12-09-2008 08:32 AM - edited 03-04-2019 12:38 AM
Hi techies,
can any one help me out , i have 2 router each at 2 diffrent AS, they are connectd through 2 diffrent service providers.the dia is enclosed, on router A i want to route seg 1 traffic on normal senirio and seg 1 & 2 traffic only when SP2 fails.
on router B , seg 2 and 1 & 2 on failing SP1 .
also on routers C ,i want to route seg 3 traffic in normal condition and seg 3 & 4 traffic only when sp2 fails.
Router D , segment 4 on normal confition & 3&4 on SP1 FAIL.
Ebgp - external protocal
Ibgp - b/n rourers in same AS.
THANKS IN ADVANCE
12-09-2008 08:37 AM
12-09-2008 10:09 AM
Hello Raahi,
because there are other ASes in the middle you can only use AS path prepending in a selective way.
RA
route-map selective_prepending permit 10
match ip address match_seg2
set as-path prepend 100 100 100
route-map selective_prepending permit 20
! empty block to allow seg1 with no changes
router bgp 100
neigh router.isp1.ipaddr remote-as ISP1.AS
neigh router.isp1.ipaddr route-map selective_prepending out
! then you have the iBGP session
neigh RB remote-as 100
the same has to be done on all the four routers: the routes with prepending applied should be the less preferred and used only in case of failure of the other link/eBGP session
the ACL can be a standard ACL or a prefix-list (in that case you need to use
match ip address prefix-list prefix-list-name inside the route-map)
Hope to help
Giuseppe
12-10-2008 03:41 AM
fine .if i configure
match ip address match_seg1
for route-map ...... 20 for router A
what would be the impact.
12-09-2008 11:14 AM
Another method to accomplish this, run iBGP between your routers within the same AS, direct LAN segment traffic to the primary router for that segment, and adjust incoming AS hops either incoming from eBGP peer or between iBGP peers. (The purpose of the latter is to make the path from other iBGP router less or equally attractive vs. via eBGP peer, but if eBGP peer drops or some route(s) lost, that route's traffic will flow across iBGP and via the other provider.)
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