We have follwoing set for routers.
1. R1 and R2 (installed at same location ) having channelised E1 controllers , and each is connected to different locations on 64 kbps serial link time slots on Ch E1. ( each timeslot connected to diffreen location.)
2. R1 and R2 are connected to each other on ethernet.
3. R1 and R2 ethernet is in the OSPF area 0.
4. All locations on channelised E1 controller that is all 64 kbps serial links are defined in area 220.127.116.11
5. All well so far so good !
6. When i am giving command on R1
# sh ip route SUB_NET_Connected_R2.
I am getting proper output as know via ospf and next hop is R2 ethernet. So that packets will be forwarded to R2 ethernet address and the R2 will forward those to corresponding serial link on channelised E1 controller. so far so good.
7. We identified one location ( R3 router ) as very critical and went for one more serial link on that router. One serial link was terminated on R1 channelised E1 controller and other link
was terminated on R2. On R2 it is not terminated on any timeslot of E1 controller but on a Channelised E1 controller all time slots clubbed together forming a point to point link. Bandwidth of both the link is configured same as 64 kbps. So ospf cost is same as 1562 for both links.
8. Now R3 is connected to R1 and also to R2 by 64 kbps link. This is causing problem. Both links are in area 18.104.22.168
9. when we check ip route ( on R1 ) for subnet connected to R2 via ch E1. , to our surpise OSPF poined all those to R3 serial link with higher metric that what was earlier with only one link on R3
Thne we admin down link R3 serial link on R2. And in 10 seconds all routes were back to normal as pointing to ethernet of R2.
10. In short traffic from R1 destined for some subnet on R2, would take route as following ( with both links on R3 are operational ) First packets will be forwared to R3 serial connected to R1 then R3 ( 64 kbps link ) will forward those to it's next serial port connected to R2 ( another 64 link ). Packets will then reach R2 and then R2 will forward those to corresponding E1 time slot.
All this will add additinal 64 link for all subnets. ( this jams both 64 kbps links connected to R3) althouhg this path is of higher metric as compared to path via ethernet connected between routes. ( so currently we have kept one link as admin down on R3 to keep the network up )
I hope I have made my point clear ! As it is too much of text.
Any similar problem if seen or came across please share, definitely appreciated.
Thanks in advance.