Routing updates issue

[viswa sai]

Hello 

I have peering with two ISPs and i am receiving full routing table + default route with ISP 1 and partial routing table with ISP 2  2.1L routes + default route.

Last week, ISP 1 was down, i am using route-maps with as path prepend for network advertisements to receive traffic on ISP 2. But i have observed a problem with few destinations, some on my clients have complained that they are unable to reach internet  and when i check in the router and issue Sh ip route mail.yahoo.com or some domains.

Its say network not in the routing table.

What could be the problem,even with partial routing table as i have a default route to ISP2. I shouldnt be having issues. But when i removed the neibour filter and received full routes from ISP2. Then it is working fine

Can anyone tell me, what could be the reason and what i have to do during that time. 

[Josh Sprang]

If you are using the default route to reach mail.yahoo.com the output is as expected..  A show ip cef <mail.yahoo.com> should show the default..  

My guess is when ISP1 goes down you do not have full visibility to the internet so you need to use the default for non provider generated routes.  

This should be fine...  I would check the route back to the host that is complaining..  Does the edge router have a route to them during the problem?  Are all the people complaining in the same subnet?  

Is ISP2 on the same or different edge router?  

To troubleshoot I would fail to ISP2 during a maint. window.  From ISP2 router do a show ip cef <IP address of complaining user>  or show ip route..  make sure you can reach that host from the edge router.  if so then:

Make sure that ISP2 is receiving all your prefixes..  show ip bgp nei <ip add of nei> advertised-routes.

Call ISP2 and make sure they are getting these prefixes..  I have seen problems where ISPs have an inbound path filter that only allows one ASN in the path from you.. 

HTH

[viswa sai]

Thanks for reply,

Both ISP 1 & ISP 2 are terminated in a single edge router. I am able to see the prefixes from global routing table via ISP 2 when the ISP 1 was down.

As i said, when i try to randomly check the Sh ip route it says network not in table... I am using AS path prefix list filter on ISP 2 to get partial routes..

Issue was resloved after i removed the prefix list filter on ISP 2 and received full routes.

I am receiving full routes on the loopback IP from the ISP 1.. So ISP 1 has two BGP peers. Should i shut one of them during the outage?? what else can i do

[Josh Sprang]

Our you sure you are receiving a default from your peers? 

Can you send an output of "show ip bgp 0.0.0.0" 

Also can you send a copy of your router bgp  config + any route-maps/prefix-lists/access lists related?

Thanks

[viswa sai]

RTR>sh ip bgp 0.0.0.0
BGP routing table entry for 0.0.0.0/0, version 475355218
Paths: (5 available, best #4, table default, RIB-failure(17))
Multipath: eBGP
  Not advertised to any peer
  4755
    121.244.206.224 from 121.244.206.224 (192.168.211.193)
      Origin IGP, localpref 110, valid, external
  4755, (received-only)
    121.244.206.224 from 121.244.206.224 (192.168.211.193)
      Origin IGP, localpref 100, valid, external
  4755, (received & used)
    14.140.128.21 from 14.140.128.21 (192.168.243.4)
      Origin IGP, localpref 100, valid, external
  9498
    125.16.0.254 from 125.16.0.254 (203.101.87.87)
      Origin IGP, localpref 120, valid, external, best
  9498, (received-only)
    125.16.0.254 from 125.16.0.254 (203.101.87.87)
      Origin IGP, localpref 100, valid, external

[paul driver]

Hello

can you post the configuration this router?

res

Paul

[Ahmed Muhi]

Hello Viswa,

Can you post the route map config that you are using with ISP 1 and ISP 2. Since the problem is resolved when you remove the route map, maybe there is something to change in it.

All the best.

[Josh Sprang]

I agree with Ahmed.  Looks like you are blocking 0.0.0.0/0 in one of the route-maps..  Can you post your config?

