How do you summarize routes in OSPF at an ABR?
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Say this first
With area <id> range on the ABR, where <id> is the area that contains the specific routes. The ABR then
sends one Type 3 for the range into the other areas instead of one per subnet, as long as at least one
component route exists. IOS also installs a discard route to Null0 for the summary on the ABR.
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Example
Area 1 holds 10.1.0.0/24 to 10.1.15.0/24. R2 is the ABR between area 0 and area 1.
router ospf 1
area 1 range 10.1.0.0 255.255.240.0On R2:
R2# show ip route ospf | include Null0
O 10.1.0.0/20 is a summary, 00:03:12, Null0In area 0 every router now sees one O IA 10.1.0.0/20 instead of sixteen /24s.
What you gain and lose
- Gain: smaller tables in other areas, and a flap of one /24 in area 1 no longer changes anything outside it.
- Lose: detail. Traffic may take a longer path through the wrong ABR if two ABRs both advertise the summary and only one has a good path to a given subnet.
Options
area 1 range 10.1.0.0 255.255.240.0 not-advertisehides the range entirely. It is a crude filter.area 1 range ... cost 50sets the cost of the summary.
Limits
You can only summarize at an ABR or an ASBR. Inside an area every router must have the full LSDB.
They will ask next
Why does IOS add the Null0 route?
To prevent a loop. If a packet arrives for
10.1.9.0/24and that subnet does not exist, R2 drops it instead of following a default route back toward the sender.Is the range configured with the area the routes are in, or the area they are sent into?
The area the routes are in.
area 1 rangesummarizes area 1's prefixes as they leave area 1.
Traps
- Using
summary-addresson an ABR for internal routes. That command is for external routes on an ASBR. - Trying to summarize on an internal router.