OSPF is a everyone knows is a link-state routing protocol. All routers inside an OSPF space keep an entire topology info of that space. A selected interface on a router could be a part of solely a single space which might result in some points if the community isn’t designed appropriately.
On this put up we’ll take a look at a state of affairs the place we’ll use the OSPF multi-area adjacency characteristic. OSPF Multi-area adjacency varieties a couple of adjacency over single hyperlink. This characteristic solely works on the OSPF point-point hyperlinks. Allow us to use the topology under to know additional:
Associated- OSPF Neighbor States
OSPF Multi-area Adjacency
Within the under topology we’ll attempt to attain 1.1.1.1/32 community from R5.
Requirement is to desire the trail R5 > R4 > R3 > R2 > R1 and never by way of path R5 > R4 > R2 > R1.
We have now configured the community diagram under as follows:
R1 – R2: Static routing
R2 – R3: OSPF Neighbors in Space 0
R2 – R4: OSPF neighbors in Space 10
R3 – R4: OSPF neighbors in Space 10
R4 – R5: OSPF neighbors in Space 10
Underneath the traditional OSPF configuration we’ll see that the trail R5 > R4 > R2 > R1 is most well-liked over the trail R5 > R4 > R3 > R2 > R1 which doesn’t meet our requirement.
Configurations and Output as under:
interface Loopback0
ip handle 1.1.1.1 255.255.255.255
!
interface FastEthernet0/0
ip handle 9.9.12.1 255.255.255.0
!
ip route 0.0.0.0 0.0.0.0 9.9.12.2
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R2:
interface FastEthernet0/0
ip handle 9.9.12.2 255.255.255.0
!
interface FastEthernet1/0
ip handle 9.9.23.2 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 0
!
interface FastEthernet1/1
ip handle 9.9.24.2 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 10
shutdown
!
ip route 1.1.1.1 255.255.255.255 9.9.12.1
router ospf 1
redistribute static metric-type 1 subnets
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R3:
interface FastEthernet0/0
ip handle 9.9.23.3 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 0
!
interface FastEthernet1/0
ip handle 9.9.34.3 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 10
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R4:
interface FastEthernet0/0
ip handle 9.9.24.4 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 10
!
interface FastEthernet1/0
ip handle 9.9.34.4 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 10
!
interface FastEthernet1/1
ip handle 9.9.45.4 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 10
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R5:
interface FastEthernet0/0
ip handle 9.9.45.5 255.255.255.0
ip ospf community point-to-point
ip ospf 1 space 10
Hint to 1.1.1.1/32 community from R5 takes path by way of R4 > R2.
R5#traceroute 1.1.1.1
Tracing the path to 1.1.1.1
VRF information: (vrf in identify/id, vrf out identify/id)
1 9.9.45.4 4 msec 20 msec 8 msec
2 9.9.24.2 8 msec 28 msec 16 msec
3 9.9.12.1 60 msec 24 msec 40 msec
R4#sh ip route 1.1.1.1
 Recognized by way of “ospf 1”, distance 110, metric 21, sort extern 1
Final replace from 9.9.24.2 on FastEthernet0/0, 00:00:53 in the past
Routing Descriptor Blocks:
* 9.9.24.2, from 9.9.23.2, 00:00:53 in the past, by way of FastEthernet0/0
Route metric is 21, visitors share depend is 1
We see leaving the whole lot to default in OSPF the route in the direction of 1.1.1.1 from R5 is most well-liked by way of R4 > R2 moderately than going from R3 as now we have decrease value/metric by way of R2 which is 21 and from R3 it is going to come out to be 22 attributable to extra-link in community. Default metric of ASBR is 20.
Now allow us to attempt to change the OSPF value on R4 connecting to R2 and take a look at tracing the trail once more. As in output under we see R4 nonetheless installs the path to 1.1.1.1 by way of R2 even when the associated fee is greater than that from R3.
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R4#sh ip route 1.1.1.1
 Recognized by way of “ospf 1”, distance 110, metric 120, sort extern 1
Final replace from 9.9.24.2 on FastEthernet0/0, 00:00:01 in the past
Routing Descriptor Blocks:
* 9.9.24.2, from 9.9.23.2, 00:00:01 in the past, by way of FastEthernet0/0
Route metric is 120, visitors share depend is 1
Hint output nonetheless is similar traversing by way of R4 > R2.
R5#traceroute 1.1.1.1
Tracing the path to 1.1.1.1
VRF information: (vrf in identify/id, vrf out identify/id)
1 9.9.45.4 20 msec 16 msec 8 msec
2 9.9.24.2 8 msec 28 msec 16 msec
3 9.9.12.1 52 msec 8 msec 64 msec
As soon as we shut the hyperlink in the direction of R2 we see the path to 1.1.1.1 is put in by way of R3 and hint additionally begins going by way of R3.
R2(config-if)#sh
R4#sh ip route 1.1.1.1
 Recognized by way of “ospf 1”, distance 110, metric 22, sort extern 1
Final replace from 9.9.34.3 on FastEthernet1/0, 00:00:03 in the past
Routing Descriptor Blocks:
* 9.9.34.3, from 9.9.23.2, 00:00:03 in the past, by way of FastEthernet1/0
Route metric is 22, visitors share depend is 1
R5#traceroute 1.1.1.1
Tracing the path to 1.1.1.1
VRF information: (vrf in identify/id, vrf out identify/id)
1 9.9.45.4 12 msec 20 msec 4 msec
 2 9.9.34.3 48 msec 12 msec 20 msec
3 9.9.23.2 48 msec 12 msec 60 msec
4 9.9.12.1 44 msec 56 msec 48 msec
The rationale behind such habits is that info wanted to achieve the ASBR is inter-area info nevertheless intra-area info to achieve the ASBR is at all times most well-liked irrespective of the OSPF value by way of two paths.One method to resolve this habits is to make use of the OSPF multi-area Adjacency.
Multi-Space ADJ can be utilized as a way to type a number of point-to-point logical adjacencies over a single hyperlink. The requirement is that there should be solely two OSPF audio system on the hyperlink, and in a broadcast community, it’s essential to manually change the OSPF community sort to Level-to-Level on the hyperlink. This characteristic permits a single bodily hyperlink to be shared by a number of Areas and creates an Intra-Space path in every of the Areas that share the hyperlink.
Configuration on R2 and R3 as under to allow OSPF Multi-Space ADJ.
R2:
interface fa1/0
ip handle 9.9.23.2 255.255.255.0
ip ospf community point-to-point
ip ospf multi-area 10
ip ospf 1 space 0
finish
R3:
interface fa0/0
ip handle 9.9.23.3 255.255.255.0
ip ospf community point-to-point
ip ospf multi-area 10
ip ospf 1 space 0
finish
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Proceed Studying:
Fundamental OSPF Configuration of HP Switches
