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CEF load balancing 2 T1 with a 2621

 
 
Mirco
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Posts: n/a
 
      07-07-2004
We have 2 full T1 lines which, individually connected to the router,
work just fine. However, when both are up at the same time, they are
not working, or at least it seems like packets take a long time to get
out.
Each T1 is routing 16 IPs and the 2 blocks are consecutive:

T1 #1 WAN: A.B.C.245/24
LAN: A.B.D.96/28
T1 #2 WAN: A.B.C.246/24
LAN: A.B.D.112/28

The following is the configuration file we are using. Could anybody
tell me what's wrong with it.

TIA,

---Mirco.

! ================================================== =====================
!
no ip name-server
!
ip subnet-zero
no ip domain-lookup
ip routing
ip cef
!
interface FastEthernet 0/0
no shutdown
ip address A.B.D.97 255.255.255.224
keepalive 10
!
interface FastEthernet 0/1
no ip address
shutdown
!
interface Serial 0/0
no shutdown
no ip address
encapsulation frame-relay
!
interface Serial 0/0.1 point-to-point
no shutdown
ip address A.B.C.245 255.255.255.0
frame-relay interface-dlci 16 ietf
!
interface Serial 0/1
no shutdown
no ip address
encapsulation frame-relay
!
interface Serial 0/1.1 point-to-point
no shutdown
ip address A.B.C.246 255.255.255.0
frame-relay interface-dlci 16 ietf
!
router rip
version 2
network A.0.0.0
passive-interface Serial 0/0.1
passive-interface Serial 0/1.1
no auto-summary
!
!
ip classless
!
! IP Static Routes
ip route 0.0.0.0 0.0.0.0 Serial 0/0.1
ip route 0.0.0.0 0.0.0.0 Serial 0/1.1
no ip http server
snmp-server community public RO
no snmp-server location
no snmp-server contact
!
! ================================================== =====================
 
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jerome benton
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      07-09-2004
http://www.velocityreviews.com/forums/(E-Mail Removed) (Mirco) wrote some stuff in message
>news:<(E-Mail Removed) .com>...


cisco.com has a detailed troubleshooting guide for load sharing on
parallel links using CEF:

http://www.cisco.com/en/US/customer/...80094806.shtml

It covers both per-packet and per-destination load sharing. There's
even a comparison of the two methods so you can select the method you
want based on your goals.

From your post, I can't tell if you meet the pre-requisite for CEF
load sharing. Like dancing the Tango, and as the name implies, load
sharing requires a pair of devices connecting 2 or more links from
source to destination, in parallel. That is, the source device (CPE
router, e.g.) and destination device (ISP edge router, e.g.) must both
support CEF and be configured for the same method on all shared
interfaces. Does your ISP have CEF and static routes configured for
each interface?

Also, from the config stub posted, you need to configure load-sharing
on the parallel link interfaces. Choose either per-packet or
per-destination to suit your needs. Use "show cef interface
[interface]" to verify CEF load-sharing.

Below is a working stub config, with IPs changed to protect the
innocent. The ISP edge router is a 7206VXR NPE300, and the CPE router
is a 2610.

ISP Edge:

interface Serial4/0/0:1
ip address 1.1.1.9 255.255.255.252
no ip directed-broadcast
no ip proxy-arp
ip load-sharing per-packet
no ip route-cache distributed
down-when-looped
no cdp enable

interface Serial4/0/0:16
ip address 1.1.1.13 255.255.255.252
no ip directed-broadcast
no ip proxy-arp
ip load-sharing per-packet
no ip route-cache distributed
down-when-looped
no cdp enable


ip route 1.1.1.224 255.255.255.240 Serial4/0/0:1
ip route 1.1.1.224 255.255.255.240 Serial4/0/0:16


CPE router:

interface Ethernet0/0
ip address 1.1.1.225 255.255.255.240
no ip directed-broadcast
!
interface Serial0/0
ip address 1.1.1.10 255.255.255.252
ip access-group 100 in
no ip directed-broadcast
ip load-sharing per-packet
no ip mroute-cache
service-module t1 timeslots 1-24
!
interface Serial0/1
ip address 1.1.1.14 255.255.255.252
ip access-group 100 in
no ip directed-broadcast
ip load-sharing per-packet
no ip mroute-cache
service-module t1 timeslots 1-24

ip route 0.0.0.0 0.0.0.0 Serial0/0
ip route 0.0.0.0 0.0.0.0 Serial0/1

....and below are the show cef interface results.

ISP edge:

Serial4/0/0:1 is up (if_number 46)
Corresponding hwidb fast_if_number 46
Corresponding hwidb firstsw->if_number 46
Internet address is 1.1.1.9/30
ICMP redirects are always sent
Per packet load-sharing is enabled
IP unicast RPF check is disabled
Inbound access list is not set
Outbound access list is not set
IP policy routing is disabled
BGP based policy accounting is disabled
Interface is marked as point to point interface
Hardware idb is Serial4/0/0:1
Fast switching type 4, interface type 22
IP CEF switching enabled
IP Feature Fast switching turbo vector
IP Feature CEF switching turbo vector
Input fast flags 0x0, Output fast flags 0x0, Flags 0xC1
ifindex 45(45)
Slot 4 Slot unit 0 VC 1
Transmit limit accumulator 0x4800022A (0x4800022A)
IP MTU 1500


