Apr
04

Hi Brian,

What is the major difference in using an E1 route over an E2 route in OSPF?

From what I’ve observed, if you redistribute a route into OSPF either E1 or E2, the upstream router will still use the shortest path to get to the ASBR regardless of what is shown in the routing table.

The more I read about this, the more confused I get. Am I missing something?

Matt

Hi Matt,

This is actually a very common area of confusion and misunderstanding in OSPF. Part of the problem is that the vast majority of CCNA and CCNP texts teach the theory that for OSPF path selection of E1 vs E2 routes, E1 routes use the redistributed cost plus the cost to the ASBR, while with E2 routes only use the redistributed cost. When I just checked the most recent CCNP ROUTE text from Cisco Press, it specifically says that “[w]hen flooded, OSPF has little work to do to calculate the metric for an E2 route, because by definition, the E2 route’s metric is simply the metric listed in the Type 5 LSA. In other words, the OSPF routers do not add any internal OSPF cost to the metric for an E2 route.” While technically true, this statement is an oversimplification. For CCNP level, this might be fine, but for CCIE level it is not.

The key point that I’ll demonstrate in this post is that while it is true that “OSPF routers do not add any internal OSPF cost to the metric for an E2 route”, both the intra-area and inter-area cost is still considered in the OSPF path selection state machine for these routes.

First, let’s review the order of the OSPF path selection process. Regardless of a route’s metric or administrative distance, OSPF will choose routes in the following order:

Intra-Area (O)
Inter-Area (O IA)
External Type 1 (E1)
External Type 2 (E2)
NSSA Type 1 (N1)
NSSA Type 2 (N2)

To demonstrate this, take the following topology:

R1 connects to R2 and R3 via area 0. R2 and R3 connect to R4 and R5 via area 1 respectively. R4 and R5 connect to R6 via another routing domain, which is EIGRP in this case. R6 advertises the prefix 10.1.6.0/24 into EIGRP. R4 and R5 perform mutual redistribution between EIGRP and OSPF with the default parameters, as follows:

R4:
router eigrp 10
 redistribute ospf 1 metric 100000 100 255 1 1500
!
router ospf 1
 redistribute eigrp 10 subnets

R5:
router eigrp 10
 redistribute ospf 1 metric 100000 100 255 1 1500
!
router ospf 1
 redistribute eigrp 10 subnets

The result of this is that R1 learns the prefix 10.1.6.0/24 as an OSPF E2 route via both R2 and R3, with a default cost of 20. This can be seen in the routing table output below. The other OSPF learned routes are the transit links between the routers in question.

R1#sh ip route ospf
     10.0.0.0/24 is subnetted, 8 subnets
O E2    10.1.6.0 [110/20] via 10.1.13.3, 00:09:43, FastEthernet0/0.13
                 [110/20] via 10.1.12.2, 00:09:43, FastEthernet0/0.12
O IA    10.1.24.0 [110/2] via 10.1.12.2, 00:56:44, FastEthernet0/0.12
O E2    10.1.46.0 [110/20] via 10.1.13.3, 00:09:43, FastEthernet0/0.13
                  [110/20] via 10.1.12.2, 00:09:43, FastEthernet0/0.12
O IA    10.1.35.0 [110/2] via 10.1.13.3, 00:56:44, FastEthernet0/0.13
O E2    10.1.56.0 [110/20] via 10.1.13.3, 00:09:43, FastEthernet0/0.13
                  [110/20] via 10.1.12.2, 00:09:43, FastEthernet0/0.12

Note that all the routes redistributed from EIGRP appear on R1 with a default metric of 20. Now let’s examine the details of the route 10.1.6.0/24 on R1.

