Can anyone advise me in the following situation?
We have 4 routers connected in a 'diamond' as follows
R1 at site 1, R2 at site2, R3 at site 3, R4 st site 4.
R1 connects to R2
R1 aLso connects to R3 via a seperate link
R2 connects to R4
R3 also connects to R4 via a seperate link
R1 is in area 0
R4 is in area 1
R2 and R3 are the ABRs between areas
The idea is that if any link fails the sites will remain connected
once ospf recalculates.
Under normal circumstances traffic between site 1 and site 4 should be
routed via R2 and only go via R3 in the event of a failure in between R1
and R2.Traffic between site 1 and 3 should be routed direct between R1 and R3
and go via R4 and R2 in the event of a failure betwwen R1 and R3
The problem we have is suboptimal routing between site 1 and site 4.
Traffic between R1 and R4 is being routed by a higher cost path (via R3)
rather than the lower cost path via R2. The reason I believe is summarization
on the ABRs.
R2 is summarising 10.1.0.0/16 into area 0
R3 is not summarising so is advertising all subnets in area 1 to area 0
As the summarised routes and the non-summarised routes are all in the routing
table on R1 the path being chosen is based on longest prefix rather than cost and hence
highest cost path is being used?
I can adjust the summary addresses but we have discontiguous subnets at site 3 and 4
site 4: 10.1.1.0, 10.1.2.0, 10.1.3.0, 10.1.4.0, 10.1.10.0. 10.1.11.0 - 10.1.99.0
site 3: 10.1.5.0, 10.1.6.0, 10.1.7.0, 10.1.8.0, 10.1.9.0, 10.1.100.0 - 188.8.131.52
Options as I see it:
Re-addressing subnets to allow a single summary from R2 and R3 into area 0 for appropriate
subnets off these routers
Using numerous area 1 range commands on r2 and r3 to summarise networks appropriately from
Distribute list on R1 to prevent certain subnets advertised by r2 or r3 from being added
to the routing table on r1
Can anyone advise if my assumptions are correct and suggest a solution?