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CEF Load Sharing Recursive Lookup

Hi,

i have this scenario:

| ----P1----P2---|

PE1----| |----PE2

| ----P3----P4---|

IP Loop PE1: 5.5.5.5

There are 2 Gigabitethernet betweenn the router PE and the router P.

PE1 advertise 1.1.1.1/32 to PE2:

PE2#sh ip route 1.1.1.1

Routing entry for 1.1.1.1/32

Known via "bgp 65000", distance 200, metric 0, type internal

Last update from 5.5.5.5 01:57:03 ago

Routing Descriptor Blocks:

* 5.5.5.5, from 4.4.4.4 , 01:57:03 ago

Route metric is 0, traffic share count is 1

AS Hops 0, BGP network version 0

PE2 has 4 link (2 towards P2 and 2 towards P4) at equal cost to reach 5.5.5.5/32 via OSPF:

PE2# sh ip route 5.5.5.5

Routing entry for 5.5.5.5/32

Known via "ospf 65000", distance 110, metric 200, type inter area

Last update from 23.23.23.24 on GigabitEthernet8/0/0, 01:13:45 ago

Routing Descriptor Blocks:

* 22.22.22.21, from IP-P1, 01:13:45 ago, via GigabitEthernet3/0/0

Route metric is 200, traffic share count is 1

23.23.23.23, from IP-P1, 01:13:45 ago, via GigabitEthernet8/0/0

Route metric is 200, traffic share count is 1

22.22.22.22, from IP-P2, 01:13:45 ago, via GigabitEthernet3/0/0

Route metric is 200, traffic share count is 1

23.23.23.24, from IP-P2, 01:13:45 ago, via GigabitEthernet8/0/0

Route metric is 200, traffic share count is 1

If I look cef table I have all the 4 link too:

PE2# sh ip cef 5.5.5.5

5.5.5.5/32, version 3101050, epoch 0, per-destination sharing

0 packets, 0 bytes

Flow: AS 0, mask 32

tag information set, shared, all rewrites owned

local tag: 1541

via 22.22.22.21, GigabitEthernet3/0/0, 14 dependencies

traffic share 1

next hop 22.22.22.21, GigabitEthernet3/0/0

valid adjacency

tag rewrite with Gi3/0/0, 22.22.22.21, tags imposed {390}

via 23.23.23.23, GigabitEthernet8/0/0, 14 dependencies

traffic share 1

next hop 23.23.23.23, GigabitEthernet8/0/0

valid adjacency

tag rewrite with Gi8/0/0, 23.23.23.23, tags imposed {390}

via 22.22.22.22, GigabitEthernet3/0/0, 15 dependencies

traffic share 1

next hop 22.22.22.22, GigabitEthernet3/0/0

valid adjacency

tag rewrite with Gi3/0/0, 22.22.22.22, tags imposed {390}

via 23.23.23.24, GigabitEthernet8/0/0, 15 dependencies

traffic share 1

next hop 23.23.23.24, GigabitEthernet8/0/0

valid adjacency

tag rewrite with Gi8/0/0, 23.23.23.24, tags imposed {390}

0 packets, 0 bytes switched through the prefix

tmstats: external 0 packets, 0 bytes

internal 0 packets, 0 bytes

But if i Look the cef table for the address 1.1.1.1/32 I have only 1 link:

PE2# sh ip cef 1.1.1.1

1.1.1.1/32, version 6477717, epoch 0, per-destination sharing

0 packets, 0 bytes

Flow: AS 0, mask 32

tag information from 5.5.5.5/32, shared, all rewrites owned

local tag: 1541

via 5.5.5.5, 0 dependencies, recursive

next hop 22.22.22.22, GigabitEthernet3/0/0 via 5.5.5.5/32 (Default)

valid adjacency

tag rewrite with Gi3/0/0, 22.22.22.21, tags imposed {390}

Recursive load sharing using 5.5.5.5/32.

The question is: why PE2 doesn't utilize all the 4 link (towards P2 and P4) to reach 1.1.1.1/32?

Thanks in advance

Rgds

Gianluca

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: CEF Load Sharing Recursive Lookup

Although the "show ip cef 1.1.1.1" command only displays one interface, the recursive loadsharing will take place as expected.

Do a "show ip cef 1.1.1.1 internal", which will show you the 4 outbound interfaces being used.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
2 REPLIES
Cisco Employee

Re: CEF Load Sharing Recursive Lookup

Although the "show ip cef 1.1.1.1" command only displays one interface, the recursive loadsharing will take place as expected.

Do a "show ip cef 1.1.1.1 internal", which will show you the 4 outbound interfaces being used.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: CEF Load Sharing Recursive Lookup

Thanks a lot!!!

It's been very helpful.

Gianluca

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