Network Engineering Asked on October 12, 2020
I have a Juniper router named r3
with one established eBGP session to router t1
(172.16.0.14
) and with second established eBGP session to router t2
(172.16.0.18
). BGP configuration of r3
can be seen below:
root@r3> show configuration protocols bgp group t1-t2
type external;
peer-as 65222;
multipath;
neighbor 172.16.0.14;
neighbor 172.16.0.18;
root@r3>
r3
receives the 1.1.1.1/32
prefix over both sessions:
root@r3> show route receive-protocol bgp 172.16.0.14 1.1.1.1/32
inet.0: 71 destinations, 84 routes (60 active, 0 holddown, 11 hidden)
Prefix Nexthop MED Lclpref AS path
* 1.1.1.1/32 172.16.0.14 65222 I
root@r3> show route receive-protocol bgp 172.16.0.18 1.1.1.1/32
inet.0: 71 destinations, 84 routes (60 active, 0 holddown, 11 hidden)
Prefix Nexthop MED Lclpref AS path
1.1.1.1/32 172.16.0.18 65222 I
root@r3>
Thanks to multipath
configuration option both the 172.16.0.14
and 172.16.0.18
are next-hops for the 1.1.1.1/32
prefix in case of the active route announced by the t1
(172.16.0.14
) which has the lower RID than the t2
. However, only the active route has two next hops calculated:
root@r3> show route 1.1.1.1 detail
inet.0: 71 destinations, 84 routes (60 active, 0 holddown, 11 hidden)
1.1.1.1/32 (2 entries, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 0
Address: 0xb634310
Next-hop reference count: 10
Source: 172.16.0.14
Next hop: 172.16.0.14 via ge-0/0/4.0, selected
Session Id: 0x0
Next hop: 172.16.0.18 via ge-0/0/7.0
Session Id: 0x0
State: <Active Ext>
Local AS: 65000 Peer AS: 65222
Age: 2:42
Validation State: unverified
Task: BGP_65222.172.16.0.14+40876
Announcement bits (4): 0-KRT 4-BGP_RT_Background 5-Resolve tree 1 7-BGP_RT_Background
AS path: 65222 I
Accepted Multipath
Localpref: 100
Router ID: 4.4.4.4
BGP Preference: 170/-101
Next hop type: Router, Next hop index: 0
Address: 0xd2ecab0
Next-hop reference count: 9
Source: 172.16.0.18
Next hop: 172.16.0.18 via ge-0/0/7.0, selected
Session Id: 0x0
State: <NotBest Ext>
Inactive reason: Not Best in its group - Active preferred
Local AS: 65000 Peer AS: 65222
Age: 6:54
Validation State: unverified
Task: BGP_65222.172.16.0.18+58923
AS path: 65222 I
Accepted MultipathContrib
Localpref: 100
Router ID: 5.5.5.5
root@r3>
Is this some sort of optimization in Junos that the recursive next hops are found only for the active route?
If you do show route 1.1.1.1/32 extensive
you should see another field Junos calls protocol next-hop which may be less confusing. In the output you've pasted, it is listing all selected, adjacent next-hops, which are determined by resolving the next-hops to the protocol next-hop.
For example, if the protocol next-hop 198.51.100.100
is several layer-3 hops away, and the RIB contains two equal OSPF-learnt routes to 198.51.100.100, the type of display you've pasted would include those two OSPF-learnt next-hops in the active route stanza.
It would be better if Junos labeled these fields differently. We always have to keep in mind next-hop has a complex meaning in the context of BGP.
Answered by Jeff Wheeler on October 12, 2020
Get help from others!
Recent Answers
Recent Questions
© 2024 TransWikia.com. All rights reserved. Sites we Love: PCI Database, UKBizDB, Menu Kuliner, Sharing RPP