HP 6125XLG - Page 87

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Configuring route summarization when redistributing routes into OSPF on an ASBR
Without route summarization, an ASBR advertises each redistributed route in a separate ASE LSA. After
you configure a summary route, the ASBR advertises only the summary route in an ASE LSA instead of
more specific routes, reducing the number of LSAs in the LSDB.
The ASBR summarizes redistributed Type-5 LSAs within the specified address range. If the ASBR is in an
NSSA area, it also summarizes Type-7 LSAs within the specified address range. If the ASBR is also the
ABR, it summarizes Type-5 LSAs translated from Type-7 LSAs.
To configure route summarization when redistributing routes into OSPF on an ASBR:
Ste
p
Command Remarks
1. Enter system view.
system-view N/A
2. Enter OSPF view.
ospf [ process-id | router-id router-id
| vpn-instance vpn-instance-name ]*
N/A
3. Configure ASBR route
summarization.
asbr-summary ip-address
{ mask-length | mask } [ cost cost |
not-advertise | nssa-only | tag tag ] *
By default, no ASBR route
summarization is configured.
The command takes effect only on an
ASBR.
Configuring discard routes for summary networks
Discard routes help prevent routing black holes when route summarization is configured on ABRs and
ASBRs.
During route summarization, an ABR or ASBR generates a discard route for the summary network. The
destination and output interface of the discard route is the summary network and interface Null 0. When
receiving packets destined for a nonexistent network that is a part of the summary network, the ABR or
ASBR discards the packets according to the discard route.
For example, Router A summarizes networks 19.1.1.0/24, 19.1.2.0/24, and 19.1.3.0/24 into network
19.1.0.0/16, and advertises the summary network to Router B. When Router B receives a packet destined
for 19.1.4.0/24, Router B forwards the packet to Router A according to the summary route. Because no
specific route to 19.1.4.0/24 exists, Router A discards the packet according to the discard route.
To configure discard routes for summary networks:
Ste
p
Command Remarks
1. Enter system view.
system-view N/A
2. Enter OSPF view.
ospf [ process-id | router-id router-id
| vpn-instance vpn-instance-name ] *
N/A
3. Configure discard routes
for summary networks.
discard-route { external
{ external-preference | suppression }
| internal { internal-preference |
suppression } } *
By default:
• The ABR or ASBR generates
discard routes for summary
networks.
• The preference of discard
routes is 255.