router1 could use
multipath with both gateways to reach a certain prefix, the RR only
sends one of the two to router1, so in effect one of two paths is not
used.  

	oli

Karol Mares <mailto:KMares at soitron.com> wrote on Thursday, September 14,
2006 10:43 AM:

> Hi Oli,
> 
> I thought , that the new feature bgp dmzlink-bw, is used for this
> purpose. As i can see, both load-balanced routes are propagate
> through the IBGP/EBGP path. 
> 
> 
> lab# show ip bgp 10.1.1.0
> BGP routing table entry for 10.1.1.0/24, version 48
> Paths: (2 available, best #2)
> Multipath: eBGP
>   Advertised to update-groups:
>      1          2
>   200
>     172.16.1.1 from 172.16.1.2 (10.1.1.1)
>       Origin incomplete, metric 0, localpref 110, valid, external,
>       multipath, best Extended Community: 0x0:0:0
>       DMZ-Link Bw 512 kbytes
>   200
>     172.16.2.2 from 172.16.2.2 (10.1.1.1)
>       Origin incomplete, metric 0, localpref 110, valid, external,
>       multipath, best Extended Community: 0x0:0:0
>       DMZ-Link Bw 625 kbytes
> 
> The output shows that a route for each exit link on lab router  to
> autonomous system 200 has been installed as a best path i
> n the BGP routing table.
> 
>                    iso
> 
> On Thu, 2006-09-14 at 09:36 +0200, Oliver Boehmer (oboehmer) wrote:
>> cisco-nsp-bounces at puck.nether.net <> wrote on :
>> 
>>> Hi,
>>>     We have a setup with 3 routers running BGP connecting to our
>>> upstream provider and iBGP with the other routers on our local
>>> network. Usually, when the "sh ip bgp xxx.xxx.xxx", is used we can
>>> see: 
>>> 
>>> - the best path
>>> - other alternative paths with the same local preference
>>> 
>>> Is it possible to also view paths with less local preference
>>> and inactive routes? At the moment we have to run the command on
>>> the router that is directly connected to a certain upstream (eBGP)
>>> to view available paths from them which we can't see from our
> iBGP routers.
>> Thanks.
>> 
>> No, BGP only sends the best path to its peers, so if you receive
>> multiple paths from eBGP peers, the knowledge of the non-best-path
>> is limited to these nodes. 
>> 
>> There are some efforts to change the BGP protocol to send multiple
>> paths (essentially to allow load-balancing), but this has not been
>> finalized and implemented anywhere yet (as far as I know)..
>> 
>> 	oli
>> 
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