05-27-2010 05:33 PM - edited 03-06-2019 11:18 AM
Hi, i've read http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080124c7d.shtml and it is stated
"When the metric of the redistributed route from multiple ASBRs are equal as illustrated in the document, the forwarding address changes the behavior of the type 5 LSA path selection. When a router receives two type 5 LSAs to the same destination with the forwarding addresses set on both LSAs, the router makes a comparison based on the metric to the forwarding addresses. The LSA with a forwarding address that offers the smaller metric is placed into the routing table.
If the metric of the redistributed routes are different, the routers prefer the route with the lowest metric and not the lowest metric to the forwarding address."
Well, I have similar case but different outcome.
Routing Bit Set on this LSA LS age: 1049 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 10.10.10.0 (External Network Number ) Advertising Router: 10.52.24.11 LS Seq Number: 80001072 Checksum: 0x8A61 Length: 36 Network Mask: /24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 10 Forward Address: 0.0.0.0 External Route Tag: 16 Routing Bit Set on this LSA LS age: 1174 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 10.10.10.0 (External Network Number ) Advertising Router: 10.52.56.1 LS Seq Number: 80001072 Checksum: 0xC008 Length: 36 Network Mask: /24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 12 Forward Address: 0.0.0.0 External Route Tag: 27 LS age: 1986 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 10.10.10.0 (External Network Number ) Advertising Router: 10.52.56.2 LS Seq Number: 80000022 Checksum: 0xDB44 Length: 36 Network Mask: /24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 27
From the above LSA Type-5, I should see the route prefers the 1st LSA as it has the lowest metric. However, it chooses 2nd LSA as it has the lower metric to ASBR.
Router#sh ip ospf border-routers OSPF Process 100 internal Routing Table Codes: i - Intra-area route, I - Inter-area route i 192.168.160.29 [1] via 192.168.160.29, Vlan750, ASBR, Area 0, SPF 92 i 192.168.160.129 [1] via 192.168.160.129, Vlan750, ASBR, Area 0, SPF 92 i 10.52.24.2 [2] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92 i 10.52.24.1 [2] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92 i 10.52.24.11 [2] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92 i 10.52.24.13 [1] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92 i 10.52.56.2 [1] via 10.52.63.246, Port-channel10, ASBR, Area 0, SPF 92 i 10.52.56.1 [1] via 10.52.63.242, Port-channel9, ASBR, Area 0, SPF 92 i 10.52.56.11 [1] via 10.52.63.130, Port-channel2, ASBR, Area 0, SPF 92
Can someone explain why?
rgds,
David Sudjiman
05-28-2010 03:03 AM
Hello David,
your LSAs are external type 1 not type 2 so metric to reach ASBR is summed to seed metric in external LSA
Metric Type: 1 (Comparable directly to link state metric)
This means that you need to calculate the SPF taking in account cost to reach ASBR
first LSA = 10 + 2
second LSA = 12 +1
it should choice the first, but there is one specific issue that I see:
first LSA :LS Seq Number: 80001072 second LSA: LS Seq Number: 80001072
This should not happen two LSAs generated by different OSPF routers should have different and unique sequence numbers for this reason the younger entry is not considered.
Are these real routers or you are running dynamips ?
Hope to help
Giuseppe
06-15-2010 06:49 PM
Giuseppe, Thanks for the explanation.
I'm aware of the metric calculation and you're right about it. This is a live in-production network.
Based on your explanation it should choose the metric first then older LSA seq #, but it's not.
Could someone please elaborate?
thx
David Sudjiman
06-15-2010 06:52 PM
It this because the next-hop
10.52.24.11 [2] and 10.52.56.1 [1]
Therefore is chooses 10.52.56.1?
thx
David Sudjiman
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