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.1.13.3 on FastEthernet0/0.13, 00:12:03 ago
  Routing Descriptor Blocks:
    10.1.13.3, from 10.1.5.5, 00:12:03 ago, via FastEthernet0/0.13
      Route metric is 20, traffic share count is 1
  * 10.1.12.2, from 10.1.4.4, 00:12:03 ago, via FastEthernet0/0.12
      Route metric is 20, traffic share count is 1

As expected, the metric of both paths via R2 and R3 have a metric of 20. However, there is an additional field in the route’s output called the “forward metric”. This field denotes the cost to the ASBR(s). In this case, the ASBRs are R4 and R5 for the routes via R2 and R3 respectively. Since all interfaces are FastEthernet, with a default OSPF cost of 1, the cost to both R4 and R5 is 2, or essentially 2 hops.

The reason that multiple routes are installed in R1’s routing table is that the route type (E2), the metric (20), and the forward metric (2) are all a tie. If any of these fields were to change, the path selection would change.

To demonstrate this, let’s change the route type to E1 under R4’s OSPF process. This can be accomplished as follows:

R4#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 1
R4(config-router)#end
R4#

The result of this change is that R1 now only installs a single route to 10.1.6.0/24, the E1 route learned via R2.

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 22, type extern 1
  Last update from 10.1.12.2 on FastEthernet0/0.12, 00:00:35 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 10.1.4.4, 00:00:35 ago, via FastEthernet0/0.12
      Route metric is 22, traffic share count is 1

Note that the metric and the forward metric seen in the previous E2 route is now collapsed into the single “metric” field of the E1 route. Although the value is technically the same, a cost of 2 to the ASBR, and the cost of 20 the ASBR reports in, the E1 route is preferred over the E2 route due to the OSPF path selection state machine preference. Even if we were to raise the metric of the E1 route so that the cost is higher than the E2 route, the E1 route would be preferred:

R4#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 1 metric 100
R4(config-router)#end
R4#

R1 still installs the E1 route, even though the E1 metric of 102 is higher than the E2 metric of 20 plus a forward metric of 2.

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 102, type extern 1
  Last update from 10.1.12.2 on FastEthernet0/0.12, 00:00:15 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 10.1.4.4, 00:00:15 ago, via FastEthernet0/0.12
      Route metric is 102, traffic share count is 1

R1 still knows about both the E1 and the E2 route in the Link-State Database, but the E1 route must always be preferred:

R1#show ip ospf database external 10.1.6.0

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 64
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.4.4
  LS Seq Number: 80000003
  Checksum: 0x1C8E
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0
        Metric: 100
        Forward Address: 0.0.0.0
        External Route Tag: 0

  LS age: 1388
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.5.5
  LS Seq Number: 80000001
  Checksum: 0x7307
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

This is the behavior we would expect, because E1 routes must always be preferred over E2 routes. Now let’s look at some of the commonly misunderstood cases, where the E2 routes use both the metric and the forward metric for their path selection.

First, R4’s redistribution is modified to return the metric-type to E2, but to use a higher metric of 100 than the default of 20:

R4#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 2 metric 100
R4(config-router)#end
R4#

The result on R1 is that the route via R4 is less preferred, since it now has a metric of 100 (and still a forward metric of 2) vs the metric of 20 (and the forward metric of 2) via R5.

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.1.13.3 on FastEthernet0/0.13, 00:00:30 ago
  Routing Descriptor Blocks:
  * 10.1.13.3, from 10.1.5.5, 00:00:30 ago, via FastEthernet0/0.13
      Route metric is 20, traffic share count is 1

The alternate route via R4 can still be seen in the database.

R1#show ip ospf database external 10.1.6.0

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 34
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.4.4
  LS Seq Number: 80000004
  Checksum: 0x9D8B
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 100
        Forward Address: 0.0.0.0
        External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 1653
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.5.5
  LS Seq Number: 80000001
  Checksum: 0x7307
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

This is the path selection that we would ideally want, because the total cost of the path via R4 is 102 (metric of 100 plus a forward metric of 2), while the cost of the path via R5 is 22 (metric of 20 plus a forward metric of 2). The result of this path selection would be the same if we were to change both routes to E1, as seen below.

