Greetings!
I am thinking about a scenario, which is maybe quite common, but I do
not know how to make that work.
Say that an AS1 is receiving full BGP table from multiple upstreams, for
example AS100 and AS200. AS1 has a customer, say AS2. There is one
Ethernet physical connection between border routers of AS1 and AS2. AS2
is paying to AS1 for upstream and receives full BGP feed. AS1 has
another customer AS3, paying for upstream also. Besides that AS1 and AS2
has a peering via some IX. AS2 is stub, so it is announcing only
prefixes with as-path ^2$. AS1 is announcing ^1$ and ^1 3$ prefixes to
its peers in the IX. AS1 preferres paths via IX by local-preferrence.
The point is how to make packets traveling from upstreams of AS1 to AS2
not to take path via IX, but via direct Ethernet connection while
traffic originating in AS1 and traffic from AS3 traveling trough AS1
take path via IX?
I have two ideas:
1) policy based routing, bind some route-map to AS1's upstream-facing
interfaces and set ip next-hop or set interface... But it does not scale
well of course.
2) put transit neighbors (upstream and customers also) into vrf, for
example:
ip vrf transit
rd 1:100
export map EXPORT_ALL
import map IMPORT_ALL
!
router bgp 1
network 1.1.1.0 mask 255.255.255.0
neighbor 2.2.2.1 remote-as 2
neighbor 2.2.2.1 route-map SET_IX_LOCPREF in
neighbor 2.2.2.1 filter-list 1
!
address-family ipv4 vrf transit
neighbor 1.1.0.1 remote-as 100
neighbor 1.1.0.1 route-map SET_TRANSIT_LOCPREF in
neighbor 1.1.0.1 description UPSTREAM1
neighbor 1.1.0.2 remote-as 200
neighbor 1.1.0.2 route-map SET_TRANSIT_LOCPREF in
neighbor 1.1.0.2 description UPSTREAM2
neighbor 2.2.2.2 remote-as 2
neighbor 2.2.2.2 route-map SET_TRANSIT_LOCPREF in
neighbor 2.2.2.2 description CUSTOMER AS2
neighbor 3.3.3.1 remote-as 3
neighbor 3.3.3.1 route-map SET_TRANSIT_LOCPREF in
neighbor 3.3.3.1 description CUSTOMER AS3
!
!
route-map SET_IX_LOCPREF permit 10
set local-preference 200
!
route-map SET_TRANSIT_LOCPREF permit 10
set local-preference 100
!
route-map EXPORT_ALL permit 10
!
route-map IMPORT_ALL permit 10
!
I spent few hours in lab experimenting with this configuration. I am
using old Cisco 1600, so there is possibility that issues I had could
come from some bug in this EoL platform... For reference, I used IOS
(tm) 1600 Software (C1600-SY-M), Version 12.2(37) RELEASE SOFTWARE (fc1)
for experiments. Problems:
1) routes in vrf transit are learned to into vrf routing table and are
announced in both directions from AS100 to AS2 and AS3 and vice-versa,
as expected. But routes from vrf transit are not exported into global
routing table nor imported from global into vrf. I tried everything (I
put some prefix- or access-list to match ip address clause in IMPORT_ALL
and EXPORT_ALL maps,...), but nothing appeared in the global table. It
should be some misconfiguration over there but I do not see that. Any
help would be appreciated.
2) Let's assume that the import and export works, so I have all transit
routes in my global table and route 1.1.1.0/24 inside vrf transit (this
is a route originated in AS2). Those routes are therefore in fact
duplicated... Is there any mechanism or chance to overcome that?
Something like default route in global table pointing into transit VRF
and triggering one extra routing decission inside VRF? Or is the
duplication somehow optimized and it won't be any problem even for full
BGP table? (O course I mean full table on real routers... 7200 or 7600.)
Is there any best-practice or common approach to that? Maybe something
completly different which I am not aware of?
Tomas
--
Tomáš Hlaváček <[EMAIL PROTECTED]>
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