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OSPF ABR

Hi,

I have some questions about ABR and intra area router. If the router gets "O IA" routes, the "via" ip address should be the ip address of ABR.

r3.3.3.3#show ip route ospf

O IA 4.0.0.0/8 [110/75] via 6.0.0.2, 00:07:59, ATM2/0.20

O IA 5.0.0.0/8 [110/65] via 6.0.0.2, 00:07:59, ATM2/0.20

please visit folloiwng link:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9ed.shtml

If the router gets "O" routes, the "via" ip address should be the ip address of intra area router.

r2.2.2.2#show ip route ospf

O 4.0.0.0/8 [110/74] via 5.0.0.1, 00:06:55, Serial0/1/0

please visit folloiwng link:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9ed.shtml

Is it correct?

Rgds

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

Re: OSPF ABR

Hello Anita,

I'm not sure what you mean here.

One thing is the OSPF router-id of the LSA owner and another is the ip next-hop to reach the route described in the LSA data structure.

The advertising router of an O IA route, an LSA type 3, is the OSPF router-id of the ABR

you can see by checking the database

Summary Net Link States (Area 1)

Link ID ADV Router Age Seq# Checksum

6.0.0.0 2.2.2.2 94 0x8000001F 0xCC43

from first link and also there is the detail of the LSA

r2.2.2.2#show ip ospf database summary 6.0.0.0

OSPF Router with ID (2.2.2.2) (Process ID 2)

Summary Net Link States (Area 1)

LS age: 216

Options: (No TOS-capability, DC)

LS Type: Summary Links(Network)

Link State ID: 6.0.0.0

!--- 6.0.0.0/8 is advertised into area 1 by the ABR(2.2.2.2).

Advertising Router: 2.2.2.2

LS Seq Number: 8000001F

Checksum: 0xCC43

Length: 28

Network Mask: /8

TOS: 0 Metric: 1

the via field in the sh ip route should be the IP address of next-hop = source ip address of the OSPF packet containing the LSA type 3.

Actually,

in

r3.3.3.3#show ip route ospf

O IA 4.0.0.0/8 [110/75] via 6.0.0.2, 00:07:59, ATM2/0.20

O IA 5.0.0.0/8 [110/65] via 6.0.0.2, 00:07:59, ATM2/0.20

6.0.0.2 is the ip address of the next-hop router but it is not necessary to be the ABR router (think of a chain of three routers in area1)

the second show is clear.

The difference can be seen at the database level but sh ip route provides the next hop for both O and O IA routes.

For an O route the LSA Adv router is that of the router that is owner of the router LSA.

For an O IA route the LSA adv router is that of the ABR that performed LSA type 3 translation.

Hope to help

Giuseppe

Re: OSPF ABR

Hello,

The next-hop shown in the routing table (shown after the word "via") for OSPF routes is a result of the SPF calculation. The advertising/originating router might be several hops away (a neighbor that floods an LSA is not necessarily the originator of the LSA). The potential recursive lookups are resolved while the SPF algorithm finds the best routes and those routes are eligible to be put in the routing table together with their associated next-hops. We might receive the same LSA due to flooding from various routers. The next-hop will be determined by the result of our SPF calculation based on the OSPF topology database we hold. As far as I know, it is not determined by which router flooded the LSA to us, though a neighbor that can flood us may have many chances to become the actual next-hop. Flooding does not normally modify LSAs (a database of a single area has the ambition to be the same in all routers in the area) and I think the philosophy of the algorithm does not require to keep track (store in memory) the router that actually handed off a particular LSA to us. We have the advertising router in the LSA and tones of other LSAs to help us figure things out.

Kind Regards,

M.

p.s. For details have a look at section "16.1.1. The next hop calculation" from RFC2328:

"This section explains how to calculate the current set of next hops to use for a destination. Each next hop consists of the outgoing interface to use in forwarding packets to the destination together with the IP address of the next hop router (if any). The next hop calculation is invoked each time a shorter path to the destination is discovered. This can happen in either stage of the shortest-path tree calculation (see Section 16.1)."..." Input to the next hop calculation is a) the destination and b) its parent in the current shortest path between the root (the calculating router) and the destination. The parent is always a transit vertex (i.e., always a router or a transit network)."

