Address Summarization

OSPF is capable of supporting classless addressing functions such as subnetting and supernetting because it includes a network mask in conjunction with the prefixes carried in the various LSAs that are used to advertise reachability. The ability to support discontiguous subnets and address summarization is one of the main benefits that OSPF provides when compared to classful protocols like RIP version 1.

Address summarization and filtering can be performed at ABRs by specifying one or more area-range statements at either the[edit protocols ospf area area-number] or [edit protocols ospf area area-number nssa] hierarchical levels, depending on what types of routes are being summarized. An area range statement contained within the nssa stanza is used only to coalesce or filter type 7 LSAs before they are translated into type 5 externals for submission to the backbone area. In contrast, an area range statement specified in the area area-number stanza is used only to summarize and filter intra-area prefixes.

Configure Summarization

Route summarization and NSSA external route filtering will be demonstrated in the following configuration tasks:

  • Summarize all the intra-area prefixes into the backbone area.

  • Block the 3/8 static route from leaving area 10.

This configuration example begins with the configuration of route summarization on r5. We first examine the current state of r3's routing table, as it relates to the prefixes assigned to area 1. This "before-and-after" approach will best demonstrate the effects of address summarization. The proposed summarization prefix and mask pair of 10.0.8/23 is first used to filter the routing table contents of r5. This allows the candidate to accurately gauge the summarization effect that will result when the same prefix/mask is configured as part of an area range statement:

lab@r3# run show route protocol ospf 10.0.8/23 inet.0: 29 destinations, 31 routes (29 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 10.0.8.0/30         *[OSPF/10] 00:04:38, metric 3                      > via at-0/1/0.35 10.0.8.4/30         *[OSPF/10] 00:04:38, metric 2                      > via at-0/1/0.35 10.0.8.8/30         *[OSPF/10] 00:04:38, metric 2                      > via at-0/1/0.35 10.0.9.6/32         *[OSPF/10] 00:04:38, metric 2                      > via at-0/1/0.35 10.0.9.7/32         *[OSPF/10] 00:04:38, metric 2                      > via at-0/1/0.35

These results demonstrate that all of area 1's internal prefixes can be correctly summarized with a 10.0.8.0/23 prefix and mask. Knowing that the summarization mask is valid, the following command is now used to configure the summarization of the prefixes in area 1:

[edit protocols ospf area 0.0.0.1] lab@r5# set area-range 10.0.8/23 
Tip 

When performing summarization for an OSPF area, do not forget to also summarize the loopback addresses, unless otherwise required by your lab scenario.

The modified configuration for area 1's ABR (r5) is shown next with the summarization changes highlighted:

[edit protocols ospf area 0.0.0.1] lab@r5# show area-range 10.0.8.0/23; authentication-type simple; # SECRET-DATA interface fe-0/0/0.0 {    authentication-key "$9$Kw2WX-UDkqfz"; # SECRET-DATA }  interface fe-0/0/1.0 {    authentication-key "$9$32M9/A0vMXxds"; # SECRET-DATA  } 

After committing the change, we again look at r3's routing table for area 1 prefixes:

lab@r3> show route 10.0.8.0/23 inet.0: 25 destinations, 27 routes (25 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 10.0.8.0/23          *[OSPF/10] 00:00:08, metric 3                        > via at-0/1/0.35

Good-the fewer routes the better! From the perspective of a backbone router, a single prefix/ mask pair now represents all of area 1's destinations. Prior to summarization, five prefix/mask pairings were needed to correctly route packets into area 1. We now add the required summarization and filtering to area 10's ABRs with the following commands:

[edit protocols ospf area 0.0.0.10] lab@r3# set area-range 10.0.4.0/22 [edit protocols ospf area 0.0.0.10] lab@r3# set nssa area-range 3/8 restrict 

A key point to this task is the need to specify an area range both at the area and NSSA levels of the configuration. The NSSA-related area range statements control type 7 translations, and so allow the summarization and filtering of external routes in a NSSA. There is no mechanism in JUNOS software to summarize or filter type 5 LSAs using area-range statements. Options for the control of type 5 LSAs include the configuration of external route aggregates at the source ASBR, or the deployment of stub areas, which automatically filter external routes.

The resulting NSSA configuration for the route summarization and NSSA external route- filtering tasks are shown next with highlighted changes:

[edit protocols ospf area 0.0.0.10] lab@r3# show nssa {      default-lsa {         default-metric 10;         metric-type 2;         type-7;     }      no-summaries;     area-range 3.0.0.0/8 restrict; } area-range 10.0.4.0/22; interface fe-0/0/1.0; interface fe-0/0/0.0; 

Once again we judge the effects of summarization by examining a backbone router that does not attach to area 10, which in this example happens to be r5:

lab@r5> traceroute 10.0.6.2 traceroute to 10.0.6.2 (10.0.6.2), 30 hops max, 40 byte packets  1 10.0.2.2 (10.0.2.2) 1.621 ms 1.526 ms 1.089 ms  2 10.0.6.2 (10.0.6.2) 1.039 ms 1.000 ms 0.624 ms

With reachability to area 10 confirmed, we now check for proper summarization:

lab@r5> show route 10.0.6.2 inet.0: 23 destinations, 25 routes (23 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 10.0.4.0/22         *[OSPF/10] 00:00:27, metric 3                       > via at-0/2/1.35 lab@r5> show route 10.0.4.5 inet.0: 23 destinations, 25 routes (23 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 10.0.4.0/22         *[OSPF/10] 00:02:03, metric 3                       > via at-0/2/1.35

The same aggregate route is returned for the area's physical and loopback addresses, which indicates that summarization is working. It may be acceptable to send two aggregates for an area-one encompassing the physical addresses and another to advertise the loopback address space-especially if the two address blocks are discontiguous.

Lastly, we confirm that the external route is being properly filtered at the NSSA boundaries with the following command issued on r5:

lab@r5> show route 3/8 lab@r5>

The lack of a route to 3/8 on r5 indicates that the external route is being properly filtered by r3 and r4.




JNCIP. Juniper Networks Certified Internet Professional Study Guide Exam CERT-JNCIP-M
JNCIP: Juniper Networks Certified Internet Professional Study Guide
ISBN: 0782140734
EAN: 2147483647
Year: 2003
Pages: 132

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