BGP Advertising problem

Answered Question
May 9th, 2008
User Badges:

Small ISP peering with upstream (ATT) and a customer to act as redundant link for that customer. I am getting routes from customer and I am advertising them to AT&T. AT&T tech says their side shows a problem with the route map but can provide no other details. They are set to accept the subnet as well as my other subnets and receive my other subnets just fine. I've been over it and over it, hopefully it's something obvious...

The 65.x.x.0 network is the one in question. The other subnets are on my network. I have routes for all of mine, but no "ip route" statements for the 65.x.x.0 as it is acquired by BGP.


!

router bgp xxxx1

no synchronization

bgp log-neighbor-changes

network 65.x.x.0 mask 255.255.255.0

network 67.x.x.0 mask 255.255.255.0

network 68.1x.x.0 mask 255.255.255.0

network 68.2x.x.0 mask 255.255.255.0

network 68.3x.x.0 mask 255.255.255.0

network 70.x.x.0 mask 255.255.254.0

network 72.x.x.0 mask 255.255.254.0

neighbor 67.x.x.8 remote-as xxxx3

neighbor 67.x.x.8 version 4

neighbor 67.x.x.8 prefix-list Cust in

neighbor 68.x.x.9 remote-as xxxx5

neighbor 68.x.x.9 version 4

neighbor 68.x.x.9 route-map bellout out

no auto-summary

!

ip as-path access-list 1 permit ^$

!

!

ip prefix-list Cust seq 5 permit 65.x.x.0/24

!

access-list 1 permit 67.x.x.0 0.0.0.255

access-list 1 permit 68.1x.x.0 0.0.0.255

access-list 1 permit 68.2x.x.0 0.0.0.255

access-list 1 permit 68.3x.x.0 0.0.0.255

access-list 1 permit 65.x.x.0 0.0.0.255

access-list 1 permit 70.x.x.0 0.0.1.255

access-list 1 permit 72.x.x.0 0.0.1.255

!

route-map bellout permit 10

match ip address 1

!

route-map bellout permit 20

match as-path 1



Correct Answer by cisco_lad2004 about 9 years 1 month ago

have checked what u advertise using this command ?


sh ip bgp neighbor x.x.x.x advertise-prefixes ?


Sam

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (2 ratings)
Loading.
cisco_lad2004 Fri, 05/09/2008 - 22:25
User Badges:
  • Gold, 750 points or more

1-ip prefix-list Cust seq 5 permit 65.x.x.0/24 ==> u accept prefix from customer.


2-network 65.x.x.0 mask 255.255.255.0 ==>u advertsie prefix via BGP.


3-route-map bellout permit 10

match ip address 1 ==> u control advertisement by route map.


all looks fine so far. so the only only thing I can think off is, do u have 65.x.x.x/24 installed in your routing table ( not BGP table).

unless it is there, it wont be advertised.

show ip bgp 65.x.x.x would show more info.

also, sh ip bgp neighbor x.x.x.x advertise-prefixes will show what is being sent out.


HTH


Sam



Harold Ritter Sat, 05/10/2008 - 07:15
User Badges:
  • Cisco Employee,

Samir,


All good points, except that the network statement is not required for 65.x.x.0/24 as it is received via BGP from customer.


Regards,

cisco_lad2004 Sat, 05/10/2008 - 07:23
User Badges:
  • Gold, 750 points or more

agreed !


not required meaning, having it does not cause any issues ?


TIA


Sam

Harold Ritter Sat, 05/10/2008 - 07:40
User Badges:
  • Cisco Employee,

Sam,


It shouldn't cause any issue as such. Just pointing out the fact that it is not necessary in this specific scenario.


Regards,

cuchara61 Mon, 05/12/2008 - 13:12
User Badges:

OK, I've taken out the network line and verified ip route and what I'm advertising and they all look right. AT&T still not seeing the 65.x.x.0 network. What about a "redistribute" command?



Giuseppe Larosa Tue, 05/13/2008 - 01:22
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello,

it looks like that ATT is only accepting IP prefixes with AS path = Your ASN only.


The customer prefix 65.x.x.0/24 should be seen by ATT with an AS path attribute of = 'YourASN CustomerASN'


So if ATT has a policy that accepts only an AS path made of ^YourASN$ it is filtering the 65.x.x.0/24 IP prefix.

Ask them to verify their incoming policy and to update it if necessary.


Another possible reason could be an unknown BGP next hop verify what is the BGP next hop for prefix 65.x.x.0/24 when advertising to ATT.


Hope to help

Best regards

Giuseppe

cuchara61 Tue, 05/13/2008 - 06:12
User Badges:

Output from bgp...


tripgw1#sho ip bgp 65.x.x.0

BGP routing table entry for 65.x.x.0/24, version 3

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Advertised to non peer-group peers:

68.x.x.9 (ATT)

custASN

67.x.x.78 from 67.x.x.78 (67.x.x.78)

Origin IGP, metric 0, localpref 100, valid, external, best


tripgw1#sho ip bgp nei 68.x.x.9 adv

BGP table version is 10, local router ID is 68.x.x.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete


Network Next Hop Metric LocPrf Weight Path

*> 65.x.x.0/24 67.x.x.78 0 0 custASN i

*> 67.x.x.0/24 0.0.0.0 0 32768 i

*> 68.x.x.0/24 0.0.0.0 0 32768 i

*> 70.x.x.0/23 68.x.x.5 0 32768 i

*> 72.x.x.0/23 68.x.x.5 0 32768 i


Will forward to ATT...

Correct Answer
cisco_lad2004 Tue, 05/13/2008 - 04:53
User Badges:
  • Gold, 750 points or more

have checked what u advertise using this command ?


sh ip bgp neighbor x.x.x.x advertise-prefixes ?


Sam

cuchara61 Mon, 10/27/2008 - 11:26
User Badges:

ATT was not accepting the redist - they have fixed. Thanks everyone...

Actions

This Discussion