Routing

Hi

 

I have two scenario:

 

1- My AS is connected to two different routers in the same AS. Now If the router in my AS recieve the same routes from the two routers then this router will consider the router-id or not in selecting the best BGP route?

 

2- My AS is connected to two different routers in different AS. Now If the router in my AS recieve the same routes from the two routers then this router will consider the router-id or not in selecting the best BGP route?

 

Thanks

Hi Dear,

 

There are the steps of route selection in BGP

 

The Route Selection Process

 

1. The Next Hop attribute value for each route must be reachable in the local routing table;

otherwise, the local router discards the route.

2. The router selects the route with the highest Local Preference attribute value.

3. The router selects the route with the shortest AS Path length.

4. The router selects the route with the smallest Origin attribute value.

5. The router selects the route with the smallest Multiple Exit Discriminator attribute value.

This step is executed, by default, only for routes from the same neighboring AS.

6. The router selects routes learned from an EBGP peer over routes learned from an IBGP

peer. If the remaining routes are all EBGP-learned routes, the router skips to step 9.

7. The router selects the route with the smallest IGP metric to the advertised BGP Next Hop.

8. If Route Reflection is used for IBGP peering, the router selects the route with the shortest

Cluster-List length.

9. The router selects the route from the peer with the smallest numerical Router ID.

10. The router selects the route from the peer with the smallest numerical Peer Address.

 

 

- So the route will be selected based on Router ID if there is it reached to step no. 9 in the evaluation in both scenario.

 

Thanks,

Mohamed Elhariry

JNCIE-M/T # 1059, CCNP & CCIP

Hi,

Only exception would be when you use "multipath". When you enable this, ROUTER_ID and PEER_ID part would be skipped in selection process.

Regards
Surya Prakash

Hi Surya

 

Thanks for the reply. Actually I read some where but I forgot where ...

 

When the same EBGP routes are recieved then router id are not compared. Instead the routes would be chosen in the order of arrival, unless you configure set protocols bgp path-selection external-router-id. 

 

On the other hand if same IBGP routes are recieved then router id is compared and the routes havign lowest peer RD would be chosen.

 

Kindly correct me if this is right

Hi aeroplane,

 

I Think you are right. By default, the router IDs of routes received from different ASs are not compared. You can change this default behavior.

 

http://www.juniper.net/techpubs/software/junos-security/junos-security95/junos-security-swconfig-interfaces-and-routing/section-bgp-overview.html

 

Also using help topic for the command you typed indicate same

 

root@R5> help topic bgp path-selection | find "Comparing the router ID"
     * Comparing the router ID between external BGP paths to determine the
       active path (external-router-id). By default, router ID comparison is
       not performed if one of the external paths is active.
     * Adding the IGP cost to the next-hop destination to the MED before
       comparing MED values for path selection.

 

I tried to test it also with small LAB and it choose the first active route as you mentioned and after clear the BGP of this neighbot it took the other one as active route and not changed after the BGP session re-estabilshed again

 

root@R5# run show route 10.0.8.0 detail     

inet.0: 22 destinations, 25 routes (22 active, 0 holddown, 0 hidden)
10.0.8.0/30 (2 entries, 1 announced)
        *BGP    Preference: 170/-101
                Next hop type: Indirect
                Next-hop reference count: 3
                Source: 10.0.9.7
                Next hop type: Router, Next hop index: 470
                Next hop: 10.0.8.10 via em3.0, selected
                Protocol next hop: 10.0.9.7
                Indirect next hop: 8a2b09c 131071
                State: <Active Ext>
                Local AS: 65412 Peer AS: 65414
                Age: 2:34       Metric2: 0
                Task: BGP_65414.10.0.9.7+179
                Announcement bits (3): 0-KRT 3-BGP RT Background 4-Resolve tree 1
                AS path: 65414 I
                Localpref: 100
                Router ID: 10.0.9.7
         BGP    Preference: 170/-101
                Next hop type: Indirect
                Next-hop reference count: 1
                Source: 10.0.9.6
                Next hop type: Router, Next hop index: 464
                Next hop: 10.0.8.5 via em2.0, selected
                Protocol next hop: 10.0.9.6
                Indirect next hop: 8a2b000 -
                State: <Ext>
                Inactive reason: Active preferred
                Local AS: 65412 Peer AS: 65413
                Age: 35         Metric2: 0
                Task: BGP_65413.10.0.9.6+179
                AS path: 65413 I
                Localpref: 100
                Router ID: 10.0.9.6
                   

Thanks,

Mohamed Elhariry

JNCIE-M/T # 1059, CCNP & CCIP

Hi,

Actually yes, but there has been changes from 10.0 JUNOS version (AFAIK). The selection process is as in below URL:

http://www.juniper.net/techpubs/en_US/junos10.2/topics/reference/general/routing-ptotocols-address-representation.html

 

Hope this helps.

 

Regards

Surya Prakash

Hi Surya

 

Thank a lot.