Cisco Support Community
Community Member

SoO for IGPs

I have a few queries regarding the utilization of SoO attribute:

1. use of SoO is understandable in case eBGP is the PE/CE routing protocol.


- If SoO is not configured, chelmford CE router will accept the the eBGP update for since the route is originated from another CE within its own AS and the chelmsford-CE might have recieved it from Newyork-CE as iBGP route. Since the weight attribute of received routes is 0 the  preference would be made on eBGP vs iBGP criteria.

- If SoO is not configured chelmsfor-CE will not be able to differentiate whether a route received is from its own site (by another CE) or from other site within the same VPN , because of asoverride configuration.

Therefore SoO is necessary in case eBGP is the PE/CE RP.

in case of IGPs/Static/connected.. do we need SoO ????

          since IGP updates from a CE will always be prefered over iBGP updates from other PEs on a PE and similarly on a CE

          and in case of eBGP it is required since eBGP route is prefered over iBGP route on a PE and on CE...

Cisco Employee

Re: SoO for IGPs


The primary function of SoO is to prevent routing loops when the site is dual homed.

"in case of IGPs/Static/connected.. do we need SoO ????"

It depends on what IGP you are trying to use. With OSPF, we dont need SoO as they have inbuilt mechanism for loop avoidance like DOWN bit and DOMAIN-TAG. With RIP/EIGRP, we need SoO to be configured.



Community Member

Re: SoO for IGPs


there are two directions for a routing loop to occur,

     i. customer network

     ii. provider network

OSPF down-bit is used to prevent routing loop in customer network. PE not accepting routes from CE whose DN bit is set

SoO is used within the provider network, PE will not accept route from another PE depending on the SoO value..

we donot need the ospf down-bit funtionallity with eBGP as the routing protocol since the PE will not accept route from CE whose as-path list contains its own AS#..

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