05-06-2009 12:11 AM - edited 03-04-2019 04:39 AM
Nordick26 or Anyone,
Hope your out there..
Listen, I came across another BGP issue that doesn't make sense..
Not as complex as my other BGP question..
Anyways..
Router A ----- Router B ---- Our Network..
1. Router A is a stub
2. ISIS running between them
3. iBGP Peering
Router A has a x.x.x.x/19 to advertize to our network..
Router B receives the x.x.x.x/19 route via iBGP considers it as best and inserts it into the routing table.
Router B receives the x.x.x.x/19 network with the next-hop of Router A loopback Addess.
The problem is that Router B does not advertise the x.x.x.x/19 in it's BGP updates with our Network..
The rest of the network doesn't see the /19 network and is unreachable..
So, I remembered what you told me..
"Example: you want to advertise (to external peer) 10.1.1.0/24, which is a summary of 4 subnet's you are using in your AS (10.1.1.0/26; 64/26; 128/26; 192/26). You're receiving this 4 prefixis via iBGP, but in other BGP can advertise this out of you AS, you need to have 10.1.1.0/24 in your routing table. This is why you need to use the static route (pointing back to router you get the more specific route from, or pointing to Null0)."
So, I created a static route to that subnet on Router B pointing to Router A and it worked.. The network see it..
But, why wasn't it propagated throughout the network via BGP??
Router B had an entry in the routing table and considered it as best??
Why??
05-06-2009 07:18 AM
Hi,
With and without the static route configured, could you post router B configuration and show ip bgp x.x.x.x/19 ?
Thanks
Laurent.
05-06-2009 07:32 AM
Hello,
I can't post the config..
But the show ip bgp command I could..
show ip bgp x.x.x.x/19
BGP routing table entry for x.x.x.x/19, version 42556205
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Advertised to update-groups:
1 3 4
Local
RouterA (metric 11) from RouterA (RouterA)
Origin IGP, metric 130, localpref 100, weight 32768, valid, sourced, best
Community: 82381832
05-06-2009 09:45 AM
From this output, the prefix is advertised to group 1,3 and 4. Use the show ip bgp update-group to see if your router is part of one of those groups.
You can also use the debug ip bgp updates out cmd on routeur B and clear softly your BGP session with router B.
Laurent.
05-06-2009 11:39 AM
Hello Mav,
just two adds to what Laurent has already noted.
the route has a BGP community associated to it that can trigger a route filter in other routers.
Note1:
the BGP community is not showed using the bgp new-format if you have enable access I would recommend to add
conf t
ip bgp-community new-format
http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp2.html#wp1015944
this allows to see BGP communities as ASN:value making the config more readable.
note2:
also you can check with
sh ip bgp neigh
if your prefix is there
Hope to help
Giuseppe
05-06-2009 11:56 PM
Giuseppe...
Grazie Mille..
It is a lot easier to read with the new-format command applied..
Thanks..
05-07-2009 11:36 AM
Hello Mav,
I would suggest you to use
sh ip bgp update-groups N
to identify to what peers is the route advertised then you need to verify on the other router.
As noted by Milan route filters can play a role here.
They can be outbound RB and inbound RC
Hope to help
Giuseppe
05-06-2009 11:55 PM
Laurent,
Thank you for the assistance..
Listen..
I tried creating a route.. A test route that could help us out.. 20.20.20.0/30 subnet.. put it on Router A to simulate the issue I'm having..
Please take a look at the output..
RouterB#show ip bgp neighbors RouterA received-routes
BGP table version is 43689367, local router ID is RouterB Loopback
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i20.20.20.0/30 Router A 130 100 0 i
Total number of prefixes 1
RouterB1#show ip route 20.20.20.0
Routing entry for 20.20.20.0/30
Known via "bgp 448563", distance 200, metric 130, type internal
Last update from RouterA 16:54:20 ago
Routing Descriptor Blocks:
* RouterA, from RouterA, 16:54:20 ago
Route metric is 130, traffic share count is 1
AS Hops 0
RouterB#show ip bgp neighbors RouterC advertised-routes | i 20.20.20.0
RouterB# (nothing)
Considers the route via BGP and inserts it into the routing table but doesn't advertise the route...
Mav
05-07-2009 04:05 AM
Hi,
is there eBGP running between routers B and C?
If yes, isn't any route-map, filter-list, prefix-list used to filter the updates sent to router C?
