The
no-summary
command does not affect intra-area advertisements, meaning the switch still accepts summary
LSAs from OSPF neighbors within its area and floods them to other neighbors. The switch can form adjacencies
with other routers regardless of whether summarization is enabled or disabled for areas on each switch.
When you use
no-summary
, the change takes effect immediately. If you apply the option to a previously
configured area, the switch flushes all of the summary LSAs it has generated (as an ABR) from the area.
NOTE:
This feature applies only when the switch is configured as an ABR for a stub area or NSSA.
To completely prevent summary LSAs from injection into the area, use
no-summary
to disable the
summary LSAs on each OSPF router that is an ABR for the area.
To implement the above operation for a stub area or NSSA, enter a command such as the following:
switch(ospf)# area 40 stub 3 no-summary
Equal cost multi-path routing (ECMP)
The ECMP feature allows OSPF to add routes with multiple next-hop addresses and with equal costs to a given
destination in the forwarding information base (FIB) on the routing switch. For example, if you display the IP route
table by entering the
show ip route
command, multiple next-hop routers are listed for the same destination
network (21.0.9.0/24) as shown in following example.
Example of show ip route command output with multiple next-hop routes
switch show ip route
IP Route Entries
Destination Gateway VLAN Type Sub-Type Metric Dist.
------------------ --------------- ---- --------- ---------- ---------- -----
1.0.0.0/8 10.0.8.1 1 static 1 1
10.0.8.0/21 DEFAULT_VLAN 1 connected 1 0
12.0.9.0/24 VLAN3 3 connected 1 0
15.0.0.0/8 10.0.8.1 1 static 1 1
21.0.9.0/24 162.130.101.2 2 ospf IntraArea 2 110
21.0.9.0/24 162.130.101.3 2 ospf IntraArea 2 110
21.0.9.0/24 162.130.101.4 2 ospf IntraArea 2 110
127.0.0.0/8 reject static 0 0
127.0.0.1/32 lo0 connected 1 0
162.130.101.0/24 VLAN2 2 connected 1 0
For a given destination network in an OSPF domain, multiple ECMP next-hop routes can be
one
of the following
types.
• Intra-area (routes to the destination in the same OSPF area)
• Inter-area (routes to the destination through another OSPF area)
• External (routes to the destination through another AS)
Multiple ECMP next-hop routes cannot be a mixture of intra-area, inter-area, and external routes. For example, in
Example of show ip route command output with multiple next-hop routes
on page 255, the multiple next-
hop routes to network 21.0.9.0/24 are all intra-area.
Also, according to the distributed algorithm used in the selection of ECMP next-hop routes:
• Intra-area routes are preferred to inter-area routes.
• Inter-area routes are preferred to external routes through a neighboring AS.
Chapter 11 Open Shortest Path First Protocol (OSPF)
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