good descrition to OSPF Down Bit

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OSPF:own Bit 10 Comments




I’ve seen the same question in several blogs, so today I will try to turn this subject inside-out.
Let’s focus on the following scenario:

                               
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The following sequence takes place, which can lead to a potential routing loop:

• R2 sends an LSA1 or LSA2 to PE1
• PE1 receives the intra-area OSPF route from R2 and redistributes it into MP-BGP.
• The receiving PE router, let’s say PE2, redistributes the MP-BGP route into OSPF as an LSA3 (remember that MPLS backbone acts as an OSPF superbackbone)
• LSA3 is propagated across the OSPF area and received by the other PE router, PE3 in our diagram, which is attached to the same OSPF area 1.
• Because PE3 has the same route learned by two protocols (OSPF and MP-BGP), it has to make a decision based on the administrative distance. Because the administrative distance of OSPF is better than the administrative distance of MP-iBGP, it selects the OSPF route. Therefore, PE3 redistributes the route OSPF back into the MP-BGP process, potentially resulting in a routing loop.
  

This is why it’s completely necessary to have a mechanism to break the loop, and OSPF down bit is that mechanism.
The down bit  is a bit located in the LSA Header, and its function is to avoid loops in MPLS-VPN environments when using OSPF as PE-CE protocol.

                               
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When PE2 redistributes the MP-BGP route into OSPF, it sets up the down bit, or downward, on the LSA3. So when PE3 receives that LSA3, it doesn’t take it into account.
R1#sh ip ospf datab            OSPF Router with ID (1.1.1.1) (Process ID 100)                Router Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Link count1.1.1.1         1.1.1.1         690         0x8000000D 0x007847 5103.103.103.3   103.103.103.3   879         0x80000009 0x006DEE 2200.200.200.2   200.200.200.2   1040        0x8000000A 0x00789F 2                Summary Net Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum2.2.2.2         103.103.103.3   108         0x80000001 0x00F5832.2.2.2         200.200.200.2   108         0x80000001 0x00DA7B10.10.22.0      103.103.103.3   98          0x80000001 0x005AFD10.10.22.0      200.200.200.2   98          0x80000001 0x003FF5R1#R1#sh ip ospf databa summ 2.2.2.2            OSPF Router with ID (1.1.1.1) (Process ID 100)                Summary Net Link States (Area 1)  Routing Bit Set on this LSA in topology Base with MTID 0  LS age: 133  Options: (No TOS-capability, DC, Downward)  LS Type: Summary Links(Network)  Link State ID: 2.2.2.2 (summary Network Number)  Advertising Router: 103.103.103.3  ...  Routing Bit Set on this LSA in topology Base with MTID 0  LS age: 133  Options: (No TOS-capability, DC, Downward)  LS Type: Summary Links(Network)  Link State ID: 2.2.2.2 (summary Network Number)  Advertising Router: 200.200.200.2R1#

PE3#sh ip ospf 100 databa            OSPF Router with ID (103.103.103.3) (Process ID 100)                Router Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Link count1.1.1.1         1.1.1.1         649         0x8000000D 0x007847 5103.103.103.3   103.103.103.3   836         0x80000009 0x006DEE 2200.200.200.2   200.200.200.2   1000        0x8000000A 0x00789F 2                Summary Net Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum2.2.2.2         103.103.103.3   65          0x80000001 0x00F5832.2.2.2         200.200.200.2   67          0x80000001 0x00DA7B10.10.22.0      103.103.103.3   55          0x80000001 0x005AFD10.10.22.0      200.200.200.2   57          0x80000001 0x003FF5PE3#PE3#sh ip ospf databa summ 2.2.2.2            OSPF Router with ID (103.103.103.3) (Process ID 100)                Summary Net Link States (Area 1)  LS age: 146  Options: (No TOS-capability, DC, Downward)  LS Type: Summary Links(Network)  Link State ID: 2.2.2.2 (summary Network Number)  Advertising Router: 103.103.103.3  ...  LS age: 148  Options: (No TOS-capability, DC, Downward)  LS Type: Summary Links(Network)  Link State ID: 2.2.2.2 (summary Network Number)  Advertising Router: 200.200.200.2  ...PE3#

This down bit is present only in the LSA3. LSA5 and LSA7 doesnt have this bit in its headers.
Let’s check this statement making PE2 and PE3 redistribute the route as LSA5. In order to do that, I will change the ospf-domain in PE1:
PE1#conf termPE1(config)#router ospf 100PE1(config-router)#domain-id 1.1.1.1PE1(config-router)#endPE1#

R1#sh ip ospf data            OSPF Router with ID (1.1.1.1) (Process ID 100)                Router Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Link count1.1.1.1         1.1.1.1         898         0x8000000D 0x007847 5103.103.103.3   103.103.103.3   1087        0x80000009 0x006DEE 2200.200.200.2   200.200.200.2   1249        0x8000000A 0x00789F 2                Type-5 AS External Link StatesLink ID         ADV Router      Age         Seq#       Checksum Tag2.2.2.2         103.103.103.3   56          0x80000001 0x009F19 34897259292.2.2.2         200.200.200.2   56          0x80000001 0x008411 348972592910.10.22.0      103.103.103.3   66          0x80000001 0x000493 348972592910.10.22.0      200.200.200.2   66          0x80000001 0x00E88B 3489725929R1#R1#sh ip ospf databa extern 2.2.2.2            OSPF Router with ID (1.1.1.1) (Process ID 100)                Type-5 AS External Link States  Routing Bit Set on this LSA in topology Base with MTID 0  LS age: 64  Options: (No TOS-capability, DC)  LS Type: AS External Link  Link State ID: 2.2.2.2 (External Network Number )  Advertising Router: 103.103.103.3  ...  Routing Bit Set on this LSA in topology Base with MTID 0  LS age: 64  Options: (No TOS-capability, DC)  LS Type: AS External Link  Link State ID: 2.2.2.2 (External Network Number )  Advertising Router: 200.200.200.2  ...R1#

