We are an ISP. We are trying to load-share traffic through our OSPG IFG to a customer with multiple leased line connections to different POPs, where he announces his routes to us with RIPv2, which are then redistributed into our OSPF. So he has multiple OSPF routes to our central core router, and these paths will have in general different OSPF path costs, and these costs will also change frequently as the network is grown. Therefore, I want to remove the OSPF path costs as a factor in the load sharing. I had thought I could do this by redistributing the customer's RIP routes with equal OSPF external-type 2 metrics at each POP.
But, although our core router has equal cost routes to all of the customer sites, it seems to be basing the selection of the paths on something called the 'forward metric'. The relationship seems to be: external typ1 1 cost = external type 2 cost + forward metric: forward metric is the accumulation of OSPF paths costs, not including the external type 2 metric used at the ASBR sourcing the route. Traffic is being shared between routes with the same 'forward metric', but not between routes with the same external type 2 costs.
Look at the following: this is the load sharing across all the routes I can acheive when the ASBR sources the routes with external tpye 1 metric, and I take care to make sure that all the OSPF path costs to the core are equal:
i79zhh-010#sh ip ro 194.209.146.64
Routing entry for 194.209.146.64/27
Known via "ospf 3303", distance 110, metric 674, type extern 1
Redistributing via eigrp 3000, ospf 3303
Last update from 164.128.44.246 on FastEthernet5/0/0, 00:40:58 ago
Routing Descriptor Blocks:
164.128.33.178, from 194.209.146.1, 00:40:58 ago, via Serial4/1/7.11
Route metric is 674, traffic share count is 1
* 164.128.33.78, from 194.209.146.1, 00:40:58 ago, via Serial5/1/1.7
Route metric is 674, traffic share count is 1
164.128.44.246, from 194.209.146.17, 00:40:58 ago, via FastEthernet5/0/0
Route metric is 674, traffic share count is 1
Then the router load shares over all three paths:
cost (674) = path cost (474) + external type 1 metric (200), as expected.
If I source the routes with external type 2 metric, I would expect:-
cost (200) = path cost (ignored) + external type 2 metric (200)
for all three routes. Instead I get:
i79zhh-010#sh ip ro 194.209.146.0 255.255.255.224
Routing entry for 194.209.146.0/27
Known via "ospf 3303", distance 110, metric 200, type extern 2, forward metric 474
Redistributing via eigrp 3000, ospf 3303 ^^^^^^^^^^^^^^^^^^
Last update from 164.128.33.78 on Serial5/1/1.7, 00:23:58 ago
Routing Descriptor Blocks:
* 164.128.33.178, from 194.209.146.1, 00:23:58 ago, via Serial4/1/7.11
Route metric is 200, traffic share count is 1
164.128.33.78, from 194.209.146.1, 00:23:58 ago, via Serial5/1/1.7
Route metric is 200, traffic share count is 1
The route
164.128.44.246, from 194.209.146.17, 00:40:58 ago, via FastEthernet5/0/0
that was previously present is still in the OSPF database, and indeed has the same metric (200) as the other two routes, but it is not put into the routing table because it has a higher 'forward metric'.
This behaviour seems to contraduct anything I can find in the Cisco documentation. I need to do this with metrics that are path independent, because if we do this for other customers, we do not want to be in the situation where the load sharing breaks for several customers if we change the network architecture.
Thanks in advance for any help
Phil
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Philip Bridge Swisscom Data/Multimedia
Voice: +41 31 688 8262 Private: +41 31 911 1694
Fax: +41 31 688 8151 mail: bridge@ip-plus.net
PGP:www-swiss.ai.mit.edu/~bal/pks-commands-beta.html(447485ED)
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