Serial4/0/0:16 is up (if_number 42)
Corresponding hwidb fast_if_number 42
Corresponding hwidb firstsw->if_number 42
Internet address is 1.1.1.13/30
ICMP redirects are always sent
Per packet load-sharing is enabled
IP unicast RPF check is disabled
Inbound access list is not set
Outbound access list is not set
IP policy routing is disabled
BGP based policy accounting is disabled
Interface is marked as point to point interface
Hardware idb is Serial4/0/0:16
Fast switching type 4, interface type 22
IP CEF switching enabled
IP Feature Fast switching turbo vector
IP Feature CEF switching turbo vector
Input fast flags 0x0, Output fast flags 0x0, Flags 0xC1
ifindex 41(41)
Slot 4 Slot unit 0 VC 16
Transmit limit accumulator 0x48000232 (0x48000232)
IP MTU 1500


CPE router:

Serial0/0 is up (if_number 3)
Internet address is 1.1.1.10/30
ICMP redirects are always sent
Per packet loadbalancing is enabled
IP unicast RPF check is disabled
Inbound access list is 100
Outbound access list is not set
Interface is marked as point to point interface
Hardware idb is Serial0/0
Fast switching type 4, interface type 43
IP CEF switching enabled
IP CEF Feature Fast switching turbo vector
Input fast flags 0x1, Output fast flags 0x0
ifindex 2(2)
Slot 0 Slot unit 0 VC -1
Transmit limit accumulator 0x0 (0x0)
IP MTU 1500


Serial0/1 is up (if_number 4)
Internet address is 1.1.1.14/30
ICMP redirects are always sent
Per packet loadbalancing is enabled
IP unicast RPF check is disabled
Inbound access list is 100
Outbound access list is not set
Interface is marked as point to point interface
Hardware idb is Serial0/1
Fast switching type 4, interface type 43
IP CEF switching enabled
IP CEF Feature Fast switching turbo vector
Input fast flags 0x1, Output fast flags 0x0
ifindex 3(3)
Slot 0 Slot unit 1 VC -1
Transmit limit accumulator 0x0 (0x0)
IP MTU 1500

HTH

--Jerome
 
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AnyBody43
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Posts: n/a
 
      07-12-2004
(E-Mail Removed) (jerome benton) wrote
> From your post, I can't tell if you meet the pre-requisite for CEF
> load sharing. Like dancing the Tango, and as the name implies, load
> sharing requires a pair of devices connecting 2 or more links from
> source to destination, in parallel.
>
> That is, the source device (CPE
> router, e.g.) and destination device (ISP edge router, e.g.) must both
> support CEF and be configured for the same method on all shared
> interfaces. Does your ISP have CEF and static routes configured for
> each interface?


I don't believe that all of this is quite correct.

" the source device (CPE > router, e.g.) and destination device
(ISP edge router, e.g.) must both support CEF"
Sorry, not correct.

The mechanism that a single device uses to route an IP packet
is independent of all other devices. The IP routing protocols
of which I have any understanding (most Interior GP's) know
nothing of load sharing other than that they may be prepared to put
more than one route into the routing table. Once a routing table
is established THEN the end device may choose for example
to do equal-cost-path load balancing, using for example per packet,
per flow, or some other algorithm.

It can be a critical part of IP troubleshooting to appreciate
that packets do not take the same outbound and return path.
A tracert for example only tells you the *outbound* path of
the particular set ot test packets. Note that it does not tell
you the path of any single packet. There is NO requirement for
any two packets to take the same path.
 
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jerome benton
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      07-13-2004
(E-Mail Removed) (AnyBody43) wrote in message news:<(E-Mail Removed). com>...
> (E-Mail Removed) (jerome benton) wrote
> > From your post, I can't tell if you meet the pre-requisite for CEF
> > load sharing. Like dancing the Tango, and as the name implies, load
> > sharing requires a pair of devices connecting 2 or more links from
> > source to destination, in parallel.
> >
> > That is, the source device (CPE
> > router, e.g.) and destination device (ISP edge router, e.g.) must both
> > support CEF and be configured for the same method on all shared
> > interfaces. Does your ISP have CEF and static routes configured for
> > each interface?

>
> I don't believe that all of this is quite correct.
>
> " the source device (CPE > router, e.g.) and destination device
> (ISP edge router, e.g.) must both support CEF"
> Sorry, not correct.
>
> The mechanism that a single device uses to route an IP packet
> is independent of all other devices. The IP routing protocols
> of which I have any understanding (most Interior GP's) know
> nothing of load sharing other than that they may be prepared to put
> more than one route into the routing table. Once a routing table
> is established THEN the end device may choose for example
> to do equal-cost-path load balancing, using for example per packet,
> per flow, or some other algorithm.
>
> It can be a critical part of IP troubleshooting to appreciate
> that packets do not take the same outbound and return path.
> A tracert for example only tells you the *outbound* path of
> the particular set ot test packets. Note that it does not tell
> you the path of any single packet. There is NO requirement for
> any two packets to take the same path.


I beleive we are both correct. IP paths are not necessarily symmetric.
However, the subject of the post is "CEF load balancing 2 T1 with a
2621" which I interpret to mean that the poster requires CEF load
balancing. The Cisco IOS, referred to by the poster, uses 4 mechanisms
to switch IP packets; namely, process switching, fast switching, Cisco
Express Forwarding, and distributed CEF. CEF uses FIB to make
destination switching decisions, and adjacency tables to maintain
layer 2 next hop information. This is all very clearly defined in the
document I referenced, and which I urge anyone with CEF curiosity to
read.

If the subject or question had been "How do I load balance multiple
links," for example, I would have responded "Choose your favorite
IGP." Perhaps the poster can clarify if they truly meant CEF load
balancing, or simply IP routing.

--Jerome
 
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