Confusion in SOO

[Deepak Sukumar]

Hi All,

          I am not  able to understand what is the use of SOO. I tried searching in Google  and some say its used with as-overide and only when BGP is used between  PE and CE to avoid loop. I tried with the following topology and my  doubt is as-path is used to avod loops ,in that case why use SOO.

Details are as follows:

CE1----Bgp---- PE1---------mp-ibgp---------PE2-----bgp---------CE2

                      CE1-------ibgp---------CE2

PE1 and PE2 are in Same AS 100. CE1 and CE2 are in AS 64500

CE1 and CE2 are having bgp session.

Before applying SOO here the details are as follows

Prefix 192.1.1.0/24 is advertised from CE1.This will be learnt in CE2 and PE1.

so on PE2 ,we will have two paths for prefix 192.1.1.0/24 . One from CE2 and Other from PE1.

PE2(config)# show protocol bgp routing-instance Test route

Status codes: * valid, > best, d dampened, h history, ! inactive

              s suppressed, l local, a aggregate, i ibgp, e ebgp

              p learnt-imported, j local-imported, S stale

Origin codes: i IGP, e EGP, ? incomplete

      Network            Peer            Next Hop        Path

      -------            ----            --------        ----

RD 650:2  (default for instance Test)

*p    15.1.0.0/24        3.3.3.3         3.3.3.3         64500 i

*>e   15.1.0.0/24        42.1.1.2        42.1.1.2        64500 i

*p    192.1.1.0/24       3.3.3.3         3.3.3.3         64500 i

*>e   192.1.1.0/24       42.1.1.2        42.1.1.2        64500 i

And in CE2 there is a entry from CE1 alone.

R2-CE2-8860(config)# show protocol bgp route 192.1.1.0/24 brief

Status codes: * valid, > best, d dampened, h history, ! inactive

              s suppressed, l local, a aggregate, i ibgp, e ebgp

              p learnt-imported, j local-imported, S stale

Origin codes: i IGP, e EGP, ? incomplete

      Network            Peer            Next Hop        Path

      -------            ----            --------        ----

*>i   192.1.1.0/24       5.5.5.5         5.5.5.5         i

Now if I apply SOO

PE2:

8860-R4-PE2(config)# show protocol bgp routing-instance Test route valid

Status codes: * valid, > best, d dampened, h history, ! inactive

             s suppressed, l local, a aggregate, i ibgp, e ebgp

             p learnt-imported, j local-imported, S stale

Origin codes: i IGP, e EGP, ? incomplete

     Network           Peer           Next Hop       Path

     -------           ----           --------       ----

RD 650:2 (default for instance Test)

*>e   15.1.0.0/24       42.1.1.2       42.1.1.2       64500 i

*>p   192.1.1.0/24       3.3.3.3         3.3.3.3         64500 i

8860-R4-PE2(config)# show protocol bgp routing-instance Test rejected-routes

Status codes: S stale

Origin codes: i IGP, e EGP, ? incomplete

Peer           Network           Next Hop       Path

----           -------           --------       ----

42.1.1.2       192.1.1.0/24       42.1.1.2       64500 i

My  doubt here is without SOO there is no loop formed.From Pe2 route if  advertised will be dropped by CE2 cause of AS-path list.If i see my own  AS i will discard it.

So what is the real use of SOO  ? Can some one explain ?

[rais]

ASN is used to avoid loops in BGP peerings. However, there are situations in which the ASN is not sufficient on its own.

SOO is useful in cases CE is multihomed to different PEs e.g. CE1 is multihomed to PE1 and PE2.  PE1 learns routes from CE1 and tags SOO to the prefixes received. PE2 learns routes from PE1 via iBGP. PE2 will not advertise routes learned from PE1 back to CE1 because PE2 also has the same SOO for CE1.

HTH.