What is the difference between E1 and E2 external routes, and which is the default?
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Say this first
E2 is the default. Its metric is only the external cost set at the ASBR, the same on every router, so it ignores how far away the ASBR is. E1 adds the internal cost to reach the ASBR, so it picks the closest exit. An E1 route always beats an E2 route for the same prefix, whatever the numbers.
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Defaults on IOS
- Redistributed routes get metric type 2 and seed metric 20.
- Routes redistributed from BGP get seed metric 1.
default-information originateproduces an E2 default with metric 1.
Tie breaking inside E2
With two ASBRs advertising the same E2 prefix with the same metric, routers compare the forward metric, the internal cost to each ASBR, and pick the lower one. So E2 is not totally blind, it just shows the same number in the routing table.
Choosing
router ospf 1
redistribute static metric 50 metric-type 1- E1 when there are several exits and each router should use the nearest one.
- E2 when there is one exit, or when the external cost should dominate, for example to keep a backup ISP exit as backup with a higher external metric.
In the routing table
O E1 198.51.100.0/24 [110/52] via 10.0.12.2, 00:02:11, GigabitEthernet0/0/0
O E2 203.0.113.0/24 [110/20] via 10.0.12.2, 00:02:11, GigabitEthernet0/0/0The E1 route shows 50 plus an internal cost of 2. The E2 route shows only its seed metric.
They will ask next
What are N1 and N2?
The same two metric types for Type 7 routes inside an NSSA. After the ABR translates them to Type 5, routers in other areas see them as E1 or E2.
Traps
- Saying E2 routes are compared by metric only. Equal E2 metrics fall back to the cost to the ASBR.
- Thinking a low E2 metric beats an E1 route. Type 1 externals always win over Type 2.