Asymmetric Routing issue

nihammimm · ‎06-29-2011

Hi,

One of the server hosted at Site-A , not able to connect the application which is hosted at Site-B Server , there is no firewall in between.Server IP's are pingable from both the end. See the below trace route from SITE-A to SITE-B and vice versa. based on below trace result we can see that there is some asymmetric route beween these two sites.

Does asymmetric routing can affect application operation?

Please advise ?

Core-SWITCH-SITE-A#traceroute 50.35.3.220 (Server-a)

1 162.28.0.9    4 msec 0 msec 0 msec
2 50.133.51.176 4 msec 0 msec 4 msec
3 50.133.51.25 44 msec 48 msec 44 msec
4 50.129.2.246   48 msec 48 msec 48 msec
5 191.168.201.30 144 msec 140 msec 144 msec
6 50.35.3.220    140 msec 140 msec 144 msec

Core-SWITCH-SITE-B#traceroute 162.18.17.28 (Server-b)

1 50.135.2.3     152 msec 152 msec 156 msec
2 50.133.51.22 136 msec 136 msec 140 msec
3 50.133.50.2   136 msec 156 msec 144 msec
4 50.133.251.175 140 msec 140 msec 140 msec
5 162.28.0.8    140 msec 144 msec 140 msec
6 162.28.17.28   148 msec 156 msec 152 msec

Regard,

Chaitra Shree · ‎06-29-2011

Hi Niham,

Asymmetric routing occurs when transmit and receive packets follow different paths between a host and the peer with which it communicates. In your scenario, if you see a different path when you traceroute to server B from server A and vice versa, it indicates asymmetric routing .

Asymmetric routing can lead to issues like unicast flooding ,missing MLS entries, latency and drops. The below link provides a detailed explanation about asymmetric routing:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801d0808.shtml#cause1

This can be tackled by applying the following:

Adjust the MAC aging time on the respective switches to 14,400 seconds (four hours) or longer :

mac-address-table aging-time seconds [vlan vlan_id]

Change the ARP timeout on the routers to five minutes (300 seconds).
Change the MAC aging time and ARP timeout to the same timeout value.

HTH,

Chaitra