Re: ODDITY - OSPF E2 preferred above E1

As I understand it with the default route to null0 the E1 route is
effectively blackholed as we're adding the internal cost to the ASBR to the
external metric....

Where as the E2 route is using only the external cost from the ASBR
therefore it does not blackhole the traffic, would need to lab it I guess..

A great article from Brain here explaining E1/E2 costs

http://blog.ine.com/2011/04/04/understanding-ospf-external-route-path-selection/

Tony

On 11 April 2013 22:03, bitje <lj_at_cli.nu> wrote:

> Hi,
>
> I have a situation where a E1 route is being overriden by a E2 route.
> The E1 route is entering the area from a mpls superbackbone and the
> E2 from an ASBR in the same area. According to what I know an E1
> route should always win from a E2 route.. I feel that I am going
> to learn something here.
>
> R5 is connected to SW2 and R4.
>
> On SW2 the following is done:
> !
> ip route 12.12.12.12 255.255.255.255 Null0
> !
> router ospf 100
> router-id 0.0.0.8
> log-adjacency-changes
> redistribute static subnets
> network 155.1.58.8 0.0.0.0 area 0
> !
>
> On R4 the following is done:
> !
> router ospf 100 vrf TEST
> router-id 0.0.0.4
> log-adjacency-changes
> redistribute bgp 100 subnets
> network 155.1.45.4 0.0.0.0 area 0
> !
> router bgp 100
> no synchronization
> bgp log-neighbor-changes
> neighbor 6.6.6.6 remote-as 100
> neighbor 6.6.6.6 update-source Loopback10
> neighbor 204.12.1.254 remote-as 54
> no auto-summary
> !
> address-family vpnv4
> neighbor 6.6.6.6 activate
> neighbor 6.6.6.6 send-community extended
> exit-address-family
> !
> address-family ipv4 vrf TEST
> no synchronization
> exit-address-family
> !
>
> On R5 the following is done:
> !
> router ospf 100
> router-id 0.0.0.5
> log-adjacency-changes
> network 155.1.45.5 0.0.0.0 area 0
> network 155.1.58.5 0.0.0.0 area 0
> Rack1R5#
>
> Some relevant show commands:
>
> Rack1R5#show ip route ospf 100
> 12.0.0.0/32 is subnetted, 1 subnets
> O E2 12.12.12.12 [110/20] via 155.1.58.8, 05:41:13, FastEthernet0/0
> Rack1R5#
>
> Rack1R5#show ip ospf database external 12.12.12.12
>
> OSPF Router with ID (0.0.0.5) (Process ID 100)
>
> Type-5 AS External Link States
>
> Routing Bit Set on this LSA
> LS age: 1494
> Options: (No TOS-capability, DC)
> LS Type: AS External Link
> Link State ID: 12.12.12.12 (External Network Number )
> Advertising Router: 0.0.0.8
> LS Seq Number: 8000000B
> Checksum: 0x79E4
> Length: 36
> Network Mask: /32
> Metric Type: 2 (Larger than any link state path)
> TOS: 0
> Metric: 20
> Forward Address: 0.0.0.0
> External Route Tag: 0
>
> Rack1R5#
>
> Rack1R5#show ip ospf database router 0.0.0.8
>
> OSPF Router with ID (0.0.0.5) (Process ID 100)
>
> Router Link States (Area 0)
>
> Routing Bit Set on this LSA
> LS age: 1533
> Options: (No TOS-capability, DC)
> LS Type: Router Links
> Link State ID: 0.0.0.8
> Advertising Router: 0.0.0.8
> LS Seq Number: 80000024
> Checksum: 0xDF9B
> Length: 36
> AS Boundary Router
> Number of Links: 1
>
> Link connected to: a Transit Network
> (Link ID) Designated Router address: 155.1.58.5
> (Link Data) Router Interface address: 155.1.58.8
> Number of TOS metrics: 0
> TOS 0 Metrics: 200
>
>
> Rack1R5#
>
> Now if I remove the static route on SW2:
> Rack1SW2(config)#no ip route 12.12.12.12 255.255.255.255 Null0
>
> And ask R5 what external route it has we see that only after
> the static from SW2 has been removed R5 will accept the E1
> route since the E2 route is no longer available:
>
> Rack1R5#show ip ospf database external
>
> OSPF Router with ID (0.0.0.5) (Process ID 100)
>
> Type-5 AS External Link States
>
> Routing Bit Set on this LSA
> LS age: 55
> Options: (No TOS-capability, DC)
> LS Type: AS External Link
> Link State ID: 12.12.12.12 (External Network Number )
> Advertising Router: 0.0.0.4
> LS Seq Number: 80000001
> Checksum: 0xF046
> Length: 36
> Network Mask: /32
> Metric Type: 2 (Larger than any link state path)
> TOS: 0
> Metric: 20
> Forward Address: 0.0.0.0
> External Route Tag: 3489661028
>
> Rack1R5#
>
> Rack1R5#show ip ospf database router 0.0.0.4
>
> OSPF Router with ID (0.0.0.5) (Process ID 100)
>
> Router Link States (Area 0)
>
> Routing Bit Set on this LSA
> LS age: 820
> Options: (No TOS-capability, DC)
> LS Type: Router Links
> Link State ID: 0.0.0.4
> Advertising Router: 0.0.0.4
> LS Seq Number: 80000016
> Checksum: 0xAB3C
> Length: 48
> Area Border Router
> AS Boundary Router
> Number of Links: 2
>
> Link connected to: another Router (point-to-point)
> (Link ID) Neighboring Router ID: 0.0.0.5
> (Link Data) Router Interface address: 155.1.45.4
> Number of TOS metrics: 0
> TOS 0 Metrics: 64
>
> Link connected to: a Stub Network
> (Link ID) Network/subnet number: 155.1.45.0
> (Link Data) Network Mask: 255.255.255.0
> Number of TOS metrics: 0
> TOS 0 Metrics: 64
>
>
> Rack1R5#
>
> Your analysis/help is appreciated.
>
> Regards
>
>
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Received on Thu Apr 11 2013 - 22:46:30 ART