Can you provide "sh ip bgp nei RouterC" output from router B?
BR,
Milan
05-13-2009 03:54 AM
Sorry for the late reply..
Trying to still figure out this problem...
The answer is no.. they are not running a eBGP relationship but a iBGP relationship..
The show command to follow:
RouterB#show ip bgp neighbors Router C
BGP neighbor is RouterC, remote AS xxxx, internal link
Description: FIRZ-NCORE-1
BGP version 4, remote router ID RouterB
BGP state = Established, up for 49w0d
Last read 00:00:00, last write 00:00:20, hold time is 180, keepalive interval is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(new)
Address family IPv4 Unicast: advertised and received
Graceful Restart Capabilty: advertised
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 26 24
Notifications: 23 2
Updates: 3737 19157612
Keepalives: 494634 494634
Route Refresh: 0 0
Total: 498401 19652272
Default minimum time between advertisement runs is 0 seconds
For address family: IPv4 Unicast
BGP table version 45189612, neighbor version 45189612/0
Output queue size : 0
Index 3, Offset 0, Mask 0x8
3 update-group member
Inbound soft reconfiguration allowed
NEXT_HOP is always this router
Community attribute sent to this neighbor
Sent Rcvd
Prefix activity: ---- ----
Prefixes Current: 370 8071 (Consumes 387408 bytes)
Prefixes Total: 6051 29163563
Implicit Withdraw: 620 21460343
Explicit Withdraw: 5323 7695149
Used as bestpath: n/a 7839
Used as multipath: n/a 0
Outbound Inbound
Local Policy Denied Prefixes: -------- -------
CLUSTER_LIST loop: n/a 6640
ORIGINATOR loop: n/a 263
Bestpath from this peer: 29074156 n/a
Bestpath from iBGP peer: 8259468 n/a
Total: 37333624 6903
Number of NLRIs in the update sent: max 341, min 0
Connections established 1; dropped 0
Last reset never
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 212.151.128.132, Local port: 179
Foreign host: 212.151.128.133, Foreign port: 21899
Enqueued packets for retransmit: 0, input: 0 mis-ordered: 0 (0 bytes)
Event Timers (current time is 0x11108B2014):
Timer Starts Wakeups Next
Retrans 498429 202 0x0
TimeWait 0 0 0x0
AckHold 10734169 7779444 0x0
SendWnd 0 0 0x0
KeepAlive 15 0 0x0
GiveUp 0 0 0x0
PmtuAger 0 0 0x0
DeadWait 0 0 0x0
iss: 663007513 snduna: 672655114 sndnxt: 672655114 sndwnd: 16327
irs: 3070273282 rcvnxt: 435114213 rcvwnd: 16198 delrcvwnd: 186
SRTT: 300 ms, RTTO: 303 ms, RTV: 3 ms, KRTT: 0 ms
minRTT: 0 ms, maxRTT: 368 ms, ACK hold: 200 ms
Flags: passive open, retransmission timeout, nagle, gen tcbs
Datagrams (max data segment is 536 bytes):
Rcvd: 19871123 (out of order: 41), with data: 19480260, total data bytes: 1659808226
Sent: 11157526 (retransmit: 202), with data: 498311, total data bytes: 9647600
05-13-2009 03:56 AM
Sorry for the late reply..
Trying to still figure out this problem...
The answer is no.. they are not running a eBGP relationship but a iBGP relationship..
What kind of information are you searching for, regarding the output show ip bgp neigh command..
Please let me know.. I will provide it
05-13-2009 04:37 AM
Hi,
so if I understand correctly there is router A peering by iBGP to router B and router B again peering by iBGP to router C?
Router B will not advertise the prefixes received from router A to router C then.
One of basic BGP rules is:
"Peering inside an AS must consist of a full mesh, because an iBGP speaker is not allowed to pass on prefixes learned from one iBGP peer to another iBGP peer."
Or am I mising something here?
BR,
Milan
05-15-2009 12:43 AM
Milan,
I think you absolutely correct..
But when I go and present my finding to my team lead he thinks it's crap...
I thinking of performing a debug on router C to find out exactly what is going on..
something like a debug ip bgp update
what do you recommend..
Mav
05-15-2009 01:23 AM
Hi Mav,
it's not a crap.
You can just show the Note shown by this link:
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#ibgp
IMHO, no debugging is necessary here, it's just the way BGP works.
BR,
Milan
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