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RSM vs 3550 Routing Issues

Hi All

When I do a "sh ip route" on our 3550 router, it only shows one route to each network that it knows of.

When I do a "sh ip route" on our RSM router, it shows four routes to each network that it knows of - one of which is always an active VLAN on the 3550.

We are running HSRP between the RSM and the 3550, as well as EIGRP, and both have exactly the same VLANs configured (taking HSRP in account, of course). We have 15 VLANs on our LAN, of which 7 are active on the 3550, and the other 8 are active on the RSM. We have a total of 11 VLANs on our remote sites as well. The gateway of last resort on all our routers are set up to be our Firewall.

When I do a traceroute from a PC on one of the active VLANs on the RSM, the trace goes via the RSM, then the 3550, and then it takes the correct (expected)route to the destination.

When I do a traceroute from an active VLAN on the 3550, the trace takes the corrected (expected) route to the destination immediately.

Why does the RSM have four routes to each destination, while the 3550 does not? Is there a way that I can force the RSM to only have one route to each network?

Regards

2 REPLIES
Cisco Employee

Re: RSM vs 3550 Routing Issues

It seesm like the there are four paralle path out of the RSM to get to those networks and the 3550 which may be connected to the Cat5000, knows only one way which may be cat5000 vlans.

Please sh ip route for any one network from both RSM and the 3550. and how they are connected to each other and to the other network and who is advertising those networks to the RSM and 3550

Re: RSM vs 3550 Routing Issues

Hi,

It sounds like EIGRP is forming an adj on every VLAN interface on the RSM.

Is the RSM connecting to another RSM / MSFC via a trunk link with same VLANs?

if so, then that explains why RSM has 4 equal cost routes to those networks. 4 is the default for maximum paths. You could configure that for 1 under the EIGRP process.

If your running EIGRP over every VLAN on a trunk between 2 RSMs or a MSFC then you might want to make only 1 VLAN active for EIGRP and rest of the VLAN interfaces passive for EIGRP. This way only one adj is formed between the routers instead of one for each VLAN exchanging same routes.

Erick

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