OSPF - Part 6

Understanding OE1 and OE2 routes
OSPF အပိုင္းတြင္ အနည္းငယ္ရွဳပ္ေသာ ေနရာျဖစ္ပါတယ္။ မစခင္ အရင္ဆံုး ေအာက္ပါ concept 2ခုကို မွတ္ထားေစလိုပါတယ္။
OE1 = Redistribute cost + Cost to ASBR
OE2 = only Redistribute cost

Ok... ရွင္လင္းခ်င္း စတင္ပါမည္။

R1 က R2 နွင့္ R3 ကို OSPF area0 အတြင္းတြင္ ခ်ိတ္ဆက္ထားပါတယ္…
R2 နွင့္ R3 က R4 နွင့္ R5 ကို OSPF area1 အတြင္းတြင္ ခ်ိတ္ဆက္ထားပါတယ္…
R4 နွင့္ R5 က အျခား routing domain (EIGRP AS10) တြင္ရွိေသာ router R6 ကို ခ်ိတ္ဆက္ထားပါတယ္…
R6 က 10.1.6.0 /24 network ကို EIGRP ျဖင့္ advertise လုပ္ၿပီး ၄င္းnetworkအား R4 နွင့္ R5 တို႔က OSPF area အတြင္းသို႔ default parameters မ်ားျဖင့္ redistribute ျပန္လုပ္ပါတယ္။

R4:
router eigrp 10
 redistribute ospf 1 metric 100000 100 255 1 1500
!
router ospf 1
 redistribute eigrp 10 subnets

R5:
router eigrp 10
 redistribute ospf 1 metric 100000 100 255 1 1500
!
router ospf 1
 redistribute eigrp 10 subnets

Result: R1 က prefix 10.1.6.0 /24 ကို OSPF E2 (default) route အျဖစ္ R2 နွင့္ R3 ဆီမွ default cost (20 for EIGRP) ျဖင့္ရယူပါတယ္။ R1 ၏ routing table output ကို ေအာက္ပါအတိုင္း ေတြ႔နုိင္ပါတယ္…

R1#sh ip route ospf
     10.0.0.0 /24 is subnetted, 8 subnets
O E2    10.1.6.0 [110/20] via 10.1.13.3, 00:09:43, FastEthernet0/0.13
                         [110/20] via 10.1.12.2, 00:09:43, FastEthernet0/0.12
O IA    10.1.24.0 [110/2] via 10.1.12.2, 00:56:44, FastEthernet0/0.12
O E2    10.1.46.0 [110/20] via 10.1.13.3, 00:09:43, FastEthernet0/0.13
                          [110/20] via 10.1.12.2, 00:09:43, FastEthernet0/0.12
O IA    10.1.35.0 [110/2] via 10.1.13.3, 00:56:44, FastEthernet0/0.13
O E2    10.1.56.0 [110/20] via 10.1.13.3, 00:09:43, FastEthernet0/0.13
                          [110/20] via 10.1.12.2, 00:09:43, FastEthernet0/0.12

Examining the details of the route 10.1.6.0 /24 on R1.

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0 /24
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.1.13.3 on FastEthernet0/0.13, 00:12:03 ago
  Routing Descriptor Blocks:
    10.1.13.3, from 10.1.5.5, 00:12:03 ago, via FastEthernet0/0.13
      Route metric is 20, traffic share count is 1
  * 10.1.12.2, from 10.1.4.4, 00:12:03 ago, via FastEthernet0/0.12
      Route metric is 20, traffic share count is 1

Result တြင္ျမင္ရသည့္အတိုင္း 10.1.6.0 /24 network အတြက္ R2 နွင့္ R3 ကုိျဖတ္ၿပီးသြားလွ်င္ metric 20 ရွိမွာျဖစ္ပါတယ္။ Forward metric က “cost to ASBR(s)” (R1 to R4 or R5) ကိုေဖာ္ျပပါတယ္။ ထို႔အျပင္ OE2 route 2ခုလံုး metric value နွင့္ forward metric တို႔တူေနပါတယ္… ဘာလို႔ FastEthernet0/0.12 route ကို best route (other is Backup route) အျဖစ္ေရြးခ်ယ္ပါသလဲ??? အေျဖကရွင္းပါတယ္... TIE Breaker (use Router ID) ကို အသံုးျပဳသြားျခင္း ျဖစ္ပါတယ္။ ေနာက္တနည္းအားျဖင့္ metric type ကိုေျပာင္းျခင္း ျဖင့္လည္း path selection လုပ္နုိင္ပါတယ္။ R4 တြင္ျပင္ဆင္ၾကည့္ပါမယ္။

R4#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 1
R4(config-router)#end
R4#

Result:
R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0 /24
  Known via "ospf 1", distance 110, metric 22, type extern 1
  Last update from 10.1.12.2 on FastEthernet0/0.12, 00:00:35 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 10.1.4.4, 00:00:35 ago, via FastEthernet0/0.12
      Route metric is 22, traffic share count is 1

ယခုအခါတြင္ metic 22 (Redistribute cost + Cost to ASBR) ရွိေသာ E1 route 1ခုတည္းသာ install လုပ္ပါတယ္။ OSPF mechanism အရ E1 နွင့္ E2 ယွဥ္လွ်င္  E1 က E2 ထက္ cost ဘယ္ေလာက္မ်ားမ်ား E1ကို prefer လုပ္ပါတယ္။ တခုသတိထားရမွာက E1 route 2ခုရွိၿပီး metric တူေနပါက Load Balancing လုပ္နုိင္ပါတယ္။ ok… Metric ကိုထပ္ျပင္ၾကည့္ပါမယ္…

R4#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 1 metric 100
R4(config-router)#end
R4#

Result: still preferring E1
R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0 /24
  Known via "ospf 1", distance 110, metric 102, type extern 1
  Last update from 10.1.12.2 on FastEthernet0/0.12, 00:00:15 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 10.1.4.4, 00:00:15 ago, via FastEthernet0/0.12
      Route metric is 102, traffic share count is 1

R1 ၏ Link-state Database ထဲတြင္ E1 နွင့္ E2 2ခုလံုးေတြ႔ေနရ ေသာ္လည္း E1 route အား routing path အျဖစ္အသံုးျပဳထားသည္ကို ေတြ႔ရွိရပါမယ္…

R1#show ip ospf database external 10.1.6.0

 OSPF Router with ID (10.1.1.1) (Process ID 1)

                Type-5 AS External Link States

Routing Bit Set on this LSA
LS age: 64
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.4.4
  LS Seq Number: 80000003
  Checksum: 0x1C8E
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0
        Metric: 100
        Forward Address: 0.0.0.0
        External Route Tag: 0

  LS age: 1388
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.5.5
  LS Seq Number: 80000001
  Checksum: 0x7307
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

ref: Brian McGahan, CCIE #8593
22/4/2011