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BGP hold down Null0 route question

I have a CIDR block I advertise over BGP, 162.212.188.0/22. I am utilizing this full /22 CIDR block on a single VLAN interface on my internal 6509 core switch. On the 7204VXR I had configured a "hold down" route for the 162.212.188.0/22 BGP advertisement, but when I tried to configure the VLAN on my 6509 as a full /22, the OSPF advertisement to the 7204VXR matched the "hold-down" null0 route, and was ignored by the 7204VXR.

What would be the proper way to provide a "hold down" route for the BGP avertisement of the /22 when the router is also getting the path to that /22 from an internal router over OSPF?

thanks

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Accepted Solutions
Cisco Employee

BGP hold down Null0 route question

Jeff,

If I understand you correctly, you have configured a static route to 162.212.188.0/22 via Null0 on your 7204 router and you are injecting that route into BGP to make sure it is constantly advertised.

Now you have created a real network 162.212.188.0/22 on a VLAN SVI and when you advertise that network in OSPF, it is not present in the 7204's routing table because, obviously, the static route is already there and OSPF route can not replace it.

There are more ways of solving this issue. One of the easiest approaches is to increase the administrative distance of the static Null0 route to be higher than OSPF's. This will make sure that as long as OSPF advertises this exact network, it will be placed into the routing table on the 7204. Only if the OSPF removes, for whatever reasons, this network from the 7204's routing table, the static route will be installed into the routing table instead. This is accomplished very easily: assuming you have not changed the administrative distance of OSPF on your 7204, the command would be:

ip route 162.212.188.0 255.255.252.0 Null0 111 ! or any number between 111 - 254 inclusive

Do you believe you could test this approach and verify that it works for you?

By the way, these Null0 routes are not called hold-down routes - their proper name is discard routes. A hold-down is a different term whose meaning strongly relates to RIP and IGRP operation (a period of ignoring any updates related to a network that has apparently become unreachable).

Best regards,

Peter

3 REPLIES
Cisco Employee

BGP hold down Null0 route question

Jeff,

If I understand you correctly, you have configured a static route to 162.212.188.0/22 via Null0 on your 7204 router and you are injecting that route into BGP to make sure it is constantly advertised.

Now you have created a real network 162.212.188.0/22 on a VLAN SVI and when you advertise that network in OSPF, it is not present in the 7204's routing table because, obviously, the static route is already there and OSPF route can not replace it.

There are more ways of solving this issue. One of the easiest approaches is to increase the administrative distance of the static Null0 route to be higher than OSPF's. This will make sure that as long as OSPF advertises this exact network, it will be placed into the routing table on the 7204. Only if the OSPF removes, for whatever reasons, this network from the 7204's routing table, the static route will be installed into the routing table instead. This is accomplished very easily: assuming you have not changed the administrative distance of OSPF on your 7204, the command would be:

ip route 162.212.188.0 255.255.252.0 Null0 111 ! or any number between 111 - 254 inclusive

Do you believe you could test this approach and verify that it works for you?

By the way, these Null0 routes are not called hold-down routes - their proper name is discard routes. A hold-down is a different term whose meaning strongly relates to RIP and IGRP operation (a period of ignoring any updates related to a network that has apparently become unreachable).

Best regards,

Peter

Community Member

BGP hold down Null0 route question

Peter,

You have it completely correct.

Thank you for your help.

Cisco Employee

BGP hold down Null0 route question

Jeff,

You are heartily welcome. Thank you!

Best regards,

Peter

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