[viswa sai]

ISP 2 configuration

 neighbor 125.x.x.2x4 remote-as 9498
 neighbor 125.x.x.2x4 ebgp-multihop 2
 neighbor 125.x.x.2x4 update-source Loopback8
 neighbor 125.x.x.2x4 activate
 neighbor 125.x.x.2x4 soft-reconfiguration inbound
 neighbor 125.x.x.2x4 route-map 18mb-bt-in in
 neighbor 125.x.x.2x4 route-map bt-18mb out
 neighbor 125.x.x.2x4 filter-list 28 in

--------------------------------------------
ip route 0.0.0.0 0.0.0.0 125.x.x.254

-----------------------------------------------

below filter list to receive partial routes

ip as-path access-list 28 permit ^9498$
ip as-path access-list 28 permit ^9498_7473_
ip as-path access-list 28 permit ^9498_3356_
ip as-path access-list 28 permit ^9498_3549_
ip as-path access-list 28 permit ^9498_3561_
ip as-path access-list 28 permit ^9498_15169_
ip as-path access-list 28 permit ^9498_32934_
ip as-path access-list 28 permit ^9498_10310_26101_
ip as-path access-list 28 permit ^9498_3257_
ip as-path access-list 28 permit ^9498_6762_
ip as-path access-list 28 permit ^9498_1299_
ip as-path access-list 28 permit ^9498_2914_

------------------------------------------------

prefix list for advertising networks via ISP 2

ip prefix-list 1nw seq 5 permit 196.x.x.0/24
!
ip prefix-list 2nw seq 5 permit 196.x.x.0/24
!
ip prefix-list 3nw seq 5 permit 196.x.x.0/24

-------------------------------------------------
route-map config

route-map 18mb-bt-in permit 10
 set local-preference 120

route-map bt-18mb permit 10
 match ip address prefix-list 1nw 2nw 3nw 4nw

[Ahmed Muhi]

Hello Viswa,

Your AS Path access list seem legit, I have one advice, why not combine the AS Path access list and the route map, so it would be instead:

route-map 18mb-bt-in permit 10

match as-path 28

set local-preference 120

And another point that caught my attention is the route map bt-18mb, the match statement is referencing a prefix list that does not exist the "4nw", can you double check this one.

All the best.

[viswa sai]

Hello Ahmed,

Thanks for your reply, So what could be the issue then?  I have a pretty similar config for ISP 1 expect the as path filter i am getting full routes.

As per my sh ip bgp , i am getting the default routes from ISPsI have also given a static route both ISPs. Can i remove those and check?.. thats a type regarding "4nw"

[Ahmed Muhi]

Hello Viswa,

Oh, I missed the static route when I first read your configuration, static routes are preferred over BGP routes because of the Administrative Distance so how are you preferring one ISP over the other. Can you give us the "show ip route 0.0.0.0".

Another point is that static routes are not removed from the routing table if the neighbor fails (i.e next hop is unreachable, but the interface is in the up/up state, this happen if you are connected to your ISP by some intermediate box like a switch) unless you use some type of SLA to confirm that the next hop is reachable and to remove the static route if the next hop becomes unreachable.

[viswa sai]

Hi

sh ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "static", distance 1, metric 0, candidate default path
  Redistributing via ospf 200
  Routing Descriptor Blocks:
  * 125.16.X.254
      Route metric is 0, traffic share count is 1
    14.1xx.1x8.x1
      Route metric is 0, traffic share count is 1

I have a doubt, if i learn a default route from ISP, how it would be?? i am not sure, whether my ISP is sending his default route to me.. ( previously i posted that config as well)

Can i remove the default static and learn via BGP?

[Ahmed Muhi]

Hello Viswa,

Your router is using the static route not the BGP route this is as we said because of the administrative distance, as you can see from the command "show ip route 0.0.0.0" it says Known via "static". From your previous post I can see you are already receiving the default route from both your ISPs.

Before removing the default route, can you tell me about this router is it your only router, I mean what was the reason for having this default route in the beginning.

Finally to see what prefixes your ISPs are advertising to you, use this command:

show ip bgp neighbor x.x.x.x received-routes

And to see what prefixes you are advertising to your ISPs use this command:

show ip bgp neighbor x.x.x.x advertised-routes

All the best!

[viswa sai]

Thank you Ahmed.  Static route has been following from WAN links on serial ports and I am not sure as why the static was given by my previous engineer. Now peers are moved to Gig E.

 I have been checking to see, how best to avoid the manual intervention during the bgp peer outage.

Onemore thing, i have a ospf domain which has a static route to the edge router. It was not learned route but was given static point to edge router for forwarding traffic.

Is it a way we can inject a static route for the distribution switches towards to the edge for forwarding the traffic.