R4#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 1 metric 100
R4(config-router)#end
R4#

R5#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R5(config)#router ospf 1
R5(config-router)#redistribute eigrp 10 subnets metric-type 1
R5(config-router)#end
R5#

R1 still chooses the route via R5, since this has a cost of 22 vs R4’s cost of 102.

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 22, type extern 1
  Last update from 10.1.13.3 on FastEthernet0/0.13, 00:00:41 ago
  Routing Descriptor Blocks:
  * 10.1.13.3, from 10.1.5.5, 00:00:41 ago, via FastEthernet0/0.13
      Route metric is 22, traffic share count is 1

R1#show ip ospf database external 10.1.6.0

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 56
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.4.4
  LS Seq Number: 80000005
  Checksum: 0x1890
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0
        Metric: 100
        Forward Address: 0.0.0.0
        External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 45
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.5.5
  LS Seq Number: 80000003
  Checksum: 0xEB0D
  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: 0

R1#

Note that the E1 route itself in the database does not include the cost to the ASBR. This must be calculated separately either based on the Type-1 LSA or Type-4 LSA, depending on whether the route to the ASBR is Intra-Area or Inter-Area respectively.

So now this begs the question, why does it matter if we use E1 vs E2? Of course as we saw E1 is always preferred over E2, due to the OSPF path selection order, but what is the difference between having *all* E1 routes vs having *all* E2 routes? Now let’s at a case where it *does* matter if you’re using E1 vs E2.

R1’s OSPF cost on the link to R2 is increased as follows:

R1#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface Fa0/0.12
R1(config-subif)#ip ospf cost 100
R1(config-subif)#end
R1#

R4 and R5’s redistribution is modified as follows:

R4#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 1 metric 99
R4(config-router)#end
R4#

R5#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R5(config)#router ospf 1
R5(config-router)#redistribute eigrp 10 subnets metric-type 1 metric 198
R5(config-router)#end
R5#

Now R1’s routes to the prefix 10.1.6.0/24 are as follows: Path 1 via the link to R2 with a cost of 100, plus the link to R4 with a cost of 1, plus the redistributed metric of 99, making this total path a cost of 200. Next, Path 2 is available via the link to R3 with a cost of 1, plus the link to R5 with a cost of 1, plus the redistributed metric of 198, masking this total path a cost of 200 as well. The result is that R1 installs both paths equally:

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 200, type extern 1
  Last update from 10.1.12.2 on FastEthernet0/0.12, 00:02:54 ago
  Routing Descriptor Blocks:
  * 10.1.13.3, from 10.1.5.5, 00:02:54 ago, via FastEthernet0/0.13
      Route metric is 200, traffic share count is 1
    10.1.12.2, from 10.1.4.4, 00:02:54 ago, via FastEthernet0/0.12
      Route metric is 200, traffic share count is 1

Note that the database lists the costs of the Type-5 External LSAs as different though:

R1#show ip ospf database external 10.1.6.0

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 291
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.4.4
  LS Seq Number: 80000006
  Checksum: 0xC9C
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0
        Metric: 99
        Forward Address: 0.0.0.0
        External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 207
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.5.5
  LS Seq Number: 80000004
  Checksum: 0xE460
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0
        Metric: 198
        Forward Address: 0.0.0.0
        External Route Tag: 0

What happens if we were to change the metric-type to 2 on both R4 and R5 now? Let’s see:

R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 2 metric 99
R4(config-router)#end
R4#

R5#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R5(config)#router ospf 1
R5(config-router)#redistribute eigrp 10 subnets metric-type 2 metric 198
R5(config-router)#end
R5#

Even though the end-to-end costs are still the same, R1 should now prefer the path with the lower redistributed metric via R4:

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 99, type extern 2, forward metric 101
  Last update from 10.1.12.2 on FastEthernet0/0.12, 00:01:09 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 10.1.4.4, 00:01:09 ago, via FastEthernet0/0.12
      Route metric is 99, traffic share count is 1

The forward metric of this route means that the total cost is still 200 (the metric of 99 plus the forward metric of 101). In this case, even though both paths are technically equal, only the path with the lower redistribution metric is installed. Now let’s see what happens if we do set the redistribution metric the same.

R4#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 10 subnets metric-type 2 metric 1
R4(config-router)#end
R4#

R5#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R5(config)#router ospf 1
R5(config-router)#redistribute eigrp 10 subnets metric-type 2 metric 1
R5(config-router)#end
R5#

Both routes now have the same metric of 1, so both should be installed in R1’s routing table, right? Let’s check:

R1#show ip route 10.1.6.0
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 1, type extern 2, forward metric 2
  Last update from 10.1.13.3 on FastEthernet0/0.13, 00:00:42 ago
  Routing Descriptor Blocks:
  * 10.1.13.3, from 10.1.5.5, 00:00:42 ago, via FastEthernet0/0.13
      Route metric is 1, traffic share count is 1

This is the result we may not expect. Only the path via R5 is installed, not the path via R4. Let’s look at the database and see why:

R1#show ip ospf database external 10.1.6.0

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 56
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.4.4
  LS Seq Number: 80000008
  Checksum: 0xB3D4
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 47
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.1.6.0 (External Network Number )
  Advertising Router: 10.1.5.5
  LS Seq Number: 80000006
  Checksum: 0xAADD
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 0

Both of these routes show the same cost, as denoted by the “Metric: 1”, so why is one being chosen over the other? The reason is that in reality, OSPF External Type-2 (E2) routes *do* take the cost to the ASBR into account during route calculation. The problem though is that by looking at just the External LSA’s information, we can’t see why we’re choosing one over the other.

Now let’s go through the entire recursion process in the database to figure out why R1 is choosing the path via R5 over the path to R4.

First, as we saw above, R1 finds both routes to the prefix with a metric of 1. Since this is a tie, the next thing R1 does is determine if the route to the ASBR is via an Intra-Area path. This is done by looking up the Type-1 Router LSA for the Advertising Router field found in the Type-5 External LSA.

R1#show ip ospf database router 10.1.4.4

            OSPF Router with ID (10.1.1.1) (Process ID 1)
R1#show ip ospf database router 10.1.5.5

            OSPF Router with ID (10.1.1.1) (Process ID 1)
R1#

This output on R1 means that it does not have an Intra-Area path to either of the ASBRs advertising these routes. The next step is to check if there is an Inter-Area path. This is done by examining the Type-4 ASBR Summary LSA.

R1#show ip ospf database asbr-summary 10.1.4.4

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Summary ASB Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 1889
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 10.1.4.4 (AS Boundary Router address)
  Advertising Router: 10.1.2.2
  LS Seq Number: 80000002
  Checksum: 0x24F3
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 1 

R1#show ip ospf database asbr-summary 10.1.5.5

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Summary ASB Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 1871
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 10.1.5.5 (AS Boundary Router address)
  Advertising Router: 10.1.3.3
  LS Seq Number: 80000002
  Checksum: 0x212
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 1

This output indicates that R1 does have Inter-Area routes to the ASBRs R4 and R5. The Inter-Area metric to reach them is 1 via ABRs R2 (10.1.2.2) and R3 (10.1.3.3) respectively. Now R1 needs to know which ABR is closer, R2 or R3? This is accomplished by looking up the Type-1 Router LSA to the ABRs that are originating the Type-4 ASBR Summary LSAs.

R1#show ip ospf database router 10.1.2.2

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Router Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 724
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.1.2.2
  Advertising Router: 10.1.2.2
  LS Seq Number: 8000000D
  Checksum: 0xA332
  Length: 36
  Area Border Router
  Number of Links: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.1.12.2
     (Link Data) Router Interface address: 10.1.12.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

R1#show ip ospf database router 10.1.3.3

            OSPF Router with ID (10.1.1.1) (Process ID 1)

                Router Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 1217
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.1.3.3
  Advertising Router: 10.1.3.3
  LS Seq Number: 80000010
  Checksum: 0x9537
  Length: 36
  Area Border Router
  Number of Links: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.1.13.1
     (Link Data) Router Interface address: 10.1.13.3
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

This output indicates that R2 and R3 are adjacent with the Designated Routers 10.1.12.2 and 10.1.13.3 respectively. Since R1 is also adjacent with these DRs, the cost from R1 to the DR is now added to the path.

R1#show ip ospf database router 10.1.1.1

OSPF Router with ID (10.1.1.1) (Process ID 1)

Router Link States (Area 0)

LS age: 948
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 10.1.1.1
Advertising Router: 10.1.1.1
LS Seq Number: 8000000F
Checksum: 0x6FA6
Length: 60
Number of Links: 3

Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.1.1.1
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1

Link connected to: a Transit Network
(Link ID) Designated Router address: 10.1.13.1
(Link Data) Router Interface address: 10.1.13.1
Number of TOS metrics: 0
TOS 0 Metrics: 1

Link connected to: a Transit Network
(Link ID) Designated Router address: 10.1.12.2
(Link Data) Router Interface address: 10.1.12.1
Number of TOS metrics: 0
TOS 0 Metrics: 100

R1 now knows that its cost to the DR 10.1.12.2 is 100, who is adjacent with R2, whose cost to R4 is 1, whose redistributed metric is 1. R1 also now knows that its cost to the DR 10.1.13.3 is 1, who is adjacent with R3, whose cost to R5 is 1, whose redistributed metric is 1. This means that the total cost to go to 10.1.6.0 via the R1 -> R2 -> R4 path is 102, while the total cost to go to 10.1.6.0 via the R1 -> R3 -> R5 path is 3.

The final result of this is that R1 chooses the shorter path to the ASBR, which is the R1 -> R3 -> R5 path. Although the other route to the prefix is via an E2 route with the same external cost, one is preferred over another due to the shorter ASBR path.

Based on this we can see that both E1 and E2 routes take both the redistributed cost and the cost to the ASBR into account when making their path selection. The key difference is that E1 is always preferred over E2, followed by the E2 route with the lower redistribution metric. If multiple E2 routes exist with the same redistribution metric, the path with the lower forward metric (metric to the ASBR) is preferred. If there are multiple E2 routes with both the same redistribution metric and forward metric, they can both be installed in the routing table. Why does OSPF do this though? Originally this stems from the design concepts of “hot potato” and “cold potato” routing.

Think of a routing domain learning external routes. Typically those prefixes have some “external” metric associated with them – for example, E2 external metric or the BGP MED attribute value. If the routers in the local domain select the exit point based on the external metric they are said to perform “cold potato” routing. This means that the exit point is selected based on the external metric preference, e.g. distances to the prefix in the bordering routing system. This optimizes link utilization in the external system but may lead to suboptimal path selection in the local domain. Conversely, “hot potato” routing is the model where the exit point selection is performed based on the local metric to the exit point associated with the prefix. In other words, “hot potato” model tries to push packets out of the local system as quick as possible, optimizing internal link utilization.

Now within the scope of OSPF, think of the E2 route selection process: OSPF chooses the best exit point based on the external metric and uses the internal cost to ASBR as a tie breaker. In other words, OSPF performs “cold potato” routing with respect to E2 prefixes. It is easy to turn this process into “hot potato” by ensuring that every exit point uses the same E2 metric value. It is also possible to perform other sorts of traffic engineering by selectively manipulating the external metric associated with the E2 route, allowing for full flexibility of exit point selection.

Finally, we approach E1. This type of routing is a hybrid of hot and cold routing models – external metrics are directly added to the internal metrics. This implicitly assumes that external metrics are “comparable” to the internal metrics. In turn, this means E1 is meant to be used with another OSPF domain that uses a similar metric system. This is commonly found in split/merge scenarios where you have multiple routing processes within the same autonomous system, and want to achieve optimum path selection accounting for both metrics in both systems. This is similar to the way EIGRP performs metric computation for external prefixes.

So there we have it. While it is technically true that “OSPF routers do not add any internal OSPF cost to the metric for an E2 route”, both the intra-area and inter-area cost can still be considered in the OSPF path selection regardless of whether the route is E1 or E2.

About Brian McGahan, CCIE #8593, CCDE #2013::13:

Brian McGahan was one of the youngest engineers in the world to obtain the CCIE, having achieved his first CCIE in Routing & Switching at the age of 20 in 2002. Brian has been teaching and developing CCIE training courses for over 10 years, and has assisted thousands of engineers in obtaining their CCIE certification. When not teaching or developing new products Brian consults with large ISPs and enterprise customers in the midwest region of the United States.

Find all posts by Brian McGahan, CCIE #8593, CCDE #2013::13 | Visit Website


You can leave a response, or trackback from your own site.

84 Responses to “Understanding OSPF External Route Path Selection”

 
  1. Matt says:

    Fantastic explanation.

    • Soske says:

      I learned something interesting today. Cold potato and hot potato. By the way any story behind why it was called that way?

  2. jdr says:

    Thanks Brian,
    Great explanation, I was looking for this for a while.

  3. Ash says:

    What a great explanation!!! Smooth.

  4. Amplebrain says:

    Interesting Read,

    This is definitely one of the myths that have been existing for so long!

    Thanks Brian

  5. Fabio says:

    Simply amazing!
    Many thanks.

  6. Berengere says:

    Perfectly explained.
    Thanks a lot!!!!

  7. Daniel says:

    Great explanation Brian. One myth busted, how about doing something like the mythbusters CCIE style? That would be cool. I have to say that the quality of posts have really improved lately. You and Petr are my favourite writers. Keep up the great posts and keep them coming at a regular pace :)

  8. stretch says:

    Nice article!

    Possible typo: I think R4 should be R3 in the phrase, “Next, Path 2 is available via the link to R4 with a cost of 1, plus the link to R5 with a cost of 1..”

  9. [...] the meantime, I came across this great article by the folks at INE. It discusses OSPF external route selection and highlights a mystery behind E1 [...]

  10. Shamim says:

    Great Post……Can you give another post for NSSA N1 and N2?

  11. Ian F says:

    WOW!!! Excellent example Brian, please include this level of detail in your new Advanced Technologies class if possible, this will be amazing!!! I think the new clases should tackle the technologies at this type of level, I mean they are called Advanced Technology classes after all :)

  12. Zabeel Musa says:

    Excellent post!

  13. Eugene Anderson says:

    Great post!

  14. AJN says:

    Brian just amazing!

  15. venky says:

    Thanks brain…every one can understand…wht a stuff…

  16. Radwan says:

    Brian , this super nice post … well-organized, detailed, step-by-step and comparative in a sleek way ! You should write some books !

    May I know what software did you use to draw the topology diagram ?

  17. Patrick says:

    my head hurts.

  18. mona says:

    great article…….thanks sooooo much

  19. Davy says:

    Very great and very clear!!!

  20. M.S.Khan says:

    Great Explanation ! Really I appriciate !!! I cleared all doubts regarding E1 and E2. Thanks a Ton.

  21. [...] A common misconception is that E2 does not use cost to ASBR when calculating metric. Read the post to understand why this is [...]

  22. Habib says:

    Brian Brain is amazing!!!!! Very Nice.

  23. Vasanthesh says:

    simply super..

  24. NIcky says:

    Brian

    You Rock,

    All details are crisp and clear.

    Thanks,
    Nicky

  25. Farooqui says:

    Amaaazzing.. Thanks alot Brian.. Crystal clear!!!

  26. Eirc says:

    Since the redistributed routes have forwarding addresses as 0.0.0.0. could you try changing the costs on R2′s f0/0.24 and R3′s f0/0.35 instead of during the redistribution. I guess R1 will prefer the less cost to R4 or R5, regardless of the path type E1 or E2.

  27. ashwani says:

    Amaaazzing.. Superb explanation….

  28. Jonathan says:

    Very well explained, as a neophyte, I’m surprised to have understood everything. One suggestion: put the IP addresses on the topology.

  29. sheno says:

    man u have a magic touch

    you convert any difficult and complicated topic to

    easy one

    how u do this !!!

  30. Matteo says:

    Hi Brian, your post clarified me greatly the differences between E1 and E2 but leaves me a question. When you say “This implicitly assumes that external metrics are “comparable” to the internal metrics”, this looks right to me (if sum two values they must be of the same type) but I don’t understand why. The external metrics are not taken from the external protocol but are setted to 20 by default or to another value with the redistribute command so why does the type of external metric matters?

    • Hi Matteo,

      You’re correct. The external metrics are not directly translated from the source protocol. This is expected as other IGPs like EIGRP use different formats for their metric calculation. What I mean by them be comparable is that since the seed metric is always 20 by default, the closest ASBR is always used as the exit point, which is a function of the internal cost not the external cost.

  31. Mahavir says:

    Hi ,

    I have one query regarding the ospf. Alway seen that E1 routes are prefer over E2 routes in Ospf, Can it possible that some time E2 routes are prefer in ospf ,if yes what is the condition for that

    • No, even if the E2 route’s metric is lower than the E1 route it still won’t be preferred.

      • rkid says:

        Hi Brian,
        But in the post you said that:
        “Regardless of a route’s metric or administrative distance, OSPF will choose routes in the following order:

        Intra-Area (O)
        Inter-Area (O IA)
        External Type 1 (E1)
        External Type 2 (E2)”

        So…which of these two statements is right – in the post or in the reply to the Mahavir’s question (sorry, maybe I’ve not understaned something…)?

  32. Patrick says:

    The ospf external route-tag value, does it correspond to a BGP AS number?

    I was going through a blog post on CLN..https://learningnetwork.cisco.com/thread/6097..

    for eg: My AS# is 3561 and external route tag value is 3489664489.. I am trying to decode this external value to match 3561, but i am not getting it..

  33. Ahamed Sadayan says:

    Brian,

    Great research and explanation on this part. I think E2 should not consider the forward metric for path selection. Otherwise there is no difference between E1 and E2. In my personal opinion I feel this might be a defect from Cisco side that need to be addressed. Is this behavior is consistent across all the networking gears in the Industry ?

    Thanks

  34. Vinicius Arcanjo says:

    Hi Brian, Thanks for explanation. Great article!

  35. Aliou says:

    timeless article, keep on reading over n over
    thanks Brian

  36. Bal Reddy says:

    Nice Posting on E1 and E2 behavior in OSPF, that I never come across in any of the text book.,.tnx Brain!!!!

  37. sandy says:

    Great Explaination…Now i got the clear clarification regards E1 and E2..thanks Brain…and we expecting same explanations more…

  38. Rolando says:

    Great article Brian, at first I thought that I fully understand ospf in ccnp level, but after reading your article, I have more digging to do…By the way your video for ccnp helps me a lot to pass the switching exam.

    Thank you.

  39. Nikhil says:

    Very Nice

  40. jacky says:

    Very nice……….

  41. Blaine says:

    Great article! I’ve been wondering why path cost still makes a difference with E2 routes. Been bugging me for awhile. I also didn’t know there was a preference order for ospf route types. Thanks for taking the time to write this up!

  42. Ujwal says:

    one word …awesome!!!

  43. Prem says:

    Amazing article!!!!, Very clearly explained…Anyone preparing for CCIE should read this once :)

  44. Wingnut says:

    Superb Thanks

  45. Umair says:

    Excellently explained! Haven’t seen such marvellous explanation before- Thanks

  46. Binoy B says:

    Hi Brian,

    I would like you to clarify. me on the below question
    Suppose R3-R5 route is a BGP–OSPF redistributed and R2 and R4 ospf area 0 , (R4 -R6 A 5-R6 are in Ospf Area 3 )

    R4 — E1 routes

    R5 – E2 (2000) route

    how can we make a subnet to be preferring R5 (eg: want to prefer 10.1.6.0/24 via L5 and all other R4

  47. brijesh says:

    grate,, method and stuff.

  48. Mark DeLong says:

    Great Explanation! Wish I found this in the ATC. I understood the forwarding metric concept as a tie breaker at a high level but it was great that you went through and showed how the cost to the ASBR was found. Also, good in the ATC would be an explanation of how this works with N1/2 routes which do not have a Type 4 LSA. Also, an explanation of who does the initial origination of the Type 4 LSA in this scenario (ASBR or ABR for the area) would be helpful as well. I would think the ABR creates the type 4 and advertises it into area 0 when it sees that the ASBR’s type 1 LSA has the e-bit set. But I just want to make sure that is how it works.

  49. Mark DeLong says:

    One quick question: In the case that you add another non-backbone area to this situation. I’m thinking that the Type 5 LSAs that get injected into the new area do not have their metric changed. But the Type 4 LSAs when injected into the new area has it’s metric changed to to account for the SPF path cost in area zero between the ABR’s and it’s advertising router id changes to the ABR that injected it into the new area. Is this right? And is this different for E1 vs. E2?

  50. brij says:

    Wonderful explanation!!!!!!!

  51. Arun says:

    g8 …. fabulous explanation

  52. Rui says:

    Excellently detailed explanation. I had never seen anywhere any mention to the “forward metric tie breaker”, so I was confused when in my network I saw routers with only one E2 route in the routing table (instead of the 2 E2 routes I expected to see).
    Thanks!

  53. Mohammad Khalil says:

    two thumbs up !

  54. dujn says:

    Excellently detailed explanation, thank you very much. Hope to see similar interesting explanation.

  55. Colin says:

    Excellent post, well explained..

  56. USMAN says:

    owesome . made another fane sir

  57. maybeccie says:

    I made this topology in GNS3…..

  58. maybeccie says:

    issue has been solved …..
    When i made my gns3 topology and i do sh ip route i see forward metric 20 …. how you people made it 2 ….. please tell me . i want to go step by step

  59. maybeccie says:

    I am using 1 Layer 2 switch of gns3 it will have ethernet ports… it cannot have Fast-Ethernet Ports …………. and All routers links terminate at this switch….
    I see this 20 forward metric on R1 … while on ur post i see its 2 …
    Just curious….

  60. maybeccie says:

    sir,
    I got ur point

  61. Atul jain says:

    Great explanation…

  62. Mohamed says:

    so amazing!!!

  63. Taran Malhotra says:

    Great Explanation!!

  64. Manish Joshi says:

    brain ur a rockstar!!!

  65. Stefan says:

    Great post, my appreciation.

  66. Vibha says:

    Brian from ine is doing very good work in network technologies.

  67. Shid says:

    Awesome post,nice explanation…..

  68. Sumit says:

    Hey Brain,
    Thank you for clear my concept, really appreciated..

  69. Sarwar says:

    Best post on e1 vs e2. Really appreciate!!

  70. delf says:

    Hi, Brain, greate post
    but i have one misunderstanding about
    “If multiple E2 routes exist with the same redistribution metric, the path with the lower forward metric (metric to the ASBR) is preferred.”
    that logic if one ASBR located in same area(intra-area ASBR), and another ASBR in different area(inter-area ASBR). In this case wil we preffer ASBR in same area or we will preffer ASBR with lowest cost regardless of area

  71. Yagya Sen says:

    Hi Brian,
    Awesome explanation. Could you please explain, in what situations one should redistribute as E1 routes and under what situation one should be redistributing as E2 routes?

  72. Sreepad says:

    Amazing! Good one.

 

Leave a Reply

Categories

CCIE Bloggers