Multiple Serial Links - Source Destination Pair

Answered Question
Mar 18th, 2009
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If I define static routes over two serial links connected to a single router, how can I check which source/destination pair is utilizing which serial link. With static routes, the traffic will be load balanced across the serial link.


Thanks.

Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Cisco_Lite,


the command has to be given in interface config mode:


ip load-sharing ?

per-destination Deterministic distribution

per-packet Random distribution


configure it on both serial interfaces


Hope to help

Giuseppe


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Giuseppe Larosa Wed, 03/18/2009 - 06:06
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Hello Cisco_lite,


you can use commands like


sh ip cef exact-route s1.s2.s3.s4 d1.d2.d3.d4


to see what link is used by source /dest pair


the static routes must be deployed in pair:

ip route 10.10.0.0 255.255.0.0 ser0

ip route 10.10.0.0 255.255.0.0 ser1


Hope to help

Giuseppe


cisco_lite Wed, 03/18/2009 - 08:38
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Thanks.


Two more questions on that


1) Isn't it possible to see the whole exact-route table without giving the source/destination selection. It would be quite good to track all communication pairs and identify which link they are routed over.


2) If there were two routers with GLBP on the inside ethernet only, along with two serial each (i.e. with load balancing by way of paired static routes), how can I find to which router gateway does the host get forwarded to.


Giuseppe Larosa Wed, 03/18/2009 - 09:44
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Hello Cisco_Lite,


1) not possible two many combinations to be shown you need to specify source and destination


2) with GLBP you need an additional step


on the PC host use arp -g to find the MAC address used to reach default gateway


from that information you can understand what AVF is used by the host and so you can know what router is effectively used


then you go to the right node and you can use the ip cef exact-route command


Hope to help

Giuseppe




cisco_lite Wed, 03/18/2009 - 11:12
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With regards to # 2, the host connected to the router is FWSM interface only. Other hosts are behind FWSM. So the host arp table does not include the router ethernet mac. Only FWSM arp table includes it.


FWSM has been configured with Active/Standby across two Cat65K switches and each router is connected to different Cat65K switch. So in this case, will GLBP benefit with its loadbalancing feature (currently default set)

Giuseppe Larosa Wed, 03/18/2009 - 11:27
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Hello Cisco_Lite,


if the FWSM is the only client in the vlan GBLP will not load balance:


once the FWSM receives the ARP reply for the default gateway it will use for all traffic


GLBP is effective only when multiple clients are in the vlan and works by answering to ARP requests in round robin from the pool of AVF (Active Virtual Forwarder) MAC addresses


in this case GLBP will behave as HSRP


Hope to help

Giuseppe


cisco_lite Wed, 03/18/2009 - 13:39
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Ok.


Going back to the original question.. when does the source/destination pair against serial link in ip cef tables gets refreshed. i.e. If there were two static routes with same destination, when would the source/destination pairs

switchover to the second serial link.

Giuseppe Larosa Thu, 03/19/2009 - 00:38
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Hello Cisco_Lite,

if I have understood correctly the question you are asking how long it takes the device to start to use the second link if the second static route is added later.


The act of adding a new ip static route is a change to the IP routing table.

The change in the ip routing table triggers the recomputation of CEF tables.

Once CEF tables are recalculated if the device has a distributed switching architecture new copies of the fresh CEF table needs to be placed in the memories of each indipendent module/linecard.


The steps are triggered but for scalability there are timers /frequency of scanning the ip routing table looking for changes and timers to propagate new CEF information.


A useful document is the following


http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html


I think it should be in the order of tens of seconds as a worst case


Hope to help

Giuseppe


cisco_lite Thu, 03/19/2009 - 13:24
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Hello Giuseppe,


I would like to put it in again.


My query is that at any point in time if there are 2 static routes, and the source-destination pair are stuck to for e.g. Serial1, when would the same source-destination pair be moved to Serial2. The static routes are already there and no new addition is required. Hence, no ip cef calculation will take place.


Similarly, I would like to know that for how long does a host remain stuck to an AVF within GLBP if there are for e.g. three/four AVFs.


Thanks.

Giuseppe Larosa Thu, 03/19/2009 - 14:28
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Hello Cisco_Lite,

sorry I haven't understood your penultimate question.


The answer to first question is that until serial1 is alive a source/dest pair associated to it uses this path.


the CEF load balancing algorithm performs a simple math operation:


(SA)lsb exor (DA)lsb exor hash


lsb = last significant bit = rightmost bit in the 32 bit IPv4 address


the hash is a seed

that changes only after a reload


the result is an index 0 or 1 that is used to decide if to use ser1 or ser2.


q2) after the ARP entry times out in the ARP table of the PC the PC performs a new ARP request and has a chance to receive a different AVF MAC in the answer.


for a router an ARP entry is timed out after 4 hours.


Hope to help

Giuseppe


cisco_lite Thu, 03/19/2009 - 15:15
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I have 2821 routers in my setup with multiple serial lines in each. I am trying to enable per-packet load sharing but I don't see the option. Is packet load sharing supported by the following IOS version.


Cisco IOS Software, 2800 Software (C2800NM-ADVIPSERVICESK9-M), Version 12.4(9)T7, RELEASE SOFTWARE (fc3)


Correct Answer
Giuseppe Larosa Fri, 03/20/2009 - 07:13
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Hello Cisco_Lite,


the command has to be given in interface config mode:


ip load-sharing ?

per-destination Deterministic distribution

per-packet Random distribution


configure it on both serial interfaces


Hope to help

Giuseppe


cisco_lite Sat, 03/21/2009 - 02:35
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Hi Giuseppe,


Is there a way that I can force the FWSM to always forward traffic to the same AVF even after the ARP times out. This is because I have two routers where one is with two serial links and other with only one. And I want FWSM to always go to the router with two serials for increased bandwidth.


And also that the traffic should go to the other AVF(2) only if AVF1 is completely down.


Thanks.

Giuseppe Larosa Sat, 03/21/2009 - 06:22
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Hello Cisco_Lite,

the easiest way to do what you want is to use HSRP instead of GLBP.

GLBP wants to do load balancing.


Using HSRP you can make the R1 with two serials preferred until it has one serial link alive


int f0/0

ip address 10.10.10.2 255.255.255.0

standby 10 ip 10.10.10.1

standby 10 priority 115

standby 10 preempt

standby 10 track ser0

standby 10 track ser1


R2:

int f0/0

ip address 10.10.10.3 255.255.255.0

standby 10 ip 10.10.10.1

standby 10 preempt

standby 10 track ser0


notice with priority 115 and default tracking decrement of 10 until one of R1's interface is alive R1 is used.

if one serial fails new priority becomes:

115-10 = 105 R1 still active

if both serials are down

R1 priority becomes 95 and R2 preempts becoming the active router.

the command standby 10 preempt is needed for getting this behaviour


Hope to help

Giuseppe




cisco_lite Sat, 03/21/2009 - 08:54
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Great. I will give it a shot. If it doesn't work I shall revert due to lack of time.


Lastly, is ip load-sharing per packet (on serials) supported in HSRP configuration or is it completely independent of HSRP/GLBP etc.

Giuseppe Larosa Sat, 03/21/2009 - 09:23
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Hello Cisco_Lite,

that HSRP config should work I used it some years ago.

I might have done some syntax errors because I wrote the commands on the post without searching an old config.


>> is ip load-sharing per packet (on serials) supported in HSRP configuration or is it completely independent of HSRP/GLBP etc.


it is indipendent


Hope to help

Giuseppe


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