Difference Between Inbound Vs Outbound Soft Reconfig

[Mitesh Manwatkar]

I am confused about the difference between the two.

How are they used?

Kindly help with this

[Richard Burts]

For inbound soft your router will keep a record of all advertisements received from neighbors before any policy was applied. When you do soft reconfig in it reads through all the received advertisements, applies the current policies, and implements any changes that it detects due to current policy changes.

For outbound soft your router reads through its route table as if it were preparing an advertisement, applies current policies, and advertises to neighbors any changes that it finds due to changes in current policy.

They both can be used when you have made configuration changes that change your BGP policies, for example when you have changed a distribute list or a prefix list that determine what to receive or what to advertise.

HTH

Rick

[paul driver]

Hello

Just like to add that inbound soft basically requires two copies of the advertised prefixes towards your router stored in memory which naturally take up valuable resources.

I A more applicable solution would be to use Outbound Route Filtering (Odr) and Route refresh in which you create a outbound filter which is advertised to your neighbouring peer which would in turn only send the actual prefixes your have defined in your acl and  with the route refresh perform a soft clearing to the bgp table without any intrusive change to the peering.

res

Paul

[Mitesh Manwatkar]

why does it require two copies...of the advertised prefixes ?

[Joseph W. Doherty]

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Posting

BTW, I believe newer BGP implementation support a BGP option that one router can request an inbound soft update without the need to maintain a local secondary copy.

[paul driver]

Hello
In BGP you have 3 tables plus the main router routing table (rib)
adj-rib-in
adj-rib-out
loc-rib

BGP is based on a tcp session, and a policy change/route update for bgp requires to be propagated to its neighboring peers so to do this the bgp session requires to be reset ( clear ip bgp *) which will cause an outage to the peering.

Soft reconfiguration (inbound/outbound) can be used so not to tear down the bgp peering. However when the soft reconfiguration is used a prefix change from your bgp peer goes into its own routers  adj-rib –out table then it can be sent to your router ( clear ip bgp * soft out)

This is then stored in your router, in the bgp adj-rib-in table which will utilize additional memory and cpu processes while its waiting for you to initiate the clear ip bgp * soft in command.then these updates go into the loc rip table and then into the router rib table itself.
 

So basically you have two copies of the routing information, the one being used by the router and the one waiting to be introduced from the adj-ribs tables.

As I have stated previous Route –Fresh has since been introduced in bgp, so by dynamically requesting your peers adj-rib-out table whenever you request it ( clear ip bgp * soft in) this negates the extra memory/cpu overhead required by soft reconfiguration inbound

I do hope I have explained this correctly , I am sure someone on these forums will correct me if I am wrong.

Res
Paul



 

[Mitesh Manwatkar]

It has definitely helped me to learn something which i had not heard of...so thank u so much for this..

regards.

[paul driver]

Hello

Please rate any post that has been helpful

res

Paul

[prajithtr_2]

Hi Mitesh,

If the policy change happens in remote side ,clearing the neighbor will make some impact because it has to update the full routes in the routing table again and it will take some time. but instead if we had pre-configured soft reconfig inbound ccommand with the neighbor ,it will keep a copy of current table in its memory ,clear it,again store it back ,So it will not take much time to update the full routing table ,so in effect no much impact.This command is used when the peer doesnt have route refresh capability.
Command sytax:
neighbor X.X.X.X soft-reconfiguration inbound
clear ip bgp X.X.X.X soft in

If we have some policy chage in our side,we can use soft out commmand .Soft reset out doesnt require any preconfiguration.
Command syntax:
clear ip bgp X.X.X.X soft out

regards

Prajith