Binding 2 T1s on a Cisco 1720

[jbober14]

Hello everyone. I'm hoping I can get some feedback on a config I have been working on. I'm binding two T1s we have in house for our backup internet connection. I'm using a cisco 1720 router for this task... if someone could look over the config and tell me if it looks like I am missing anything that would be appreciated.

Config:

version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Router
!
enable secret 5 xxxxxxxxxx
!
memory-size iomem 25
ip subnet-zero
ip name-server 65.

http://www.x.zzz

ip name-server 65.

http://www.y.zzz

!
interface Multilink1
description multilink PPP connection
no ip address <--- I'm waiting on ISP to get me the serial IP address
no ip redirects
no ip proxy-arp
ip nat outside
no ip route-cache
no cdp enable
ppp multilink
multilink-group 1
!
interface Serial0
description first multilink connection
no ip address
no ip redirects
no ip proxy-arp
encapsulation ppp
no ip route-cache
load-interval 30
no keepalive
shutdown
no fair-queue
no cdp enable
ppp multilink
multilink-group 1
!
interface Serial1
description second multilink connection
no ip address
no ip redirects
no ip proxy-arp
encapsulation ppp
no ip route-cache
load-interval 30
no keepalive
shutdown
no fair-queue
no cdp enable
ppp multilink
multilink-group 1
!
interface FastEthernet0
description public interface to bonded T1
ip address 67.xxx.yyy.zzz 255.255.255.248 <---Gateway IP
ip nat outside
no ip route-cache
shutdown
speed auto
full-duplex
no cdp enable
!
ip classless
ip route 0.0.0.0 0.0.0.0 Multilink1
no ip http server
!
no cdp run
!
line con 0
line aux 0
line vty 0 4
!
no scheduler allocate
end

 

[unclerico]

be sure to no shut all of your interfaces. remove the nat configuration from the mu1 interface since you already have it specified under your f0 interface (do you have a firewall behind this device and is it also NATing??). specify the next hop IP address as opposed to the exit interface for your default route.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[jbober14]

unclerico thanks for the response... the NATing was disabled, this was unnecessary due to the fact I have the firewall doing so. Good catch on that thanks!

As for specifying the next hop IP address on the exit interface that will not be an issue, but my question is why? I'm just curious if you could provide some background info on what the benefit is to using the next IP hop rather than the interface itself for the route.

Thanks again

 

[unclerico]

no problem. a couple of things are wrong with specifying the exit interface as opposed to the next-hop ip:
1) proxy-arp is involved which isn't always enabled (most times it's not)
2) each and every new destination address will result in an ARP request on the segment. if your gateway has a route to the destination it will reply on the destinations behalf. this leads to excessive taffic and table lookups

if you really want to optimize traffic, specify both the exit interface and the next-hop ip address. this will require fewer table lookups:

ip route 0.0.0.0 0.0.0.0 mu1 <next_hop_ip>

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[jbober14]

I'll do that. Thanks again.

 

[jbober14]

The router is working wonderfully. I appreciate the feedback.