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Monday, January 14, 2013

cef and load balancing...

routing protocols will load balance x equal cost paths based on your version of ios... cef will thwart this effort...


r4 has 2 equal cost paths as depicted in the topo...

R4(config)#do sh ip eigrp topo 1.1.1.0/24
EIGRP-IPv4 Topology Entry for AS(1)/ID(4.4.4.4) for 1.1.1.0/24
  State is Passive, Query origin flag is 1, 2 Successor(s), FD is 2560002560
  Descriptor Blocks:
  172.16.1.3 (FastEthernet1/1), from 172.16.1.3, Send flag is 0x0
      Composite metric is (2560002560/2560000000), route is External
      Vector metric:
        Minimum bandwidth is 1 Kbit
        Total delay is 100 microseconds
        Reliability is 1/255
        Load is 255/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 3.3.3.3
      External data:
        AS number of route is 1
        External protocol is OSPF, external metric is 3
        Administrator tag is 0 (0x00000000)
  172.16.2.3 (FastEthernet0/0), from 172.16.2.3, Send flag is 0x0
      Composite metric is (2560002560/2560000000), route is External
      Vector metric:
        Minimum bandwidth is 1 Kbit
        Total delay is 100 microseconds
        Reliability is 1/255
        Load is 255/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 3.3.3.3
      External data:
        AS number of route is 1
        External protocol is OSPF, external metric is 3
        Administrator tag is 0 (0x00000000)

with cef enabled one path will be chosen, and there will be no load balancing...

 R4(config)#do sh ip route 1.1.1.1
Routing entry for 1.1.1.0/24
  Known via "eigrp 1", distance 170, metric 2560002560, type external
  Redistributing via eigrp 1
  Last update from 172.16.1.3 on FastEthernet1/1, 00:24:22 ago
  Routing Descriptor Blocks:
  * 172.16.2.3, from 172.16.2.3, 00:24:22 ago, via FastEthernet0/0
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
    172.16.1.3, from 172.16.1.3, 00:24:22 ago, via FastEthernet1/1
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1

note the asterisk... the asterisk should bounce between the routes, either per packet or per destination, depending on your determination...

R4(config)#do ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/58/68 ms
R4(config)#do sh ip route 1.1.1.1
Routing entry for 1.1.1.0/24
  Known via "eigrp 1", distance 170, metric 2560002560, type external
  Redistributing via eigrp 1
  Last update from 172.16.1.3 on FastEthernet1/1, 00:26:03 ago
  Routing Descriptor Blocks:
  * 172.16.2.3, from 172.16.2.3, 00:26:03 ago, via FastEthernet0/0
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
    172.16.1.3, from 172.16.1.3, 00:26:03 ago, via FastEthernet1/1
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
R4(config)#do ping 1.1.1.1      
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/59/68 ms
R4(config)#do sh ip route 1.1.1.1
Routing entry for 1.1.1.0/24
  Known via "eigrp 1", distance 170, metric 2560002560, type external
  Redistributing via eigrp 1
  Last update from 172.16.1.3 on FastEthernet1/1, 00:26:11 ago
  Routing Descriptor Blocks:
  * 172.16.2.3, from 172.16.2.3, 00:26:11 ago, via FastEthernet0/0
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
    172.16.1.3, from 172.16.1.3, 00:26:11 ago, via FastEthernet1/1
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1

the damn asterisk doesn't float...

turn cef off and now your asterisk is buoyant...

R4(config)#no ip cef
R4(config)#do sh ip route 1.1.1.1
Routing entry for 1.1.1.0/24
  Known via "eigrp 1", distance 170, metric 2560002560, type external
  Redistributing via eigrp 1
  Last update from 172.16.1.3 on FastEthernet1/1, 00:27:05 ago
  Routing Descriptor Blocks:
  * 172.16.2.3, from 172.16.2.3, 00:27:05 ago, via FastEthernet0/0
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
    172.16.1.3, from 172.16.1.3, 00:27:05 ago, via FastEthernet1/1
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
R4(config)#do ping 1.1.1.1      
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/60/68 ms
R4(config)#do sh ip route 1.1.1.1
Routing entry for 1.1.1.0/24
  Known via "eigrp 1", distance 170, metric 2560002560, type external
  Redistributing via eigrp 1
  Last update from 172.16.1.3 on FastEthernet1/1, 00:27:19 ago
  Routing Descriptor Blocks:
    172.16.2.3, from 172.16.2.3, 00:27:19 ago, via FastEthernet0/0
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
  * 172.16.1.3, from 172.16.1.3, 00:27:19 ago, via FastEthernet1/1
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
R4(config)#do ping 1.1.1.1      
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 40/63/88 ms
R4(config)#do sh ip route 1.1.1.1
Routing entry for 1.1.1.0/24
  Known via "eigrp 1", distance 170, metric 2560002560, type external
  Redistributing via eigrp 1
  Last update from 172.16.1.3 on FastEthernet1/1, 00:27:33 ago
  Routing Descriptor Blocks:
  * 172.16.2.3, from 172.16.2.3, 00:27:33 ago, via FastEthernet0/0
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
    172.16.1.3, from 172.16.1.3, 00:27:33 ago, via FastEthernet1/1
      Route metric is 2560002560, traffic share count is 1
      Total delay is 100 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1500 bytes
      Loading 255/255, Hops 1
R4(config)#

but you can turn on ip load sharing for cef...

R4(config-if)#do sh cef int f0/0
FastEthernet0/0 is up (if_number 2)
  Corresponding hwidb fast_if_number 2
  Corresponding hwidb firstsw->if_number 2
  Internet address is 172.16.2.4/24
  ICMP redirects are always sent
  Per packet load-sharing is disabled
  IP unicast RPF check is disabled
  IP policy routing is disabled
  BGP based policy accounting on input is disabled
  BGP based policy accounting on output is disabled
  Hardware idb is FastEthernet0/0
  Fast switching type 1, interface type 18
  IP CEF switching enabled
  IP CEF switching turbo vector
  IP CEF turbo switching turbo vector
  IP prefix lookup IPv4 mtrie 8-8-8-8 optimized
  Input fast flags 0x0, Output fast flags 0x0
  ifindex 2(2)
  Slot  Slot unit 0 VC -1
  IP MTU 1500
R4(config-if)#ip load-shar ?
  per-destination  Deterministic distribution
  per-packet       Random distribution

refs:

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a0080094806.shtml

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094820.shtml



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