Solved: OSPF route preferred over BGP route

rick505d3 · ‎05-02-2021

Hi,

In the network below, the objective is redundant reachability to prefix 1.1.1.1 (say public cloud hosting) routers csr2 and csr3 (internal network).

In the order of operations, BGP was configured and verified first, followed by OSPF configuration. The working state is like this - csr2 and csr3 can reach 1.1.1. directly via BGP:

csr2#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 1
1, (received & used)
10.0.12.1 from 10.0.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0

csr2#show ip ospf database ext 1.1.1.1

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Type-5 AS External Link States

  LS age: 82
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0x9704
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 12

  LS age: 45
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x8B0B
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 13

csr3#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 2
  1, (received & used)
    10.0.13.1 from 10.0.13.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

csr3#show ip ospf data ext 1.1.1.1

            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Type-5 AS External Link States

  LS age: 214
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0x9704
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 12

  LS age: 176
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x8B0B
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 13

Now, shut/no shut the BGP neigh 10.0.12.1 on csr2 - or shut/noshut the interface long enough for bgp to go down. The state now looks like this:

csr2#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 7
Paths: (2 available, best #2, table default)
  Not advertised to any peer
  Refresh Epoch 2
  1, (received & used)
    10.0.12.1 from 10.0.12.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external
      rx pathid: 0, tx pathid: 0
  Refresh Epoch 1
  Local
    10.0.23.3 from 0.0.0.0 (2.2.2.2)
      Origin incomplete, metric 1, localpref 100, weight 32768, valid, sourced, best
      rx pathid: 0, tx pathid: 0x0


csr2#show ip ospf data ext 1.1.1.1

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Type-5 AS External Link States

  LS age: 358
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x8B0B
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 13

csr2#show ip route 1.1.1.1
Routing entry for 1.1.1.1/32
  Known via "ospf 1", distance 110, metric 1
  Tag 13, type extern 2, forward metric 1
  Redistributing via bgp 2
  Advertised by bgp 2 match internal external 1 & 2
  Last update from 10.0.23.3 on GigabitEthernet3, 00:28:14 ago
  Routing Descriptor Blocks:
  * 10.0.23.3, from 3.3.3.3, 00:28:14 ago, via GigabitEthernet3
      Route metric is 1, traffic share count is 1
      Route tag 13

csr3#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 3
  1, (received & used)
    10.0.13.1 from 10.0.13.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

csr3#show ip ospf data ext 1.1.1.1

            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Type-5 AS External Link States

  LS age: 375
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x8B0B
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 13

csr3#show ip route 1.1.1.1
Routing entry for 1.1.1.1/32
  Known via "bgp 2", distance 20, metric 0
  Tag 1, type external
  Redistributing via ospf 1
  Advertised by ospf 1 subnets tag 13
  Last update from 10.0.13.1 00:36:12 ago
  Routing Descriptor Blocks:
  * 10.0.13.1, from 10.0.13.1, 00:36:12 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
      Route tag 1
      MPLS label: none

Here, if I shut/noshut csr2:G3 (to drop/re-estab ospf adj), then csr2 will route to 1.1.1.1 via bgp again.

What's causing this behavior and how to fix this?

Thanks,

Rick.

rick505d3 · ‎05-03-2021

Hi Elliot,

Duplicate post got created when this one got marked as Spam, don't know why, and not visible on the forum for some time. @Moderator , please delete this post. Actual post with discussion / solution HERE.

Regards,

Rick.

View solution in original post

Elliot Dierksen · ‎05-03-2021

It is kind of hard to say for sure without seeing the configs, but I wonder if the problem is that you are sourcing BGP routes from a loopback interface, but there isn't a route to that IP (note the "incomplete" in the BGP table). I don't see the BGP ASN's in there either, but an iBGP route (same ASN) would be less preferred than an OSPF route based on admin distance (iBGP = 200, OSPF = 110, eBGP = 20).

rick505d3 · ‎05-03-2021

Hi Elliot,

Duplicate post got created when this one got marked as Spam, don't know why, and not visible on the forum for some time. @Moderator , please delete this post. Actual post with discussion / solution HERE.

Regards,

Rick.

paul driver · ‎05-03-2021

Hello
Duplicate post -

Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul