External routes: LSA types 4 and 5, E1 and E2

A partner network that only speaks EIGRP

EIGRP (Enhanced Interior Gateway Routing Protocol) and OSPF exchange different routing information. A router running only OSPF cannot read the partner's EIGRP messages as OSPF routes. Redistribution lets R5 take an eligible route learned from another source and advertise it into OSPF.

The company connects to a partner that runs EIGRP and owns 10.50.0.0/16. R5 speaks both protocols. Users everywhere need to reach the partner, but OSPF routers only know OSPF routes. R5 must redistribute the intended partner route into OSPF, and routers in the normal areas have to work out both the route and how to reach R5.

R5 becomes an autonomous system boundary router (ASBR), the router introducing external reachability. R2 and R4 are area border routers (ABRs), joining area 0 to areas 1 and 2. An area contains detailed OSPF topology records. A link-state advertisement (LSA) is one of those stored records, carried inside an OSPF update packet.

How one partner route travels through three areas

Why type 5 LSAs cross areas unchanged

A type 5 AS-external LSA describes the imported prefix, mask and external metric. R5 remains its advertising router as it floods through normal areas. ABRs do not originate a replacement type 5 at each boundary, although normal ageing, acknowledgement, refresh and retransmission still happen. Stub and NSSA boundaries do not admit these type 5 records.

A type 4 ASBR-summary LSA answers a different question: how far is the ASBR from this ABR? R4 describes R5 to area 0, and R2 describes reachability to R5 in area 1. R1 adds its own cost to R2. R1 therefore knows both the external prefix and a usable path towards its originator.

For these metrics, each of the four router-to-router outgoing interfaces from R1 to R5 costs 1. The partner-facing interface on R5 runs EIGRP and is outside OSPF, so the external LSA has forwarding address 0.0.0.0: use R5 as the forwarding target. This field is not a default route; the destination remains 10.50.0.0/16.

E2 versus E1 metrics

With E2, routers still know the internal cost to the ASBR (the forward metric) and use it to compare otherwise eligible E2 paths with the same external metric, for example from two ASBRs. More advanced ASBR-path preference rules can apply before that cost comparison; E2 does not simply mean the nearest exit in every design.

Route type beats metric

For the same prefix, OSPF prefers, in order:

1. intra-area (O),
2. inter-area (O IA),
3. external type 1 (O E1),
4. external type 2 (O E2).

Compare eligible paths and their route type before metrics. An NSSA uses N1/N2 route codes and has additional type 7 preference rules; do not use this normal-area list as a complete NSSA tie-break algorithm.

Redistributing the partner routes on R5

These Cisco IOS/IOS XE style commands begin in global configuration mode. EIGRP process 100 must already have the exact 10.50.0.0/16 route installed on R5. The filter below allows only that prefix; it does not permit every route R5 might learn from the partner. A route map applies the matching rule to redistribution.

Redistributing in both directions (OSPF into EIGRP and EIGRP into OSPF) on more than one router can create routing loops. Real networks use route maps and tags to control it; that is CCNP material.

Finding the type 4 and type 5 LSAs from R1

R1#show ip ospf database
            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         802         0x80000006 0x00A3F1 6
2.2.2.2         2.2.2.2         795         0x80000004 0x0051C2 2

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.23.0       2.2.2.2         790         0x80000001 0x00E21B
10.0.34.0       2.2.2.2         790         0x80000001 0x00D93A
10.2.45.0       2.2.2.2         790         0x80000001 0x0077C5

                Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
5.5.5.5         2.2.2.2         512         0x80000001 0x00F06D

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.50.0.0       5.5.5.5         515         0x80000001 0x00B8D4 0

R1#show ip route ospf | include 10.50
O E2     10.50.0.0/16 [110/20] via 10.1.12.2, 00:08:35, GigabitEthernet0/1

When external routes go missing or take the long way

• The ASBR has the routes but OSPF doesn't advertise them. No redistribute, or (on old IOS) no subnets, so only classful networks went across.
• External routes reach some areas but not one. That area is a stub, totally stubby or NSSA area, which blocks type 5 LSAs by design.
• Traffic uses a far exit. Two ASBRs advertise the same prefix as E2 with different seed metrics, so internal distance is ignored. Use E1, or equal seed metrics.

In practice, when an external prefix is present in every area but one, check that area's type before touching the ASBR: a stub flag is the usual answer.

Three misreadings of E1, E2 and type 4

• Expecting type 4 LSAs in the ASBR's own area. They are only needed in other areas.
• Thinking E2 routes ignore distance completely; the forward metric still breaks ties.
• Comparing an E1 and an O IA route by metric. The route type decides first.

Rules for external routes

Short lab: compare E2 and E1

In the isolated five-router lab, apply the filtered E2 example on R5. On R1, run show ip ospf database external 10.50.0.0 and show ip route 10.50.0.0. Confirm mask /16, advertising router 5.5.5.5, forwarding address zero, external metric 20 and installed E2 metric 20.

Replace only the redistribution line with the filtered E1 alternative. After convergence the route should be E1 metric 24. The seed did not change; the additional 4 is R1's internal cost to R5. Restore the E2 line and confirm metric 20 returns. Test an assigned partner host only after verifying the return route and permitting policy.

Sources: RFC 2328 external path calculation; Cisco forwarding-address explanation. Examples are documentation-based and not validated on a physical router image.

Test yourself on external routes