[c-nsp] 3750 CEF and equal cost multihoming

[Oliver Boehmer (oboehmer)]

Matt Bazan <> wrote on Thursday, January 06, 2005 6:04 PM:

> Have a mystery I'd like to solve:  On one of my 3750 switches I've
> setup two static routes with equal metrics (on two separate routed
> ports) via their respected interfaces (gi1/0/19, gi1/0/21) to a /27
> network.  So, I've got two routes in my routing table that look like
> this: 
> 
>  XX.XXX.XXX.XXX/27 is directly connected, GigabitEthernet1/0/19
>                    is directly connected, GigabitEthernet1/0/21

You don't want this. Please always specify a next-hop along with the
interface, i.e.

ip route XX.XXX.XXX.XXX 255.255.255.224 GigabitEthernet1/0/19 a.b.c.d
ip route XX.XXX.XXX.XXX 255.255.255.224 GigabitEthernet1/0/19 e.f.g.h

where a.b.c.d and e.f.g.h are the respective next-hop addresses.

[...]

> Is it possible to have the router only use the interface that has the
> arp entry mapped to it?

No. If you set-up routing like you did, you end up arp'ing for every
destination address in this network range. This can create quite a high
ARP activity which you want to avoid. When you specify the next-hop, the
router only needs to arp once, and you'll be set.

	oli