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Community Member

BGP Help #2

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..


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), which is a summary of 4 subnet's you are using in your AS (; 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 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??


Cisco Employee

Re: BGP Help #2


With and without the static route configured, could you post router B configuration and show ip bgp x.x.x.x/19 ?



Community Member

Re: BGP Help #2


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


RouterA (metric 11) from RouterA (RouterA)

Origin IGP, metric 130, localpref 100, weight 32768, valid, sourced, best

Community: 82381832

Cisco Employee

Re: BGP Help #2

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.


Hall of Fame Super Silver

Re: BGP Help #2

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.


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

this allows to see BGP communities as ASN:value making the config more readable.


also you can check with

sh ip bgp neigh advertised-routes

if your prefix is there

Hope to help


Community Member

Re: BGP Help #2


Grazie Mille..

It is a lot easier to read with the new-format command applied..


Hall of Fame Super Silver

Re: BGP Help #2

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


Community Member

Re: BGP Help #2


Thank you for the assistance..


I tried creating a route.. A test route that could help us out.. 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

Routing entry for

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

RouterB# (nothing)

Considers the route via BGP and inserts it into the routing table but doesn't advertise the route...


Re: BGP Help #2


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?



Community Member

Re: BGP Help #2

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:, Local port: 179

Foreign host:, 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

Community Member

Re: BGP Help #2

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

Re: BGP Help #2


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?



Community Member

Re: BGP Help #2


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..


Re: BGP Help #2

Hi Mav,

it's not a crap.

You can just show the Note shown by this link:

IMHO, no debugging is necessary here, it's just the way BGP works.



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