Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

bgp commands

Could somebody help me understand the network and redistribute commands as used in context of BGP?

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

bgp commands

Hi,

Yes you're correct only the BGP best routes(in the BGP table) will be advertised not taking filtering into account of course.

Then the peer receiving the routes(if they are not filtered) will install them in the RIB if it doesn't know about them from a source with a better AD.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
18 REPLIES
Silver

bgp commands

The network command will check prefixes from the Route Table and add it to the BGP table.

The redistribute command will check prefixes from the Route Table, depending of what protocol are you redistributing (OSPF, RIP, IS-IS, EIGRP connected, statics...) from,  and add it to the BGP table.

One of the differences is that the Origin attribute by default will be IGP in case of the Network command and Incomplete in case of redistribution.

The Network command gives you better control about what prefixes you would like to add. On the other hand, redistribution could be better in case you want to add a lot of prefixes and using the network command for every prefix would be cumbersome.

Hope this helps,

Jose.

New Member

bgp commands

Thanks Jose.

Suppose I am running bgp on router A...Router A has 10.1.1.1/32 loopback on it...Router A is also learning 10/8 subnetworks using ospf...then if I do "network 10.1.1.0 mask 255.255.255.0" under router bgp configuration, what networks (if at all) will bgp advertise? What if I do "redistribute ospf 1 subnets" under router bgp configuration?

Purple

bgp commands

Hi,

if you have no auto-suumary which is the default then to inject a route in the BGP table with the network command the mask must correspond to the subnet mask in the routing table so the /32 won't get injected for sure with a mask of 255.255.255.0, only the corresponding  /24s that are in the routing table will get injected.

Now if you redistribute ospf then only the OSPF routes will get injected.

if you have auto-summary then classful network will be injected but not the subnets.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
New Member

bgp commands

Thanks Cadet.

How does the routing information flow in this case? Specifically, how routes from routing table (known via ospf) go to bgp table and further to routing table (as bgp known routes) and in what sequence? How is the information advertised to other bgp peers?

New Member

bgp commands

HI Kashish,

In this case you have OSPF routes in the routing table say 10.0.0.0/8 as OPSF routes with an AD of 110. When you redistribute this route into BGP the same route 10.0.0.0/8 will be populated into the BGP table. The router now has the same route from two routing protocols ie OSPF and BGP and it needs to select the best route which it does based on the AD. In this case since the BGP routes are redistributed locally the AD of the 10.0.0.0/8 BG route is 200. And hence the OSPF roue wins and remains in the global routing table. So if you have a BGP peer this route will not be advertised to that peer.

HTH !!!

Regards

Umesh Shetty

Purple

bgp commands

Hi,

So if you have a BGP peer this route will not be advertised to that peer.

WRONG  if this was the case what would be the use of redistribution ?

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
New Member

bgp commands

I am confused and still looking for an answer for:

How does the routing information flow in this case? Specifically, how  routes from routing table (known via ospf) go to bgp table and further  to routing table (as bgp known routes) and in what sequence? How is the  information advertised to other bgp peers?

Purple

bgp commands

Hi,

they stay as OSPF routes in the routing table but get injected in the BGP table then get advertised to peers that can install them as BGP in the routing table provided these BGP peers don't learn these eoutes from another protocol with better( lower) AD

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
New Member

bgp commands

So it means that bgp will advertise best routes (from its bgp table) to its peer and it is not always that these best routes will also be present in ip routing table as bgp routes??

New Member

bgp commands

Hi Alain,

I was wrong you were right..the routes are locally originated hence will be advertised...thnx for correcting me here.

Kashish,

This document will help you in understanding basic route selection criteria in cisco routers, later on reading through redistribution will help you understand things better...

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml

Regards

Umesh Shetty

Silver

bgp commands

Not exactly related to this case but, by default, BGP advertises routes even though are not the best or are not in the routing table. For example:

- a prefix is learned via OSPF and internal BGP. The AD of OSPF is better 110 to 200, it will give a RIB failure but still the prefix will be propagated to the eBGP peer.

The reason why is advertising these prefixes in this case is because by default BGP sends inactive routes by default, and you may modify this behavior with: bgp suppress-inactive

http://www.cisco.com/en/US/docs/ios/12_2s/feature/guide/fs_sbair.html

Best Regards,

Jose.

Purple

bgp commands

Hi,

rib-failures can still be best routes( greater than sign), these are 2 different things and routes that are not best routes are not propagated.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
Silver

bgp commands

Cadet,

What I was trying to say is the best routes regarding the RIB, not BGP best routes.

Best Regards,

Jose.

New Member

bgp commands

Cadet,

Your answers have helped...Could you confirm my understanding here:

bgp propagates best routes from its bgp table to its bgp peers...And it is not always true that these best routes from bgp table will also go to routing table of that router....so the decision of what routes will be advertised is taken based on bgp table and not ip routing table?

New Member

bgp commands

Could anybody confirm my understanding here?

Purple

bgp commands

Hi,

Yes you're correct only the BGP best routes(in the BGP table) will be advertised not taking filtering into account of course.

Then the peer receiving the routes(if they are not filtered) will install them in the RIB if it doesn't know about them from a source with a better AD.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.

bgp commands

Nice explanation Cadet.

New Member

bgp commands

Thanks Cadet!!

353
Views
20
Helpful
18
Replies