PE3#sh ip ospf 100 datab            OSPF Router with ID (103.103.103.3) (Process ID 100)                Router Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Link count1.1.1.1         1.1.1.1         851         0x8000000D 0x007847 5103.103.103.3   103.103.103.3   1038        0x80000009 0x006DEE 2200.200.200.2   200.200.200.2   1202        0x8000000A 0x00789F 2                Type-5 AS External Link StatesLink ID         ADV Router      Age         Seq#       Checksum Tag2.2.2.2         103.103.103.3   7           0x80000001 0x009F19 34897259292.2.2.2         200.200.200.2   9           0x80000001 0x008411 348972592910.10.22.0      103.103.103.3   17          0x80000001 0x000493 348972592910.10.22.0      200.200.200.2   19          0x80000001 0x00E88B 3489725929PE3#PE3#sh ip ospf 100 databa extern 2.2.2.2            OSPF Router with ID (103.103.103.3) (Process ID 100)                Type-5 AS External Link States  LS age: 30  Options: (No TOS-capability, DC)  LS Type: AS External Link  Link State ID: 2.2.2.2 (External Network Number )  Advertising Router: 103.103.103.3  ...  LS age: 32  Options: (No TOS-capability, DC)  LS Type: AS External Link  Link State ID: 2.2.2.2 (External Network Number )  Advertising Router: 200.200.200.2  ...PE3#

In order to check if LSA7 has this bit, I change the area type and redistribution in PE2 and PE3:
PE2(config)#router ospf 100PE2(config-router)#redis bgp 65001 sub nssa-onlyPE2(config-router)#area 1 nssaPE2(config-router)#endPE2#

PE3(config)#router ospf 100PE3(config-router)#redis bgp 65001 sub nssa-onlyPE3(config-router)#area 1 nssaPE3(config-router)#endPE3#

R1#sh ip ospf databa            OSPF Router with ID (1.1.1.1) (Process ID 100)                Router Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Link count1.1.1.1         1.1.1.1         75          0x80000010 0x00189E 5103.103.103.3   103.103.103.3   80          0x8000000B 0x000F45 2200.200.200.2   200.200.200.2   80          0x8000000C 0x001AF5 2                Type-7 AS External Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Tag2.2.2.2         103.103.103.3   10          0x80000001 0x003E54 34897259292.2.2.2         200.200.200.2   10          0x80000001 0x000869 348972592910.10.22.0      103.103.103.3   223         0x80000002 0x00A0CF 348972592910.10.22.0      200.200.200.2   190         0x80000001 0x006CE3 3489725929R1#R1#sh ip ospf datab nssa-external 2.2.2.2            OSPF Router with ID (1.1.1.1) (Process ID 100)                Type-7 AS External Link States (Area 1)  Routing Bit Set on this LSA in topology Base with MTID 0  LS age: 28  Options: (No TOS-capability, No Type 7/5 translation, DC)  LS Type: AS External Link  Link State ID: 2.2.2.2 (External Network Number )  Advertising Router: 103.103.103.3  ...  Routing Bit Set on this LSA in topology Base with MTID 0  LS age: 28  Options: (No TOS-capability, No Type 7/5 translation, DC)  LS Type: AS External Link  Link State ID: 2.2.2.2 (External Network Number )  Advertising Router: 200.200.200.2  ...R1#

The down bit helps prevent routing loops between MP-BGP and OSPF when LSA3 are used, but not when external routes are announced.
So then, how are routing loops avoided when external routes are present? Well, with LSA5 and LSA7 there is a new field checked to avoid loops. This is called the tag field:
PE3#sh ip ospf 100 datab            OSPF Router with ID (103.103.103.3) (Process ID 100)                Router Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Link count1.1.1.1         1.1.1.1         834         0x80000010 0x00189E 5103.103.103.3   103.103.103.3   837         0x8000000B 0x000F45 2200.200.200.2   200.200.200.2   839         0x8000000C 0x001AF5 2                Type-7 AS External Link States (Area 1)Link ID         ADV Router      Age         Seq#       Checksum Tag2.2.2.2         103.103.103.3   19          0x80000001 0x003E54 34897259292.2.2.2         200.200.200.2   11          0x80000004 0x00026C 348972592910.10.22.0      103.103.103.3   19          0x80000001 0x00A2CE 348972592910.10.22.0      200.200.200.2   11          0x80000004 0x0066E6 3489725929

When a PE redistributes a route from MP-BGP into OSPF as LSA5 or LSA7, it adds a tag to the route (tag 3989725929 by default). So if another PE receives an LSA5 or LSA7 with this tag, it doesn’t redistribute it back into MP-BGP: Of course, we may change the value of the tag to any other value. This is done when configuring the redistribution under ospf process:
PE2#conf termPE2(config)#router ospf 100PE2(config-router)# redistribute bgp 65001 subnets tag 100PE2(config-router)#endPE2#

I hope it’s a little bit clearer now

                               
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