3 REPLIES
Hall of Fame Super Silver

Re: OSPF ABR

Hello Anita,

I'm not sure what you mean here.

One thing is the OSPF router-id of the LSA owner and another is the ip next-hop to reach the route described in the LSA data structure.

The advertising router of an O IA route, an LSA type 3, is the OSPF router-id of the ABR

you can see by checking the database

Summary Net Link States (Area 1)

Link ID ADV Router Age Seq# Checksum

6.0.0.0 2.2.2.2 94 0x8000001F 0xCC43

from first link and also there is the detail of the LSA

r2.2.2.2#show ip ospf database summary 6.0.0.0

OSPF Router with ID (2.2.2.2) (Process ID 2)

Summary Net Link States (Area 1)

LS age: 216

Options: (No TOS-capability, DC)

LS Type: Summary Links(Network)

Link State ID: 6.0.0.0

!--- 6.0.0.0/8 is advertised into area 1 by the ABR(2.2.2.2).

Advertising Router: 2.2.2.2

LS Seq Number: 8000001F

Checksum: 0xCC43

Length: 28

Network Mask: /8

TOS: 0 Metric: 1

the via field in the sh ip route should be the IP address of next-hop = source ip address of the OSPF packet containing the LSA type 3.

Actually,

in

r3.3.3.3#show ip route ospf

O IA 4.0.0.0/8 [110/75] via 6.0.0.2, 00:07:59, ATM2/0.20

O IA 5.0.0.0/8 [110/65] via 6.0.0.2, 00:07:59, ATM2/0.20

6.0.0.2 is the ip address of the next-hop router but it is not necessary to be the ABR router (think of a chain of three routers in area1)

the second show is clear.

The difference can be seen at the database level but sh ip route provides the next hop for both O and O IA routes.

For an O route the LSA Adv router is that of the router that is owner of the router LSA.

For an O IA route the LSA adv router is that of the ABR that performed LSA type 3 translation.

Hope to help

Giuseppe

Re: OSPF ABR

Hello,

The next-hop shown in the routing table (shown after the word "via") for OSPF routes is a result of the SPF calculation. The advertising/originating router might be several hops away (a neighbor that floods an LSA is not necessarily the originator of the LSA). The potential recursive lookups are resolved while the SPF algorithm finds the best routes and those routes are eligible to be put in the routing table together with their associated next-hops. We might receive the same LSA due to flooding from various routers. The next-hop will be determined by the result of our SPF calculation based on the OSPF topology database we hold. As far as I know, it is not determined by which router flooded the LSA to us, though a neighbor that can flood us may have many chances to become the actual next-hop. Flooding does not normally modify LSAs (a database of a single area has the ambition to be the same in all routers in the area) and I think the philosophy of the algorithm does not require to keep track (store in memory) the router that actually handed off a particular LSA to us. We have the advertising router in the LSA and tones of other LSAs to help us figure things out.

Kind Regards,

M.

p.s. For details have a look at section "16.1.1. The next hop calculation" from RFC2328:

"This section explains how to calculate the current set of next hops to use for a destination. Each next hop consists of the outgoing interface to use in forwarding packets to the destination together with the IP address of the next hop router (if any). The next hop calculation is invoked each time a shorter path to the destination is discovered. This can happen in either stage of the shortest-path tree calculation (see Section 16.1)."..." Input to the next hop calculation is a) the destination and b) its parent in the current shortest path between the root (the calculating router) and the destination. The parent is always a transit vertex (i.e., always a router or a transit network)."

Re: OSPF ABR

Hello,

Just to comment on something I wrote, in particular the phrase "I think the philosophy of the algorithm does not require to keep track (store in memory) the router that actually handed off a particular LSA to us". Although I think this is not required, the router actually gives us this information if we issue a "sh ip route" for a route known via OSPF (output says something like "last update from router X some time ago"), so this information is stored in memory in cisco routers. Anyway, this might well be an implementation choice and the implementors can comment better on it.

Kind Regards,

M.

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