From: Jonathan Hays (jhays@xxxxxxxx)
Date: Sun May 19 2002 - 03:13:33 GMT-3
Yes - I know what you are thinking - this:
http://www.cisco.com/warp/public/104/3.html#12.0
"External Route Summarization
External route summarization is specific to external routes that are
injected into OSPF via redistribution. Also, make sure that external
ranges that are being summarized are contiguous. Summarization
overlapping ranges from two different routers could cause packets to be
sent to the wrong destination. Summarization is done via the following
router ospf subcommand:
summary-address ip-address mask
This command is effective only on ASBRs doing redistribution into OSPF."
--- The above CCO verbiage WOULD seem to indicate that the command is effective only one way. However, don't forget, this is an ASBR which also has the duty to tell external networks about what routes are available in the OSPF cloud.The configuration you gave is only the part where IGRP is redistributed into OSPF:
router ospf 100 router-id 4.4.4.4 log-adjacency-changes summary-address 135.1.20.0 255.255.255.0 redistribute igrp 100 subnets
I haven't seen the write-up in question, but you are correct that the above configuration by itself does nothing to redistribute routes into IGRP. To do that statements like the ones below are needed:
router igrp redistribute ospf 100 metric 10000 2000 255 1 1500 subnets
And the trick here, is that since the 135.1.20.0/22 network has been summarized in OSPF as a /24, then *that* /24 is what gets redistributed into IGRP. This "trick" is necessary because IGRP won't understand the classless subnets. Essentially, you are using the OSPF ASBR's summary network statement to "adjust" the mask of OSPF classless networks to something that IGRP will understand.
HTH,
Jonathan
-----Original Message----- From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Todd Spencer Sent: Saturday, May 18, 2002 4:54 PM To: ccielab@groupstudy.com Subject: FATKID 502 Expert Redistribution
I am trying to do the FatKid 502 Expert Redistribution Lab, and I don't understand the solution provided. I will try to briefly explain.
The problem has to do with the classic VLSM to FLSM problem (IGRP/OSPF). The lab requires that OSPF area 4 be configured with a /22 subnet, and the IGRP domain be configured with a /24. OSPF area 4 is using the following subnet 135.1.20.x/22 and the IGRP domain is using 135.1.1.x/24. The OSPF configuration below is taken from the solution:
router ospf 1 summary-address 135.1.20.0 255.255.255.0 redistribute igrp 1 subnets network 135.1.14.0 0.0.0.255 area 4 network 135.1.20.0 0.0.3.255 area 4
I am unsure what the summary-address command is doing in this configuration. How is it able to summarize a /22 into a /24? How is the /22 subnet getting into the IGRP domain? If anyone has done this lab and can provide assistance, it would be greatly appreciated.
-Todd
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