This article explains how SRX advertises the aggregate route to its EBGP peer when there is only 1 contributing route active in its routing table.

TOPOLOGY

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SRX-A====ospf====SRX-B====EBGP===SRX-C===EBGP===SRX-D

+ SRX-B is receiving route 10.10.2.177/32 from SRX-A via OSPF.

+ SRX-B is receiving route 10.10.2.178/32 from SRX-C via EBGP.

+ We are adding an aggregate route 10.10.2.176/28 on SRX-B and advertising it to SRX-C via EBGP.

User@srx-B> show route                                        

inet.0: 11 destinations, 11 routes (11 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

0.0.0.0/0         *[Static/5] 2d 10:44:16

                   > to 10.10.10.1 via ge-0/0/0.0

10.10.2.176/28 *[Aggregate/130] 00:02:57

                      Discard

10.10.2.177/32 *[OSPF/150] 00:00:04, metric 0, tag 0

                   > to 192.168.1.1 via ge-0/0/1.0

10.10.2.178/32 *[BGP/170] 00:07:24, localpref 100

                     AS path: 131 132 I, validation-state: unverified

                   > to 192.168.2.2 via ge-0/0/2.0

10.10.10.0/24  *[Direct/0] 2d 10:44:16

                   > via ge-0/0/0.0

10.10.10.3/32  *[Local/0] 2d 10:44:16

                      Local via ge-0/0/0.0

192.168.1.0/24    *[Direct/0] 2d 10:44:16

                   > via ge-0/0/1.0

192.168.1.2/32    *[Local/0] 2d 10:44:16

                      Local via ge-0/0/1.0

192.168.2.0/24    *[Direct/0] 2d 10:44:16

                   > via ge-0/0/2.0

192.168.2.1/32    *[Local/0] 2d 10:44:16

                      Local via ge-0/0/2.0

224.0.0.5/32      *[OSPF/10] 00:00:14, metric 1

                      MultiRecv


User@srx-B> show route advertising-protocol bgp 192.168.2.2   

inet.0: 10 destinations, 10 routes (10 active, 0 holddown, 0 hidden)

 Prefix                 Nexthop             MED    Lclpref   AS path

* 10.10.2.176/28     Self                                   131 132 I


Now, if the BGP route 10.10.2.178/32 becomes inactive for some reason and the only active contributing route in SRX routing table is the one learnt from OSPF.

User@srx-B> show route   
inet.0: 11 destinations, 11 routes (10 active, 0 holddown, 1 hidden)

+ = Active Route, - = Last Active, * = Both
0.0.0.0/0         *[Static/5] 2d 09:25:02

                   > to 10.10.10.1 via ge-0/0/0.0

10.10.2.176/28 *[Aggregate/130] 00:00:08

                      Discard

10.10.2.177/32 *[OSPF/150] 00:00:08, metric 0, tag 0

                   > to 192.168.1.1 via ge-0/0/1.0

10.10.10.0/24  *[Direct/0] 2d 09:25:02

                   > via ge-0/0/0.0

10.10.10.3/32  *[Local/0] 2d 09:25:02

                      Local via ge-0/0/0.0

192.168.1.0/24    *[Direct/0] 2d 09:25:02

                   > via ge-0/0/1.0

192.168.1.2/32    *[Local/0] 2d 09:25:02

                      Local via ge-0/0/1.0

192.168.2.0/24    *[Direct/0] 2d 09:25:02

                   > via ge-0/0/2.0

192.168.2.1/32    *[Local/0] 2d 09:25:02

                      Local via ge-0/0/2.0


SRX-B will continue to advertise aggregate route 10.10.2.176/28 to SRX-C via BGP but the AS PATH of the aggregate route changes to I indicating that it is an IGP route:

User@srx-B> show route advertising-protocol bgp 192.168.2.2   



inet.0: 11 destinations, 11 routes (10 active, 0 holddown, 1 hidden)

 Prefix                 Nexthop             MED    Lclpref   AS path

* 10.10.2.176/28     Self                                   I

  

This is an expected behaviour as the only active contributing route in SRX routing table is an OSPF route and OSPF does not have any path attribute concept, so the aggregate route will be advertised to EBGP neighbour as IGP route. In the above case, peer device will instal the route successfully in its routing table.